TRANSMITTER OPERATIONS AND MAINTENANCE [AM-FM Broadcasting Equipment, Operations, and Maintenance (1974)]

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In the majority of am-fm broadcast installations today, transmitter and studio operations are combined. However, in the event of transmitter problem revealed the remote-control monitoring devices, the duties of the personnel often become divided-those concerned with studio maintenance and those concerned with transmitter maintenance.

In any event, it is desirable from an orientation standpoint to treat transmitter installations as a separate, but correlated, study from studio installations. In the instance of remote control from the studio, it is still the responsibility of the operator to analyze any malfunction, to remove the transmitter from the air if it is out of tolerance (this is automatic in many instances), and to place the standby transmitter (or the properly operating portion of a parallel transmitter) on the air.

During the regular broadcast day, the transmitter operator keeps the circuits properly tuned, maintains correct power input to the final stage, logs meter readings in accordance with current FCC rules ( which also aids in forestalling trouble), checks the frequency, and maintains the modulation at a level consistent with good engineering practice and the type of pro gram in progress.

The first discussion to follow will pertain to the all-important operation of the broadcast transmitting installation in order to achieve the best results possible from the finely engineered equipment available and in use today.

Operating practice at the transmitter is just as important to the final result of overall performance as it is at the broadcast studio. The operation of the transmitter and associated speech-input equipment may be shown to be a highly specialized art, and we have chosen the term "operational engineering" to define the content of the special study undertaken in this part of the Section.

Following the operations sections, the testing and maintenance of transmitting equipment will be discussed. Proof-of-performance measurements are detailed, and a suggested preventive-maintenance routine is given.


It is true that the primary purpose of the transmitter operator is to keep the station on the air. But with the increased demands for higher-fidelity program transmission, the day when a typical "ship operator" possessing thorough technical understanding could step into a broadcast installation has passed. The operator of a broadcast transmitting plant has a specialized range of duties requiring a technical education as well as a thorough under standing and appreciation of the more intangible values of program material.

A number of his fundamental duties are, of course, strictly technical in nature. In brief, they consist of turning on the transmitter before the beginning of the daily program schedule, checking all meter readings to make proper adjustments, checking level with the studio, shutting down the transmitter after sign-off, repairing and maintaining equipment, and testing for noise and distortion levels. During the daily operating schedule, he consistently monitors the program with a monitoring amplifier and speaker, adjusts line-amplifier gain in accordance with good engineering practice pertaining to percentage modulation ( the transmitter operator does not normally "ride gain" as does the studio operator), maintains correct transmitter tuning, logs all meter readings as required by the FCC, and corrects any trouble that develops in the shortest possible time in order to keep the station on the air.

The transmitter operator in all but the lowest-power local stations is usually scheduled to be on duty at least 30 minutes prior to air time for the purpose of getting the equipment ready for the broadcast day. The start of an operator's day ( when on duty in person at the transmitter) may be outlined as follows:

1. Power is applied to the audio rack, including such measuring equipment as the frequency and modulation monitors. The audio line is used as a program loop and opened by inserting a patch cord into the line jacks. This removes the line from the input to the line amplifier and prevents any test program that might be on the line from the studio from being applied to the transmitter when it is first turned on.

2. A visual inspection of all relays in antenna-phasing cabinets ( where used) and in coupling houses at the antenna is performed. Relay armatures are manually operated to ascertain freedom of movement.

All rf meters are observed for bent hands or zero set.

3. An inspection of all safety gaps is carried out, including antenna and transmission-line lightning gaps for approximate correct spacings.

4. Air-cooling systems usually start the blower motors when the filament-on switches are operated. Transmitter filaments are next turned on and filament voltages checked. Minimum voltage should first be applied to large power tubes that have tungsten-type filaments; then the voltage should be run up to normal after about 3 to 5 minutes.

This procedure is automatic in some transmitters and helps to lengthen the usable life of such power tubes. Tubes with thoriated tungsten or oxide-coated filaments, such as those used in the low-power stages, are always operated at normal filament voltage for maximum tube life.

5. Plate voltage can then be applied to low-power units or exciter units (power installations of 1 kW or more) to check for proper excitation to the final stage.

6. Low power is then applied to the final stage. All meter readings are checked for normal low-power operation. If everything is normal, high power is applied and meter readings are checked.

7. Filament and line voltages are checked and adjusted for high-power operation. Final adjustment is made on the final stage for optimum meter readings regarding resonance and power input.

8. Since the control-room operator sometimes has circuits "hot" with his own testing procedure, the transmitter operator plugs a patch cord from the program line to a monitor amplifier to ascertain continuity of the program line.

He then notifies the control operator to stand by for an overall circuit test. When this has been done, the transmitter operator removes the patch cord, automatically restoring the input line amplifier. A test tone may then be fed from the studio to check the overall continuity of the circuits from the studio to the transmitter modulators. In the event of remote-control operation, the studio operator has full control over the transmitter input.

Level checks with the studio are not required as a daily procedure after an initial installation has been made, tested, and operated for some time, since with properly operating equipment the level remains nearly the same over a period of time. At regular intervals, however, it is desirable to use a signal generator to check the frequency characteristics of the line and transmitting equipment. In this connection, it is advisable for the transmitter operator to understand the difference in modulator power requirements for sine waves and the complex waveforms of speech or music program content.

It will be remembered from circuit theory that for a class-C modulated amplifier the power requirement for complete sinusoidal modulation is 50 percent of the dc power input to the modulated tube or tubes. Fig. 11-8 (Section 11) shows how the peak factor of speech or music waves varies greatly from that of a pure sine wave, when "peaked" the same on a VU meter. This peak factor of program waves is 10 to 15 dB more than that of a sine wave. That is, the ratio of peak to rms voltage is far greater for complex waveforms than for sine waves. The average power for complete modulation of a transmitter over a period of time is far less than the aver age power required for complete modulation by means of a signal genera tor. It is a well known fact that for program signal waves the modulator power required may be 25 percent or less of the dc power input to a class-C stage. Therefore, if a signal generator is used at the studio for frequency runs or level checks, the transmitter operator must realize that if he adjusts the gain on the line amplifier to give 100 percent modulation on sine waves, the same adjustment will be 10 to 15 dB high for program signals.

The gain adjustment must be lowered accordingly to the point that experience has dictated for program modulation before the actual program schedule starts. In the past, this has led to some confusion among transmitter operators.

This difference in peak factor between program and sine waves is also noticed when comparing the percent of antenna-current increase with 100-percent modulation. It is true that the antenna-current increase should be approximately 22.5 percent over no modulation when a sine wave is applied to the transmitter at 100 percent modulation. Antenna current in creases for 100 percent program modulation, however, will be much less, due not only to the difference in peak factor, but also the sluggishness of the thermocouple rf meter action. This slowness of action is due to the heating effect of the two dissimilar metals on which the action of the meter depends.


It is only natural that the program level being sent by wire from the studio be of utmost importance from a strictly operational point of view to the transmitter operator. With competent studio personnel, the line-amplifier gain adjustment may be set for 100-percent modulation on program peaks at the start of the day and left at that adjustment. Many times, how ever, a transmitter operator, who in some cases may not appreciate musical and dramatic values, will become piqued with the control operator when the program level is very low. He should realize, however, that broadcast stations are not strictly for "communications," but are intended to bring entertainment into the home with as much of the original content as possible consistent with the state of the art. Certain types of programs, sym phony concerts in particular, are meant for those listeners in the primary service area and not intended to override the noise level at some secondary service point. If the monitor speaker is turned up in volume consistent with that of the interested listener at home for these types of programs, the transmitter operator will be able to use good judgment as to whether the signal is or is not entirely too low in level to be usable for proper modulation.

Program Levels

In relation to the study of program levels, it is of primary importance to understand the characteristics of the indicating meters used at both ends of the transmission system. These meters differ in characteristics because of the different functions which they are intended to perform. The standard VU meter, used in most broadcast studios today, is an rms-indicating full-wave rectifier device intended to give a close visual approximation of the sound waves emanating from the speaker. However, the concern at the transmitter is with modulating voltages, and a semi-peak indicating device is necessary and required by the FCC. If peaks of the program signal con tent should be excessive and occur in rapid succession, danger of circuit component breakdowns would arise as well as severe adjacent-channel interference. Therefore, since the peak factor of program waves is high, the modulation meter is a peak-indicating device. It is also necessary that a phase-reverse switch be incorporated in the modulation-meter circuit to switch the polarity of the input to the metering circuit so that either the positive or negative side of the modulated envelope may be monitored separately. Thus, it is obvious that there are two distinct types of level meters, namely, a full-wave rms meter at the studio and a half-wave peak meter at the transmitter. In addition to these meters, there is a limiting-type amplifier (in most modern installations) which is used at the transmitter as a line amplifier. This has meters which measure the amount of compression ( full-wave peak meter) and output level in VU ( full-wave rms meter).

The number of different types of indicating meters should not confuse the operator as long as the proper interpretation is given to the readings.

Fig. 14-1 shows the indication of a program peak at a given instant on the various meters involved. The studio VU meter has registered 100; the compression meter at the transmitter shows the normal 5-dB limiting; the line-amplifier output meter shows 100; and the modulation meter would show either 100-percent modulation on positive peaks or, if set to monitor negative peaks, might show only 60-percent modulation. This, of course, could be just reversed with a change in polarity of the microphone output or any connection in between.

It is a well known fact that speech waves are not equal in positive and negative peaks regardless of the type of microphone used. This may be observed from the graph of the speech wave shown in Fig. 14-2. Two speakers working from opposite sides of a bidirectional microphone and peaked the same on the studio VU meter will not give equal indications on the modulation meter when it is set to indicate a certain peak (either positive or negative) because of the negative-peak effect.

Assume, for example, that the modulation-monitor switch is set to monitor the negative peaks, and the indication of one voice is close to 100 per cent. The indication of the voice on the other side of the microphone (therefore of opposite polarity at the microphone output transformer) may indicate only 40 to 50 percent, with the amplitude of the studio VU meter remaining the same. For this reason, it is obvious why misunderstandings sometimes arise between studio and transmitter personnel regarding the comparative levels of two or more voices.

Fig. 14-1. Indications of a given program peak on several meters.

What indication exists at the transmitter plant to show a true indication of comparative levels from the studio? It has been shown that the half-wave reading of the modulation meter, which depends on the polarity of operation, is not a true indication of comparative levels from the studios. The VU meter at the output of a limiting amplifier would not be a true indication since the output level is limited by the compression taking place in the amplifier for signals over a predetermined level. The compression meter, although a full-wave indicating device, is a peak-reading instrument, and, since the peak factor of program waves varies considerably, it is not an absolutely accurate indication of comparative levels. It is, however, the most reliable indication (within limits) existing at the transmitter, since it is full-wave rectified and is limited by only wire-line characteristics. If two voices, for example, show about the same amount of compression, the comparative levels (not loudness) may be considered very nearly the same.

100-Percent Modulation ( AM)

Fig. 14-3A shows an oscillographic pattern of a carrier modulated 100 percent by a sine-wave tone. This illustration shows what constitutes positive and negative modulation of the carrier. It may be seen that negative, or trough, modulation cannot attain more than 100 percent of the avail able range, whereas positive, or peak, modulation may go over 100 percent.

When a carrier is thus modulated with a pure tone, the degree of modulation is:

m Eav- Emin=

Eav where, m is the degree of modulation, Ea, is the average envelope amplitude, E,n1,, is the minimum envelope amplitude.

0.001 Sec

Fig. 14-2. Curves indicating peak factors of voice and sine waves.

The peaks and troughs of the envelope will be equal. When the minimum envelope amplitude (negative peak modulation) is zero in the foregoing equation, m is 1.0, and the degree of modulation is complete, or 100 percent expressed in percentage modulation.

When the envelope variation is not sinusoidal, such as is true for pro gram signals, the positive and negative peaks will not be equal, and the percentage of modulation differs for the peaks and troughs of modulation as follows:

Positive peak modulation = E°'" Ea° E"° 100

Negative peak modulation- Emit' 100 where, E"° is the average envelope amplitude, Emi" is the minimum envelope amplitude, E. is the maximum envelope amplitude.

Thus, it is possible to understand why the trough modulation cannot exceed 100 percent, since the minimum voltage cannot be less than zero. It may be seen, however, that the positive peak voltage may be more than twice the average (or carrier) voltage, in which case the positive peak modulation will exceed 100 percent. What important information does this hold for the transmitter operator? First, it should be clarified in the operator's mind that overmodulation can take place on the negative ( trough) modulation as well as on the positive (peak) modulation. It is true that the degree of modulation can never exceed unity on the negative peaks, but it can exceed unity on the positive peaks. Complete modulation (of a class-C stage) , however, requires that the peak values of the modulating voltage equal the dc plate voltage of the modulated stage. Fig. 14-3B shows an oscillographic pattern of a carrier wave with modulating voltage exceeding the dc plate voltage and causing overmodulation of the carrier. It is true that the positive modulation peaks exceed unity while the negative peaks are cut off by the excessive negative modulating voltage and cannot exceed unity. This excess energy, however, which allows the voltage applied to the rf-amplifier plate circuit to become negative with respect to ground, causes radiation in the form of spurious frequencies, resulting in "splatter" and adjacent-channel interference.

Positive Negative

(A) 100 % modulation. (B) Overmodulation.

Fig. 14-3. Modulation envelopes.

This actually is overmodulation in its severest form, since positive peaks may extend beyond 100-percent modulation without amplitude distortion, whereas clipping of the negative peaks will cause severe amplitude distortion. It will be remembered that the bandwidth occupied by the carrier and sidebands depends ( for amplitude modulation) not on the degree of modulation, but on the highest frequency being transmitted. Amplitude distortion resulting from negative-peak overmodulation generates a number of distortion harmonics that may extend high enough to spread the sidebands into adjacent channels.

This discussion has been presented in order to show the transmitter operator that the negative side of the modulation is the most important peak to monitor on the modulation meter. It should be held under 100 per cent at all times. It is well to remember that a modulation meter of the vacuum-tube-voltmeter type will not be able to indicate over 100 percent (negative) on the meter because the peaks cannot attain more than this value. This is why an oscilloscope is sometimes used at a broadcast transmitter to show negative peak overmodulation, since the negative peak clip ping shows up as white lines across the center of the modulated pattern.

When the usual vacuum-tube-voltmeter type of modulation indicator is used, the flasher should be set for 95-percent modulation so that the warning is given usually before the meter ever swings to this value. A flasher will respond on a much faster increment of program peaks than will the indicating meter movement.

NOTE: FCC rules at the time of this writing limit positive peak modulation to 125 percent. Always check current FCC rules.

FM Modulation-Monitor Interpretation

Peak amplitude variations in the positive and negative directions show up on an fm transmitter monitor as a decided difference in amplitude of the plus and minus excursions of the meter. In this case, however, we are not concerned with either peak in relation to distortion, since modern fm transmitters are able to over-modulate greatly either plus or minus without inherent distortion. Distortion does occur, however, in the receiver when overmodulation over noticeable periods of time occurs, since few receivers have the capability of providing faithful reproduction of a modulating signal that produces a frequency swing much in excess of the maximum of 150 kHz (-x-75 kHz, which is defined as the value of 100-percent modulation for fm broadcasting).

The fm transmitter operator, therefore, is concerned with preventing overmodulation on either peak. For this reason, it is of utmost value to have all studio microphones connected so that their maximum polarity occurs on a definite side, either plus or minus on the modulation monitor. Otherwise, the transmitter operator must check his peaks at each change of microphone before he is certain that the maximum peaks are not over 100 percent.

Automatic Correction of Nonsymmetrical Audio Peaks

The problem of peak polarity in proper modulating techniques, particularly important in AM transmission, has been stressed. A special passive device designed to distribute equally nonsymmetrical audio peaks, particularly those produced by certain inherent characteristics of the human voice, is shown in Fig. 14-4. By removing asymmetrical energy, this unit permits higher average modulation, thus giving the effect of higher transmitter power with no change in the program signal level.

Courtesy Kahn Research laboratories, Inc.

Fig. 14-4. A device for removing asymmetrical energy from audio peaks.

Long-line telephone circuits normally correct for speech asymmetry, and music seldom contains unbalanced waveforms. Thus, asymmetrical modulation peaks are mainly caused by live or tape-recorded voice programs originating locally or over relatively short telephone lines. Since the main purpose of this device is to distribute nonsymmetrical energy equally with out disturbing symmetrical sources, the remaining modulation problem is thereby removed. Local voice program levels can be raised to equal those of other programs without danger of overmodulation.

This unit also offers another advantage for users of compressor, limiter, or uniform-level type amplifiers. With reduced peak energy, limiting will take place only during higher average modulation levels. When speech clippers are employed, the unit removes the low-frequency bounce normally produced by dc shifts in clipped nonsymmetrical waves.


The limiting amplifier, or compression amplifier, is a very important link in a broadcast installation. However, its effect may not be advantageous if the wrong operational interpretation is given to the main purpose for which it is designed. This type of amplifier, as designed for use in a broad cast installation, is intended as a peak limiting device, the amount of gain reduction being a function of the program peak amplitude. In order to prevent material reduction in the dynamic range of the signal, the peak gain reduction should not be more than 3 to 5 dB. A broadcast limiting amplifier, therefore, should not be considered as a volume limiter, but as a peak limiter intended to prevent adjacent-channel interference and over loading of transmitter components.

In some installations, a special agc amplifier is used for automatic announcer override of musical background. In this application (which can be considered a small step toward automation) , the normal program reaches the agc input at -35 dB, for example. The announce circuit has added amplification ahead of the agc amplifier so that the signal is at a level of -20 dB, for example. Since the agc tends to hold the output constant, about 15 dB of program reduction occurs so that the announce portion is at normal level and gain riding of background is not necessary. The reader should bear in mind that the main limiter amplifier has a normal limiting of only 3 to 5 dB and is not to be confused with the operation previously noted, where the actual compression of the announce portion may be as much as 15 to 25 dB.

It is true that doubling the output power of a transmitter raises the signal intensity 3 dB. It is also true that the limiter amplifier raises the signal level about 3 dB on program peaks. To those familiar with volume indications of program circuits, however, this 3-dB increase on speech or music is of small consequence. As far as the transmitter is concerned, the operator should think of this amplifier as a protective device to limit peaks caused by line transmission and those program peaks that escape the action of the control-room operator.

That the primary purpose of a limiting amplifier may be defeated by erroneous operation is a very important fact for the broadcast operator to know. Seriously detrimental effects will result if this amplifier is operated as a volume-compression device to attempt to provide a coverage area greater than a given power and transmitter location warrant. The attack time of peak limiting (about 0.001 second) is determined by a resistor-capacitor charging circuit with the inherent characteristics of a low-pass filter. At high frequencies, and where the duration of the peak is short compared to this operating time, a portion of the peak energy will escape limiting action. If the average signal level is so high that a great amount of compression takes place at all times, a larger amount of adjacent-channel interference will result, defeating one of the main purposes of the amplifier.

This has been quite noticeable in practice when the program content consists of music from dance orchestras of brass instruments where high peak powers at high frequencies are prevalent. A limiting amplifier operated properly for broadcast service will show about 3 to 5 dB of intermit tent gain reduction as indicated by the peak-reading meter used to show the amount of program peak compression. The operator must realize that for certain types of programs, such as symphonies, liturgical music, and operas, the average audio signal may be very low over a period of time-even with limiting amplifiers in use. Dynamic range is just as important to high fidelity transmission of these types of programs as is the frequency range. Another consideration is the recovery time, or time required to restore the gain to normal after a peak has momentarily reduced the gain. Optimum recovery time varies for different types of program material. Piano music, for example, sounds unnatural when the recovery time is too short, because the effect is similar to inadequate damping of the strings after they are struck or to holding the sustaining pedal too long on the loud notes.

As the recovery time is lengthened, however, the gain will be reduced (in effect) a greater proportion of the total time, and unnatural transmission of certain passages in music will result. If limiting amplifiers are operated properly and not subjected to more than the specified amount of peak load, however, they will serve their primary purpose quite satisfactorily without introducing undesirable effects.

When thinking of a compression amplifier as a means of increasing the service area of a transmitter, keep in mind the known facts concerning the psychological differences that exist in listening habits for various types of programs. A lower relative signal level is tolerable for dance music, news broadcasts, etc., where the average audio level is high over a period of time.

In this case where listeners well outside the primary service area of the station may be numerous, the maximum amount of peak limiting may be used to help raise the signal-to-noise ratio at the receiving point. However, symphony broadcasts, choral music, certain liturgical music, opera, etc., where the average audio signal may be very low over a period of time, will appeal only to those listeners who are adequately served with strong carrier signals. In the interests of preserving the original dramatic effects of this type of program, it simply is not technically feasible for a broadcaster to attempt to set a fixed value of coverage area for all types of program material. Similarly, the engineer responsible for the transmission of programs should not attempt to operate all equipment in the same manner regardless of the type of programs being transmitted.


When an antenna is properly tuned, the reactive component is cancelled, leaving only the resistive component. The power input is the magnitude of the square of the rf current times the antenna resistance in which the cur rent is measured. The antenna resistance is determined during the original installation and is part of the data filed with the FCC. This should be checked at periodic intervals.

There are two basic methods of measuring the antenna impedance, the rf-bridge method and the substitution method. In general, a greater degree of skill is necessary in using the rf bridge than in using the substitution method. However, the bridge method is more accurate and reliable. It re quires becoming thoroughly acquainted with the particular bridge used. It is highly desirable to obtain initial guidance from an experienced user of the bridge.

Bridge Method

Fig. 14-5 shows the equivalent circuit of a shielded rf bridge such as is used for antenna work. The rf impedance is measured either by a substitution method on the bridge (not to be confused with the substitution method without a bridge as described later) or directly by the unity-ratio bridge method.

Bridge Input From RF Oscillator Note: Capacitors with dashed connections represent capacitance of shield.

Fig. 14-5. Equivalent circuit of shielded rf bridge.

Method 1 (Bridge-Substitution Method)--The unknown impedance is placed in parallel with C5R5. Then R8 is adjusted to zero, and Cs is adjusted to some convenient value. The bridge is then balanced ( zero output at the output terminals) with a convenient resistance and capacitance in the X arm. The unknown impedance is then removed. The resistive and reactive components are now determined from the changes required in the calibrated Cs and Rs controls to restore the balance. Obviously, in the use of any bridge, the manufacturer's instructions and precautions must be followed carefully.

Method 2 (Unity-Ratio Bridge Method)

The bridge is first balanced with Rs shorted to ground. The balance is made by adjusting Cs or by the use of an external capacitance from terminal A or C to ground as required.

The unknown impedance is then connected to the X arm. The bridge is again balanced by adjusting C8Rs to match the unknown impedance, indicating the resistive and reactive component values. With this particular type of bridge, when the unknown impedance is inductive, it must be made capacitive by shunting it with a known capacitance (shown in Fig. 14-5 as C') so that balance may be obtained by the capacitance in the standard (S) arm.

Refer to Fig. 14-6 (T section of a single-antenna tuning unit). The reactance values for the circuit can be calculated as follows:

X1 = +-VR1R2 X2=+VR1R2 X3=-VR1R2 where,

X1 is the reactance of L1, X2 is the reactance of L2, X3 is the reactance of C, R1 is the line impedance, R2 is the antenna resistance.


Fig. 14-6. Representation of single T section, antenna tuning unit.

First, it is necessary to determine the impedance of the antenna at the operating frequency so that the proper values of the inductance and capacitance arms may be computed. Impedance measurement of the antenna is made directly at the antenna-tower input terminal with the line disconnected. Usually, measurements are made at frequencies above and below the operating frequency, and the resistance at the operating frequency is determined from a graph of antenna resistance versus frequency.

As an example, suppose a tower has a height of 190 electrical degrees and 28 ohms of resistance and +j25 ohms of reactance. The transmission-line impedance is 70 ohms. Substitution of these values in the preceding equations gives:

- /(70) (28) _ x/1960 = 44 ohms (approx)

Therefore, X1, X2, and X3 should have a value of 44 ohms at the operating frequency.

The antenna reactance in this case is +j25 ohms. It is assumed that this reactance is a part of the value of X2. Therefore, subtract the 25 ohms of positive antenna reactance from 44 ohms, giving 19 ohms to be obtained in X2. When X1 and X3 have been adjusted to 44 ohms, an impedance match is produced between the transmission line and the antenna resistance. Adjustment of X2 to 19 ohms cancels the antenna reactance, which would cause a loss of efficiency and a high vswr.

The rf impedance bridge is used in making these adjustments by connecting it across the input of the matching network with the transmission line disconnected. When measurements indicate that an input impedance of 70 ohms has been obtained (with antenna connected), the transmission line is reconnected for the final check before applying power from the transmitter.

Care should be taken that the capacitive branch uses capacitors having a high enough current rating to withstand the current at that point. Usually two or more capacitors of the proper value (to total the required value) are used to handle a current larger than one could safely carry.

Substitution Method When an impedance bridge cannot be obtained for the necessary length of time, a method commonly known as the substitution method is used to tune the antenna system. Fig. 14-7 shows the arrangement of coupling circuit and tuning unit necessary for this procedure.

Fig. 14-7. Substitution method for adjusting antenna system.

It may be seen that it is necessary to switch the line from the tuner to a substitution resistance, which should equal the line impedance, so that a change in line current indicated by the meter (M) is noted when the switching action occurs. The tuner is properly adjusted when no change occurs.

Obviously, full power must not be applied to the line, since the voltage rating of a nonreactive resistor is comparatively low. Therefore, connections must be made from a low-power stage in the transmitter, or an accurate rf oscillator may be employed.

Capacitor C is first tuned for a maximum current reading in the line meter (M) with the switch in the resistor position. The tuner input is then switched in, and C is adjusted again for maximum line current. The change in the capacitance of C indicates whether the antenna circuit reactance is capacitive or inductive. If it is necessary to increase the capacitance, the load is capacitively reactive. Conversely, if the capacitor must be de creased to increase the line current, the load is inductively reactive. If no change in C is necessary, the load is resistive.

Final Check of Antenna Match

Whichever method of tuning has been employed, a final check of match conditions should be made before full power is applied to the circuits. The measuring equipment is removed, and low-range thermal milli-ammeters are inserted at each end of the ungrounded conductor of the line. Sufficient power is then applied from a low-power stage of the transmitter so that the meters give an adequate reading. When the tuning adjustments are correct, these readings agree within 15 percent, showing a proper feeding match between the line and tower networks.


The inverse field is the un-attenuated ground wave (the ground-wave field intensity that would exist considering only the effect of distance) and is used for comparison purposes. The rms value of this field is the radius of a circle (Fig. 14-8) which has the same area as the pattern formed by all the inverse-field-strength values at 1 mile in all horizontal directions from the antenna. The inverse field is found by taking measurements of the actual attenuated field with a field-strength meter; this information is then used to determine the un-attenuated field strength at 1 mile in the direction of each radial. The rms value is then:

E = E102 + E202 + ... + E3002 36 where, Fe is the field strength,

E10 is the field strength at an azimuth angle of 10° (un-attenuated) ,

E20 is the field strength at an azimuth angle of 20°, etc.

Less computation is involved if the actual field is first plotted on polar paper and a polar planimeter used to determine the equivalent-area circle.

The unattenuated ground wave is inversely proportional to the distance and proportional to the square root of the power. Thus if 100 mV/m is produced at 1 mile with 1 kW, 5 kW will produce times 100 mV/m, or 223.5 mV/m from the same antenna.

Fig. 14-8. Inverse field.

Fig. 14-9. Block diagram of a field-strength meter.

Fig. 14-9 is a fundamental block diagram of a field-strength meter. A loop antenna into which a known oscillator amplitude may be injected for calibration of the meter is normally employed for AM measurements. The measurement of the received signal may be taken from the calibrated attenuator and/or gain controls. Most of the later meters for standard AM frequencies are designed for direct reading in millivolts per meter or microvolts per meter. It is obviously essential to become thoroughly familiar with the equipment before field-strength measurements are attempted.

The following requirements govern the taking and submission of data on the field intensity produced. First, have available a sufficient quantity of log-log graph paper. For a direct match to the FCC Documentary Graphs use K&E Ground-Wave Field Intensity paper No. 61729. Other graph papers that may be used are Dietzgen 3 x 3 cycle log No. 340-L33, or K&E No. 359-120. The steps to be followed are prescribed by the FCC as follows (be sure to check the current rules):

1. Beginning as near to the antenna as possible without including the induction field and to provide for the fact that a broadcast antenna is not a point source of radiation (not less than one wavelength or 5 times the vertical height in the case of a single-element antenna or 10 times the spacing between the elements of a directional antenna) , measurements shall be made on eight or more radials, at intervals of approximately one-tenth mile up to 2 miles from the antenna, at intervals of approximately one-half mile from 2 miles to 6 miles from the antenna, at intervals of approximately 2 miles from 6 miles to 15 or 20 miles from the antenna, and few additional measurements if needed at greater distances from the antenna. When the antenna is rurally located and unobstructed measurements can be made, there shall be as many as 18 or 20 measurements on each radial. However, when the antenna is located in a city where unobstructed measurements are difficult, measurements shall be made on each radial at as many unobstructed locations as possible, even though the number of intervals is considerably less than stated above, particularly within 2 miles of the antenna. In cases when it is not possible to obtain accurate measurements at the closer distances ( even out to 5 or 6 miles due to the character of the intervening terrain) , the measurements at greater distances should be made at closer intervals. (It is suggested that "wave tilt" measurements may be made to determine and com pare locations for taking field-intensity measurements, particularly to determine that there are no abrupt changes in ground conductivity or that reflected waves are not causing abnormal intensities.)

2. The required data should be plotted for each radial in accordance with either of the two following methods:

(A) Using log-log coordinate paper, plot the field intensity as the ordinate and the distance as the abscissa.

(B) Using semilog coordinate paper, plot the field intensity times distance as the ordinate on the log scale and the distance as the abscissa on the linear scale.

3. Regardless of which of the methods is employed, the proper curve to be drawn through the points plotted shall be determined by comparison with the curves in FCC Paragraph 73.184 as follows: Place the sheet on which the actual points have been plotted over the appropriate graph in FCC paragraph 73-184. Hold it to the light if necessary and adjust until the curve most closely matching the points is found.

This curve should then be drawn on the sheet on which the points were plotted, together with the inverse-distance curve corresponding to that curve. The field at 1 mile for the radial concerned shall be the ordinate on the inverse-distance curve at 1 mile.

Fig. 14-10. Example of field-strength plot.

Fig. 14-10 shows a typical graph; in this instance the inverse field at 1 mile is found to be 210 mV/m. Dot this in on the 1-mile abscissa and the 210-mV/m ordinate. Since the inverse field varies inversely with the distance, place another dot on the 10-mile abscissa at the 21-mV/m ordinate (one-tenth of 210). The entire inverse curve may then be drawn with a straightedge through these two points.

4. When all radials have been analyzed, a curve shall be plotted on polar-coordinate paper from the fields obtained which gives the inverse- distance field pattern at 1 mile. The radius of a circle whose area is equal to the area bounded by this pattern is the effective field.

5. While making the field-intensity survey, the output power of the station shall be maintained at the licensed power as determined by the direct method. To do this it is necessary to determine accurately the total antenna resistance (the resistance variation method, the substitution method, or the bridge method is acceptable) and to measure the antenna current by an ammeter of acceptable accuracy.

The complete data taken in conjunction with the field-intensity measurements should be recorded, including the following:

1. Tabulation by number of each point of measurement to agree with the map required in item 2 below, the date and time of each measurement, the field intensity (E) , the distance from the antenna (D), and the product of the field intensity and distance (ED) (if data for each radial are plotted on semilogarithmic paper, see above) for each point of measurement.

2. Map showing each point of measurement numbered to agree with tabulation required above.

3. Description of method used to take field-intensity measurements.

4. The family of theoretical curves used in determining the curve for each radial properly identified by conductivity and dielectric constants.

5. The curves drawn for each radial and the field-intensity pattern.

6. Antenna resistance measurement:

(A) Antenna resistance at operating frequency.

(B) Description of method employed.

(C) Tabulation of complete data.

(D) Curve showing antenna resistance versus frequency.

7. Antenna current or currents maintained during field-intensity measurements.

8. Description, accuracy, date, and by whom each instrument was last calibrated.

9. Name, address, and qualifications of the engineer making the measurements.

10. Any other pertinent information.

Fig. 14-11. Example of field-strength log for directional antenna.


A station license may require periodic field-strength measurements (particularly in the case of directional arrays). Even if it is not specifically required, it is a good practice to make these measurements at least quarterly.

The following data are required:

1. Monitor-point identification. On the log for field-strength measurements, this may be only an identifying number. However, a complete description for each point must be on file at the station, usually a duplicate of the original proof-of-performance data filed with the FCC for license application.

2. Specified unattenuated field strength at 1 mile (inverse field) .

3. The inverse field at 1 mile obtained.

4. Actual received field strength at monitoring point logged.

An example of a field-strength log is shown in Fig. 14-11.

It is a good practice to make a complete list of components employed in the antenna system, such as meter types and calibration, types and manufacture of coils and capacitors, etc., along with the actual bridge measurements of reactance for the portions of the coils and (variable) capacitors used. This allows quick replacement and adjustment in case of damage by electrical storms or other causes. Keep a record of all dial settings on a matching or phasing unit. Keep all components and relay contacts clean.


The important characteristics of any modern broadcast installation are an adequate frequency range to convey as much of the original sound as possible, low noise and distortion levels necessary for the required dynamic range, and dependability of performance. One of the most important pieces of auxiliary equipment in the transmitting plant is the instrument used to determine noise and/or distortion over the usable frequency range. Several manufacturers supply such equipment, and most stations are equipped with a means of checking noise and distortion.

Adjustments and maintenance must be performed to assure that the following FCC requirements are met. Always check the latest revisions of FCC rules.

1. The total audio-frequency distortion from microphone terminals to the antenna output, including the microphone amplifier, must not exceed 5 percent harmonics (voltage measurements of arithmetical sum or rss) when modulating from 0 to 84 percent, and not over 7.5 percent harmonics (voltage measurements of arithmetical sum or rss) when modulating 85 to 95 percent. (Distortion shall be measured with modulating frequencies of 50, 100, 400, 1000, 5000, and 7500 Hz up to the tenth harmonic or 16,000 Hz, or any intermediate frequency that readings on these frequencies indicate is desirable.)

2. The audio-frequency transmitting characteristics of the equipment from the microphone terminals (including microphone amplifier un less microphone frequency correction is included, in which event proper allowance shall be made accordingly) to the antenna output must not depart more than 2 dB from the response at 1000 Hz be tween 100 and 5000 Hz.

3. The carrier shift (current) at any percentage of modulation must not exceed 5 percent.

4. The carrier hum and extraneous noise ( exclusive of microphone and studio noise) level (unweighted rss) must be at least 45 dB below 100-percent modulation for the frequency band of 30 to 20,000 Hz.

Refer to Fig. 2-14 (and associated text in Section 2) for a typical audio-oscillator feed used for a broadcast proof of performance. Remember that this proof requires the signal-generator output to feed a studio microphone preamplifier input.

Fig. 14-12 shows a block diagram of a typical noise and distortion meter, which is also used to check frequency response. In measuring harmonic distortion, the following takes place:

Fig. 14-12. Block diagram of noise-distortion meter.

A. The amplitude of a single-frequency sine wave is measured.

B. A tuning circuit is adjusted to suppress the fundamental frequency.

This is done by a sharply tuned circuit and bridge-balance control to achieve at least 80 dB of suppression at the fundamental frequency.

C. The remaining measured amplitude is the total harmonic distortion.

Normally the transmitter output measurement is taken from a special output of the modulation monitor designed for this purpose. However, an AM detector may also be provided for direct off-the-air measurement as shown in Fig. 14-12.

Always be sure of the "back-to-back" characteristics of the audio oscillator and noise-distortion meter before taking complete proofs or after tube changes or other servicing of the measuring equipment. The direct combination of these instruments should indicate a frequency response flat within 2 dB from 30 to 15,000 Hz. The distortion should measure less than 0.2 percent from 60 Hz to 20 kHz and not more than 0.35 percent from 20 to 50 Hz. The noise should measure at least 70 dB below the output level of the audio oscillator. If it does not, substitute new tubes one at a time (allowing good warmup time) or follow the manufacturers' instructions for necessary adjustments to bring the equipment within the above specifications or the specifications for the particular equipment used.

For frequency-response runs, always consider the back-to-back response of the measuring equipment in calibration of the readings for the input level at the studio. For example, if the back-to-back response is down 2 dB at 5000 Hz relative to 1000 Hz (reference frequency) , then feed an input two decibels higher to the studio at this frequency.


Audio Oscillator

-50 dBm Mic Preamp Amplifier Gain Control Adjusted for 1001%Modulation and Left in This Position for Remainder of Tests All Gains Adjusted for Desired Percentage Modulation of Transmitter All Transmitter Line or STL Terminal Gear Including (Apprac) Limiter Amp for Line at (Limiter Off) 100% Modulation Studio Path From Mic to Output Turn Limiter Section of Limiter Amp Off if Included at Studio.

Bypass Any AGC Amplifier.

Dummy Load Optional RF Pickup for Diode When Included in Noise- Distortion Meter Diode Noise-

Distortion Meter Transmitter Freq Monitor Detector Output Modulation Monitor

Fig. 14-13. Block diagram of setup for AM proof of performance.


Frequency-Response Runs

Fig. 14-13 shows a typical setup for a complete proof-of-performance run. The FCC rules require an overall response (from studio microphone input to transmitter output) flat within 2 dB from 100 to 5000 Hz. They further require that measurements be made at 25, 50, 85, and 100 percent (or the highest attainable) modulation from 50 to 7500 Hz. A step-by-step procedure follows.

1. With the audio oscillator set at 1000 Hz, feed-50 dBm to a studio microphone input (Fig. 14-13). If the oscillator meter indicates zero dBm at the adjusted gain, the calibrated attenuators will total 50 dB.

Usually the oscillator meter has a full-scale reading of +15 to +17 dBm. It is advisable to adjust the oscillator gain to obtain, say, +10 dBm and set the attenuators for a total of 60 dB to result in a -50 dBm output. Usually the back-to-back measurement previously de scribed gives the best results with this type of operation, particularly when distortion and noise measurements are taken. Usually there are three sets of calibrated attenuators-tens, units, and tenths-so that precise readings may be made.

Courtesy National Association of Broadcasters

Fig. 14-14. Form for recording frequency-response data.

2. Adjust the associated faders and master gain control for reference line output (an indication of zero on the studio-line VU meter) . As discussed in Section 2, this is actually +4 to +12 VU. All faders and the master gain control should be in approximately the normal operating positions. The term "approximately" must be used since the microphone-fader setting will in most cases actually be somewhat higher due to the high peak factor of speech waves compared to the sine-wave rms value.

3. Be sure that any agc amplifier is bypassed (patched around) and that if a limiting amplifier is employed at the studio, the limiter section is turned off.

4. At the transmitter, adjust the line or limiter amplifier to obtain 100-percent modulation. The limiter section of the limiter amplifier must be off.

5. Record the oscillator attenuator reading. Refer to Fig. 14-14 for an example of tabulated response data in the form suggested by the NAB (National Association of Broadcasters) . Note that the attenuator reading for 100-percent modulation at 1000 Hz is simply recorded along the entire top row. Assume this to be 60 for illustrative purposes. Copy the figure 60 also in row 2 for 1000 Hz.

6. Tune the oscillator to 30 Hz, and readjust the attenuators (if necessary) to again obtain 100-percent modulation. Record the new attenuation figure in row 2 under 30 Hz. Repeat this procedure for the other frequencies listed.

7. Fill in row 3 by subtracting the readings in row 2 from those in row

1. This is a record of the response variation. Note that, for example, it was necessary to reduce the attenuation 1.8 dB at 30 Hz, this indicates that a 1.8-dB higher level was required at 30 Hz relative to 1000 Hz to obtain 100-percent modulation. Therefore the response of the system is down 1.8 dB at this frequency relative to the reference frequency, and is so plotted on the graph (Fig. 14-15) .

8. The entire process is repeated at the lower percentages of modulation required. Two methods may be used:

(A) The audio-oscillator attenuators may be raised in value to obtain the new (lower) percentage modulation, or (B) The faders on the console may be adjusted to obtain the new percentage, retaining the-50 dBm input to the microphone preamplifier at the reference frequency of 1000 Hz.

Remember that the basic idea for a proof run is to prove that the system can be brought within specifications by the proper adjustments or servicing.

A proof is a record of the performance as it exists at the time of measurement. This is no guarantee that a single tube in the system may not deteriorate the next hour, day, or week to result in different performance. How ever, the more often such tests are made and proper remedial steps taken to correct deficiencies, the better the overall results will be.

When a system fails to meet response specifications, first ascertain whether the trouble is at the studio, at the transmitter, or in the line. When the transmitter is at a separate location from the studio, feed the audio oscillator directly into the line. If the trouble persists, run a characteristic curve on the line (or STL) itself to isolate the fault or determine if the re- suit is cumulative. If the trouble is at the studio, it is a simple matter to run studio-only frequency-response checks.

Fig. 14-15. Form for plotting frequency-response curves.

Noise Level

Hum and noise must be at least 45 dB below the level representing 100-percent modulation between 30 Hz and 20 kHz. The reference frequency is 1000 Hz, and this measurement may actually be performed in Step 5 of the preceding procedure by removing the oscillator (after the reference level has been established) and terminating the microphone-amplifier in put with a resistor equal to its input impedance. Leave all gains as originally set, but be sure that no other source faders are open. The noise-distortion meter is then placed in the noise mode and its sensitivity increased to obtain the noise level in the unmodulated carrier. The microphone input level should be no lower than -50 dBm.

The input tubes of preamplifiers are the most common cause of noise. If an oscilloscope is available, connect it to the scope terminals of the noise-distortion meter, and determine if the noise is caused by a hum component.

Many amplifier or preamplifier power supplies employ "hum" potentiometers which should be adjusted while the noise measurement is watched. Use the same trace-down procedure for hum and noise as that previously de scribed for running down the cause of an improper frequency response.

Audio-Frequency Harmonic Distortion

Fig. 14-16 illustrates the NAB-suggested forms for recording the audio-

frequency harmonic content, which is normally measured in terms of percentage. The measurements must be taken at the modulation percentages indicated and at the specified single sine-wave frequencies. The noise-distortion meter must be capable of measuring throughout the harmonic spectrum required by the FCC. The harmonic distortion must not exceed 5 percent up to 84-percent modulation, or 7.5 percent for modulation percentages greater than 84 percent. A typical procedure for the measurement of audio-frequency harmonic distortion follows.

1. The reference modulation at the reference frequency (1000 Hz) is obtained as in the frequency-response procedure. The noise-distortion meter is then placed in the set-level position, and the meter is adjusted to 100-percent calibration.

2. The noise-distortion meter is then placed in the distortion position, and the tuning and bridge-balance controls are adjusted for minimum reading. The sensitivity is next increased in steps of 10 dB, and these controls are readjusted for minimum each time.

3. The percentage is read directly on the most sensitive scale possible to obtain a minimum reading by use of the tuning and balance controls.

Excessive harmonic distortion can be caused by tubes, low power-supply voltages, or overdrive of any amplifier or chain. Again, this is a case of tracing down the source of distortion first to the transmitter only, line or STL, or studio. Also, if excessive noise is present, the distortion measurement is apt to be high because of the noise level. It is important to keep the microphone input level from the audio oscillator no lower than -50 dBm.

Carrier Shift

Carrier shift is a change in the average value of the modulated rf carrier compared to the average value of the unmodulated carrier. A shift in the upward direction is called a positive carrier shift; a shift downward is a negative carrier shift. Excessive carrier shift results in unwanted harmonics and additional sideband frequencies with consequent interference on adjacent channels.

Fig. 14-17 illustrates a convenient form for use in recording the carrier shift data. A tone of 400 Hz is required. Assume, for example, that the rectified unmodulated carrier measures 1 volt. This is recorded in row 1.

Under modulation, the voltage drops to 0.95 volt. This is recorded under the first reading in row 2. The difference voltage is 0.05 volt, recorded in row 3. The ratio of the number in row 3 to the corresponding number in row 1 is 0.05/1, or 0.05. This is 5-percent carrier shift (negative) .

In practice, the rf input meter on the modulation monitor (which is normally adjusted to 100 percent in operation) may be used to measure this carrier shift. In the preceding example, if the modulated carrier exhibits a 5-percent negative carrier shift under modulation, the input meter will indicate 95 percent rather than 100 percent.

Courtesy National Association of Broadcasters

Fig. 14-16. Form for recording harmonic-distortion data.

Fig. 14-17. Form for recording carrier-shift and noise and hum data.

The maximum carrier shift at any of the specified degrees of modulation must be less than 5 percent in either the positive or negative direction.

Excessive carrier shift may result from any of the following: overmodulation (check accuracy of modulation monitor with oscilloscope), improper grid bias, poor grid-bias supply regulation, poor plate-supply regulation, defective power-supply filters, faulty neutralization, or improper rf excitation

Spurious Radiations and RF Harmonics

Spurious radiations and rf harmonics must be kept at a minimum and must never be of sufficient amplitude to cause undue interference to other services. Fig. 14-18 is a suggested form for these measurements and is self explanatory.

Applicable FCC Rules (always check latest rulings) are as follows:

1. Any emission appearing on a frequency removed from the carrier by between 15 kHz and 30 kHz inclusive shall be attenuated at least 25 dB below the level of the unmodulated carrier.

2. Any emission appearing on a frequency removed from the carrier by more than 30 kHz and up to and including 75 kHz shall be attenuated at least 35 dB below the level of the unmodulated carrier.

3. Any emission appearing on a frequency removed from the carrier by more than 75 kHz shall be attenuated at least [43 + 10Logio (power, in watts)] dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

Fig. 14-18. Form for recording spurious-radiation and harmonic data.

(1) Record attenuator reading for the 1000-Hz reference signal in his row.

(2) Record the attenuator readings for the specified frequencies in this row.

(3) Record the AF response variations, which are obtained by subtracting row (2) from row (1), in this row. These final figures are to be used in plotting the graphs.

NOTE: The figures inserted in this example indicate pre-emphasis measurement.

Fig. 14-19. Example of data from frequency-response check of fm transmitter.


The same technique as that previously described for AM is used for fm proof of performance; the essential difference is the broadened and more stringent requirements of performance. The frequency-response measurement may be taken either with or without de-emphasis. It is a good engineering practice to run this check both ways. This proves that the station de-emphasis network in the monitor circuit is essentially complementary to the 75-microsecond pre-emphasis.

Fig. 14-19 is an example of tabulated data obtained by measuring the transmitter output without de-emphasis. Just as in AM measurements, the entry in row 1 is the attenuator setting for the 1000-Hz reference frequency to obtain the specified modulation. Recorded in row 2 are the actual settings required at the specified frequencies to maintain the reference modulation. The numbers in row 3 are obtained by taking the difference between each entry in row 1 and the corresponding entry in row 2. Note: that as in the AM measurements the plus attenuator setting indicates that the response is lower. These figures are plotted on the standard 75-µs pre-emphases curve (Fig. 14-20) . Note in this example that the original run, where the 1000-Hz reference is 0 dB, results in the values for frequencies of 5000 Hz and 10,000 Hz falling outside the tolerable limits. Note, how ever, that at 5000 Hz (farthest out of the range) a shift of-0.8 dB brings this measurement within the limits. Therefore a new curve may be drawn (as shown by solid dots) with an axis shift of-0.8 dB, and the proof is satisfactorily completed. If this shift had resulted in other points falling outside the limits, remedial measures would have been needed to correct the fault. Measurements must be taken at least at the frequencies specified in Fig. 14-19 and for modulation percentages of approximately 25, 50, and 100 percent. The response is then run with standard de-emphasis, and the overall curve is drawn as shown in Fig. 14-15 for AM, except for the ex tended frequency range.

Fig. 14-20

......... allowable distortion (measured through a standard 75-1.ts de-emphasis circuit) is as follows:

50-100 Hz: 3.5 percent 100-7500 Hz: 2.5 percent 7500-15,000 Hz: 3.0 percent

The output noise level for fm is measured in two categories, fm noise and AM noise. The method of measuring fm noise is the same as that de scribed for AM in the preceding section. This includes any noise in the entire system that would result in frequency modulation of the carrier. Just as ir_ AM, fm noise is measured in dB below the level corresponding to 100-percent modulation, which for fm broadcast is a frequency swing of ±75 kHz. This measurement must be made with standard 75-us de-emphasis. The indicating instrument must have ballistic characteristics similar to those of a standard VU meter. The fm noise must be at least 60 dB below 100-percent modulation, with a 75-µs de-emphasis circuit employed.

It is necessary to obtain the AM noise level in terms of what would correspond to 100-percent amplitude modulation of the transmitter, but it is obviously not possible to amplitude modulate the fm transmitter to 100 percent. Some other means must be used to calibrate the noise meter.

Fig. 14-21. One method of measuring AM noise level of fm transmitter.

The arrangement shown in Fig. 14-21 may be used for this purpose. A diode rectifier rectifies a small amount of rf energy from the output of the transmitter. A dc voltage proportional to the carrier output of the transmitter appears across the 600-ohm resistor with the switch in the rf position. Should the carrier output be amplitude modulated, there would also appear an ac voltage that would be proportional to the percentage of modulation. If the carrier were amplitude modulated 100 percent, this ac volt age (rms) would be equal to 0.707 times the dc voltage. (The rectifier is a peak-voltage rectifier.) Thus an external calibration of the noise meter can be set up. This is done with an audio oscillator having a 600-ohm output adjusted so that there appears across the output an ac voltage equal to 0.707 times the rectified dc voltage. This voltage is fed into the noise meter, and the latter is adjusted for full-scale deflection. Thus calibrated, it is ready for use in measuring the AM modulation level that appears across the output of the diode rectifier. The actual steps follow.

1. A diode rectifier is coupled to the output of the transmitter (Fig. 14-21) . In some transmitters, one-half of the audio-monitor coupling links may be used. Adjust R1 to obtain a convenient reading, such as 1 volt, on M1.

2. With the switch in the rf position, measure the dc output voltage of the diode rectifier by means of voltmeter M1.

3. Throw the switch to the of position, and adjust the output of the audio oscillator so that voltmeter M2 indicates an ac voltage equal to 0.707 times the dc voltage just measured. The noise meter is then adjusted to zero dB. (The reference is now set.)

4. Return the switch to the rf position, and read the noise level indicated by the noise meter. This is the measurement of the a-m noise level.

This level must be at least 50 dB below 100-percent modulation, with a 75-µs de-emphasis circuit employed.

Fig. 14-22 illustrates an alternate method of measuring AM noise with out the conversion factor of 0.707 described previously. Only one voltmeter is required. The procedure is as follows:

1. Open S2 and close S1. Adjust the trimmer capacitor (if used) and R1 to obtain a convenient voltmeter reading, for example 2 volts.

2. Open S1 and close S2. Adjust R2 to obtain exactly the same voltage.

3. Calibrate the noise meter for 0 dB reference.

4. Open S2 and close S1. Take the noise reading according to the instructions for using the noise-distortion meter.

Fig. 14-22. Alternate method of measuring AM noise level of fm transmitter.

Spurious emissions are measured essentially as previously outlined for AM transmitters. The results must be as shown in Table 14-1.

Table 14-1. Spurious-Emission Limitations (FM)

Frequency Difference

From Carrier Required Level 120 to 240 kHz 240 to 600 kHz

More than 600 kHz

At least 25 dB below unmodulated carrier

At least 35 dB below unmodulated carrier

At least [43 + 10 log10 (power in watts)] dB below level of unmodulated carrier, or 80 dB, whichever is the lesser attenuation


The basic principles of SCA and compatible stereo have been covered in previous Sections. This section is concerned with engineering standards, transmitter adjustments, and proof-of-performance measurements.

Engineering Standards for Subsidiary Communications Multiplex Operations

The following standards for SCA operation by fm stations are from paragraph 73.319 of the FCC Rules and Regulations.

A. Frequency modulation of SCA subcarriers shall be used.

B. The instantaneous frequency of SCA subcarriers shall at all times be within the range of 20 to 75 kHz, provided, however, that when the station is engaged in stereophonic broadcasting the instantaneous frequency of SCA subcarriers shall at all times be within the range 53 to 75 kHz.

C. The arithmetic sum pf the modulation of the main carrier by SCA subcarriers shall not exceed 30 percent, provided, however, that when the station is engaged in stereophonic broadcasting the arithmetic sum of the modulation of the main carrier by the SCA subcarriers shall not exceed 10 percent.

D. The total modulation of the main carrier, including SCA subcarriers, shall meet the requirements of 73.268 (85 to 100 percent modulation total).

E. Frequency modulation of the main carrier caused by the SCA subcarrier operation shall, in the frequency range 50 to 15,000 Hz, be at least 60 dB below 100 percent modulation, provided, however, that when the station is engaged in stereophonic broadcasting, frequency modulation of the main carrier by the SCA subcarrier operation shall, in the frequency range 50 to 53,000 Hz, be at least 60 dB below 100 percent modulation.

Measuring Cross Talk

The following is the RCA in-plant procedure for testing transmitters employing subchannel SCA before shipment. It appears here through the courtesy of RCA.

The measurement of special multiplex parameters should begin with main-to-subchannel cross talk. To set the reference, the subchannel should be modulated 100 percent (±7.5 kHz deviation) with a 400-Hz tone. Set the distortion analyzer for noise measurement, and adjust the meter deflection to 0 dB. The filter is set to pass a band of frequencies from 50 to 15,000 Hz (Fig. 14-23) .

Fig. 14-23. Test setup for checking exciter and subcarrier-generator performance.

Now remove the subcarrier modulation and apply modulation to the main channel using several frequencies ranging from 50 to 15,000 Hz.

Adjust each frequency to 85-percent modulation if one subchannel is used.

(If two subchannels are used, the main channel should be modulated 70 percent instead.) Read the cross talk for every frequency on the distortion analyzer.

Next, modulate the main channel again with 400 Hz at 85 percent, and carefully adjust all multiplier stages of the exciter to give a minimum cross talk reading. Be sure not to detune the tuned circuits too much. Also touch up the monitor tuning (rf and IF coils and discriminator) , and be sure that the monitor and separate subcarrier adapter, if they are used, are fed with the right amplitudes and are not overdriven.

If it is necessary, repeat the steps previously indicated; however, a slight adjustment of the first tripler coil should correct any excessive cross talk on 67 kHz. By observing the waveform oscilloscope, make sure that the meter indication of the distortion analyzer represents the components required.

Certain beat frequencies generated in the monitor may give a false impression. It should be noted that the above adjustment is greatly simplified because only three tuned circuits may contribute to cross talk. In some in stances, it may be advantageous to shift the modulator grid-tuning capacitor slightly to give 2 to 3 dB better cross-talk reduction. The subcarrier modulator itself requires no tuning (in the RCA system).

Sub-to-main channel cross talk is measured in a similar way. This time the 0-dB reference is 100 percent modulation (±75 kHz deviation) by a 400-Hz tone on the main channel. This modulation then is removed, and tones from 50 to 6000 Hz are applied to the subcarrier generator, giving 100 percent (±7.5 kHz) modulation on the subchannel. Slight cross talk may result from improper balancing of the main-channel modulator tubes.

Therefore, the modulator-grid tuning, after an initial setting for maximum swing, should ultimately be tuned for minimum sub-to-main channel cross talk. This setting requires only a very slight change. The exciter multiplier tuning has practically no effect on sub-to-main channel cross talk.

To measure inter-subcarrier cross talk, one proceeds in the same way, set ting a reference level for the channel in which cross talk is being measured by first modulating it with a 400-Hz tone at 100 percent modulation (±7.5 kHz).

The cross talk measurement for the stereo subchannel is made by the same procedure as before, except that the main channel is modulated 90 percent instead of 100 percent (81 percent if SCA is included with stereo operation). The subchannel is amplitude modulated, and the suppressed-carrier sidebands frequency modulate the main channel.

To check the requirement in FCC paragraph 73.322(c), a "standard stereo receiver" or a reliable station receiving-type stereo monitor may be used. However, it is also possible to check the subcarrier generator itself.

An oscilloscope connected to a suitable point in the adder stage (addition of 38-kHz subchannel and 19-kHz pilot) may be used. The scope must have dual-beam provisions. The procedure is as follows:

1. Connect one scope input to the 19-kHz grid of the adder stage and the other input to the 38-kHz grid.

2. Be sure that no modulation occurs (no audio signal) .

3. Since the adder follows the balanced modulator, it is necessary (in order to obtain the 38-kHz subcarrier) to unbalance this circuit. This has no effect on the measurement.

(A) Subcarrier and pilot subcarrier.

(B) Proper detected main-channel output.

(C) Detected main-channel output in which L-R sidebands lag L + R.

(D) Detected main-channel output where L-R channel lacks sufficient gain.

Fig. 14-24. Stereo waveforms.

Fig. 14-25. RCA BTE 15A fm exciter.

4. See Fig. 14-24A. Note that each time the pilot crosses the zero axis, the subcarrier, which is the second harmonic of the pilot, crosses the zero axis in the positive direction.

The detector output waveform from the receiver should appear as in Fig. 14-24B. The waveform is a composite of the L + R signal and the L-R sideband signal for a 400-Hz left-only input signal. The zero axis should be straight for all modulating frequencies.

Fig. 14-24C indicates that although the amplitudes of L + R and L-R are correct, the L-R sidebands lag L + R. In this particular case the lag is 6°, which is twice that allowed by the FCC. If the tilt is in the opposite direction, the L-R sidebands lead the L + R signal. In general, the phase is within the 3° tolerance if the amplitude of either half of the waveform is 9 times the measurable height of tilt. Thus, if the scope gain is adjusted to allow 4.5 cm of deflection for half of the waveform, the tilt amplitude should be no more than 0.5 cm.

An improper amplitude ( insufficient gain) of the L-R sideband signal is indicated by the waveform of Fig. 14-24D. The phase is correct. If the bottom trace bowed upward instead of dipping, excessive gain of the L-R sideband signal would be indicated.

All stereo transmitters are accompanied by detailed instructions, and the maintenance engineer is obligated to become thoroughly familiar with the necessary adjustments of his particular installation to bring it within FCC specifications on proof runs. These procedures naturally vary according to the circuitry.

For example, the BTE 10B exciter in the above example is a tube-type unit (of which many are still in use) requiring more adjustments than the more recent RCA type BTE 15A all solid-state unit (Fig. 14-25) .

Modulation of the temperature-compensated basic on-frequency oscillator is achieved by applying the composite stereo or SCA signals from the BTS 1B and BTX 1B generators, respectively, to a pair of push-pull varicap diodes which are coupled to the frequency-determining resonant circuit of the basic oscillator. The output of the basic oscillator is isolated from the following buffer amplifier by a 10-dB resistive attenuator. Thus, the stability and modulation characteristics of the basic direct fm oscillator are not disturbed by following rf power amplifiers.

The output of the buffer amplifier, approximately 500 mW, is used to drive the 15-Watt, three-stage rf amplifier as well as the binary divider chain in the afc circuit. The basic oscillator, buffer amplifier, and afc circuit are mounted inside a shielded enclosure. The rf power amplifier is also completely shielded.

Automatic frequency control (afc) for the on-frequency basic oscillator is achieved by taking a sample of the buffer output and applying it to a chain of 14 divide-by-two frequency dividers. A low-frequency reference crystal operating at 1/1024 of the desired output frequency has its frequency divided by 16 in a binary chain. Integrated circuits operating in the saturated mode are used in both binary dividing chains. The outputs from the reference and basic-oscillator binary dividers are phase compared in a time-sharing IC comparator. The output of the circuit, which represents the afc error voltage, is filtered and applied to another pair of varicap diodes coupled to the basic-oscillator tuned circuit. Thus, the basic oscillator is phase locked to the 1024th harmonic of the oven-controlled reference crystal.

An off-frequency detector is incorporated in the design of the BTE 15A fm exciter. When the basic oscillator frequency is not phase locked to the reference crystal, an ac component appears in the afc output. This voltage is rectified to operate a relay the contacts of which can be used to turn off the fm transmitter.

Two multimeters are located on the hinged door in front of the regulated power supply section. One of these meters is used to indicate power supply and operating voltages within the exciter and 15-watt rf amplifier.

The second meter is a peak-reading voltmeter that is used to indicate key modulating signals.

The rf power output of the BTE 15A can be continuously adjusted from 7 to 15 watts by means of a front-panel control. The primary power is turned on with a circuit breaker. The rf output is turned on with a front-panel switch or by jumping contacts available on the rear of the unit. The exciter will tolerate load mismatches from short circuit to open circuit without damage to the output transistor. Another safety feature prevents turning on the 41-kHz SCA subcarrier when the stereo generator is in the stereo mode.

In the Model BTS 1B stereo generator, the left and right input channels are identical, each having resistive input terminations, isolating transformers, 15-kHz low-pass filters, and an operational amplifier for obtaining pre-emphasis. The pre-emphasis is convertible from 75 to 50 microseconds in the field, or can be removed entirely. The left and right channels can be matched to within I/2 percent gain difference and I/2° phase difference from 30 to 15,000 Hz, including the 15-kHz low-pass filters. These filters are less than 0.5 dB down at 15 kHz, and more than 50 dB down at 19 kHz and above. This insures an absolute minimum of disturbance to the pilot carrier and subcarrier regions by the program material.

The pre-emphasized and filtered left and right audio signals are applied to a switching modulator which alternately switches between the two audio channels. The balanced and symmetrical 38-kHz switching signal is de rived from a buffered 38-kHz output of a bistable multivibrator. The negligible amount of second harmonic (76 kHz) in the 38-kHz switching signal assures a minimum of interference to a 67-kHz SCA channel. The 76 kHz crystal-controlled signal driving the binary divider assures a frequency stable 38-kHz stereo subcarrier.

The output of the switching modulator, along with the sinusoidal pilot (less than 1 percent distortion), is applied to a phase-linear filter to remove the third and all higher-order harmonic components of the switching signal. The complete composite stereo signal, or a left or right monaural signal, is selected by relays and applied to the input of an operational amplifier. The output of this amplifier is then applied to the wideband input of the BTE 15A fm exciter.

Switching between monaural right, monaural left, or stereo may be accomplished by front-panel push buttons on the BTS 1B or by momentary remote-control contact closures. The selected mode is indicated by front-panel lamps. Left, right, and composite program outputs are also applied to a peak-reading meter on the main frame of the fm exciter.

The Model BTX 1B SCA generator, using all hermetically sealed metal-cased integrated circuits and transistors, is designed to operate on either the 41-kHz or 67-kHz SCA channels. The audio input is applied to a resistive terminating pad and then to an isolating transformer before being amplified. An optional 5-kHz low-pass filter may be inserted in the input to pre vent higher-order lower sidebands of the 67-kHz subcarrier from penetrating the upper regions of the stereophonic spectrum.

The audio amplifier includes an active pre-emphasis network which may be easily changed from 75 microseconds to 50 or 150 microseconds or adjusted for a flat response. The audio sensitivity of the BTX 1B is sufficiently high that line amplifiers are not required.

The processed audio input signal is then applied as modulation to a direct-fm SCA generator that includes a temperature-compensating circuit for extreme frequency stability. A vernier center-frequency control is avail able on the front panel.

Following this generator are a series diode muting gate, a buffer amplifier, and a wideband low-pass filter to remove subcarrier harmonics. The total harmonic content of the subcarrier output is less than 1 percent, and the incidental AM is less than 5 percent peak with 10 percent subcarrier modulation. The output of the low-pass filter is applied to another buffer amplifier and output level control for application to the multiplex input of the BTE 15A fm exciter.

A sample of the pre-emphasized audio is used to drive a peak-reading multimeter on the main frame of the exciter. Automatic muting of the subcarrier is accomplished in the following manner. A portion of the pre-emphasized audio is applied to a variable-gain amplifier and, with an adjustable time constant, peak detected to operate a Schmitt trigger circuit.

The output of the Schmitt trigger is shaped with a low-pass filter and used to turn on or off the series diode muting gate. When audio is applied to the input of the BTX 1B, the muting diode gate is turned on to allow the subcarrier output to appear. In the absence of audio, the Schmitt trigger pauses for a selected time interval before turning off the diode muting gate.

The subcarrier envelope rise and fall times are constant and so chosen as to minimize clicks and pops in an SCA multiplex receiver. The amount of Schmitt-trigger delay is adjustable with a front-panel control. With this control, subcarrier muting can be adjusted to occur from 0.5 to 5 seconds after the audio input is removed. Two transistors are used to operate front-panel lamps to indicate the on-off status of the subcarrier. Also, a front-panel switch provides manual control of the subcarrier output or the use of the automatic muting feature. The subcarrier also can be turned on or off remotely.

Stereo Adjustments and Proof of Performance

Fig. 14-26 presents a suggested setup at the studio for stereo proof-of-performance measurements. The audio generator, step attenuator, and VU meter are usually incorporated in one unit. The balanced Y pad splits the common audio signal into two paths for simultaneous left and right channel (microphone input) feeds. Switches S1 and S2 allow either left only, right only, or left plus right feeds. Switch S3 allows either normal or re verse polarity feed to the right-channel input. Note that in the normal position, the A output of the signal generator feeds terminals 1 and 3 of the input transformers, while the B output feeds terminals 2 and 4. Thus the two inputs are being fed with a common in-phase signal, where L = R.

With S3 in the reverse position, the polarity of feed to the right channel input is 180° out of phase with the left channel input, or L =-R. This is necessary for certain adjustments and measurements as described later.

When only the transmitter is to be tested to prepare for overall proof-of-performance measurements, the left and right inputs are to the inputs of the stereo generator. A typical setup at the transmitter for overall proof-of-performance runs is illustrated in Fig. 14-27.

Audio frequency-response and noise-distortion measurements are taken exactly as outlined earlier for mono fm, except that the procedure is repeated for the opposite channel. This simply means that the left and right channels must individually meet requirements of the FCC for the various modulation levels just as in mono fm specifications. We are then ready to take the remaining measurements which have to do with the composite signal: separation and cross talk between channels.

Left & Right

Input Channels

1 tMic Inputs)

Left In Right In

Fig. 14-26. Studio setup for stereo measurements.

Fig. 14-27. Transmitter setup for stereo measurements.

The following is an outline of a typical preliminary adjustment procedure in preparing for stereo proof-of-performance runs. It is assumed that the signal generator is at the transmitter.

STEP 1: Connect the oscilloscope vertical input to the composite-signal output of the stereo generator. Sync the scope internally. Apply a left-only signal of 400 Hz at the specified input level of the stereo generator. Turn the pilot amplitude control to minimum.

STEP 2: The pattern on the scope should be as in Fig. 14-28A. If the base line is not flat (Fig. 14-28B) , adjust the left-channel (L + R) gain control (sometimes termed "separation" control) to obtain a flat base line.

STEP 3: Apply a right-only 400-Hz signal and repeat the above adjustment for the right-channel gain control. Sometimes both controls are used; sometimes only an L + R gain control is incorporated. The point is that when the two gains are equal, the base line will be flat.

CAUTION: When a sampling type of modulator is used, a large number of odd-order harmonics (3rd, 5th, etc.) of the 38-kHz switching rate exist. For this reason, an output filter is employed to confine component frequencies below 75 kHz. Sometimes an output-filter phase-linearity control is incorporated to set the driving impedance of the filter for maxi mum phase-shift linearity over the range of 50 Hz to 53 kHz. Misadjustment of this control will cause a left-only or right-only signal to appear very similar to the waveform of Fig. 14-28B. If it is not possible to obtain a perfectly flat base line by adjusting the L + R and/or L-R gains, investigate this possibility. More recent solid-state stereo generators employing the sampling tye of modulation have fixed filtering networks that require no adjustment. The same problem can occur, of course, with a component change or failure within the filtering network.

(A) L R and L-R gains equal.

(B) L R and L-R gains unequal.

Fig. 14-28. Setting separation controls.

STEP 4: Reverse the polarity of the right-channel input (Fig. 14-26) , and feed both channels so that L = -R.

Turn the pilot amplitude control to normal. With the scope time base set at about 200 µs/cm, the pattern of Fig. 14-29A should be obtained. This is termed the L-R "butterfly" and can be used to adjust the pilot phase control accurately. Remember that proper phase is indicated when a zero crossing of the pilot signal occurs at precisely the same time as the positive-going zero crossing of the (sup pressed) 38-kHz subcarrier (Fig. 14-24A) .

STEP 5: Expand the time base of the scope to around 50 µs/cm. Trigger the scope externally with the signal-generator frequency or the 19-kHz pilot. If the pilot phase is correct, a perfect diamond pattern such as that of Fig. 14-29B will be displayed. This indicates proper zero crossings of the pilot and subcarrier sine waves. If the phase is not correct, the four crossings do not form a diamond pattern, and a distorted presentation such as that of Fig. 14-29C is obtained. Adjust the pilot phase control so that the pattern of Fig. 14-29B is obtained. Always recheck the pilot gain after pilot phase adjustments are made.

STEP 6: Move the scope to the modulation monitor position. The scope display from the composite output of the modulation monitor should be identical to that obtained from the composite output of the stereo genera tor. If a different pilot phase is indicated, adjust the pilot phase control in the monitor to agree with the phase shown at the composite output of the stereo generator, or follow specific instructions of the monitor manufacturer.

Measuring Separation

Feed a signal of 1 kHz to either the left or right (only) input. With the modulation monitor set in the Total position, modulate the transmitter 100 percent. Switch the modulation meter to the channel opposite that being fed; the modulation indication should be at least 29.7 dB below the 100-percent modulated channel (actually 90 percent with 10-percent pilot) .

This condition should prevail over a frequency range of 50 Hz to 15 kHz.

Measuring Cross Talk

The left and right input channels of the stereo generator are connected in parallel and in phase. The frequencies at which cross talk is to be measured modulate the main (L + R) channel without modulation of the subchannel (L- R) . With the main channel at 90-percent modulation (10-percent pilot modulation) , cross talk of the main channel into the sub-channel is then indicated on the modulation monitor in the L-R position.

The percent modulation in this channel should be a maximum of 1 percent, indicating -40 dB of cross talk.

Next, reverse the phase of the signal to the right channel, and set L-R to 90-percent modulation. Cross talk of the subchannel into the main channel is now indicated by the L + R position of the modulation monitor.

Problems in Stereo Proof of Performance

The problems of frequency response, fm and AM noise of the fm carrier, and distortion are no different from those previously described for the mono fm transmitter, except that the number of measurements is essentially doubled. As in mono fm, poor performance is isolated to transmitter only, lines or STL from studio to transmitter, or the studio.

The cross-talk specification is usually the most difficult to meet. This measurement should first be made on the transmitter alone to check adjustments of the stereo generator, exciter and power amplifiers, and antenna system. If the cross-talk measurement deteriorates between the stereo-generator output and the final-stage output, check for proper circuit tuning and stage neutralization.

After the stereo generator and following rf stages have been optimized, move the signal generator to the terminal gear where the STL or wire lines normally feed. Unequal phase shifts and/or gains through agc or limiting amplifiers can cause major cross-talk problems. Gain must be kept to within one percent and phase to within one degree if cross-talk specifications are to be met.

Next, the signal generator is moved to the input of the STL or lines at the studio. Thus any additional factors in this area will influence the measurements. The final step is to feed the left- and right-channel microphone in puts for the regular overall proof-of-performance measurements.

(A) Time base of 200 us/cm. (B) Time base of 50 us/cm, correct phase.

Fig. 14-29. L- R display to. setting pilot phase.

(C) Time base of 50 µs/ cm, incorrect phase.


Of course, the primary purpose of any preventive-maintenance schedule is to reduce as much as possible the likelihood of a failure of any component part of the broadcasting installation during the broadcast day. Regular maintenance schedules are in effect at most broadcast stations and do much to increase the useful life of equipment and anticipate many tube and parts failures that would otherwise occur.

Preventive maintenance on any sort of equipment may be defined as a systematic series of operations performed periodically on the equipment in order to prevent breakdowns. This type of maintenance may be divided into two phases: work performed while the equipment is functioning, and work performed during the normal shutdown periods. This discussion is concerned only with preventive maintenance procedures carried out during the shutdown period.

The importance of preventive maintenance cannot be overestimated.

The owners of a broadcast station depend on its being on the air every second of its scheduled periods of transmission. It is very important that the personnel maintain the equipment so that lapses in the transmission will be kept to a minimum.

Cleanliness of the equipment is of the utmost importance since collections of dust and dirt can cause a number of troubles. This is particularly true in the higher-power stages of transmitters, since accumulation of foreign matter over a period of time reduces the effectiveness of the insulation to a point where leakage currents and arc-overs are common. High-voltage contacts have an extreme tendency to collect dirt (this is the principle used in electronic smoke eliminators), and the higher relative humidity existing in summer or in southern locations tends to aggravate this characteristic. A dusting and clean-up procedure, then, is desirable at a transmitter plant. A source of dry air under pressure is a common means of blowing out dirt, dead insects, and the like from inaccessible corners and from variable tuning capacitors. Insulators, safety gaps, etc., should be polished with a dry cloth. Carbon tetrachloride may be used to loosen excessive dirt and grime.

Transmitter Maintenance Schedule

So that preventive maintenance is effective, it must be performed at regular intervals; that is, certain portions of the equipment must be inspected for certain things every day in older-type equipment, while other parts of the equipment need be inspected only weekly or monthly. The following is a comprehensive maintenance schedule that may be considered a guide to anyone desiring to set up a means of preventing breakdowns. Naturally, this listing may include items that are unnecessary at a particular station, but it has been compiled with the thought that very precaution should be taken.


1. Hourly read all meters and check power-tube filament voltages.

2. Check air-cooled anode temperatures. Check water temperature of water-cooled tubes if used.

3. Check for correct cabinet temperature of air around high-voltage rectifiers.

4. After shutdown, make a general inspection for overheated components, such as capacitors, inductors, transformers, relays, and blowers.

5. Investigate any peculiarities of meter readings.

6. In case of overloads, examine safety gaps and transmitter components for arc pits, etc. Clean and polish surfaces where arcs have occurred.

Reset gaps if necessary. Investigate causes of outages.

7. In the event of lightning or heavy static discharges, inspect the trans mission line, terminating equipment, and the antenna including the gaps. Polish pitted surfaces.

8. If gas-filled coaxial line is used, check pressure.

B. WEEKLY (In addition to items in daily inspection)

1. Immediately after shutdown, check antenna-terminating components for signs of overheating.

2. Clean antenna-tuning apparatus. Check for arc pits, etc. Clean and polish gaps and adjust if necessary.

3. Test antenna monitor rectifier tubes.

4. Calibrate remote antenna meters against meters at the antenna.

5. Clean transmitter using a vacuum cleaner.

6. Clean component parts of transmitter.

A. Brush terminal boards.

B. Clean insulators with carbon tetrachloride.

C. Clean power tubes and high-voltage rectifiers with tissue and alcohol or distilled water.

7. Check filament voltages and dc voltages at the sockets of all tubes that are not completely metered by panel meters.

8. Check air-flow interlocks for proper operation. Check all door interlocks for proper operation.

9. Check operation of grounding switches. Examine mechanical operation and electrical contacts.

10. Inspect blowers for loose impellers, free rotation, and sufficient oil.

11. Inspect relays for proper mechanical and electrical operation. If necessary, clean and adjust components.

12. Inspect air filters; clean if excessive dirt has accumulated.

13. Check all sphere and needle gaps. Clean any pits or dirt. Check gap spacings.

14. Check filter-bank surge resistors with an ohmmeter.

15. Check any power-tube series resistors with an ohmmeter.

16. Check power-change switches if used; check for serious arcing during day-night antenna changeover if used.

17. Make general performance checkup (distortion, noise, and frequency response) . Observe modulated waveform on cro.

18. Check neutralization by disabling crystal oscillator and observing grid currents.

19. Check proper operating voltage for pure-tungsten-filament tubes.

Determine lowest permissible voltage as follows:

A. AM transmitters-distortion and carrier-shift checks.

B. Fm transmitters-decrease filament voltage until output begins to drop.

C. Operate filaments approximately 1 percent above the filament voltage determined in A or B.

20. If water cooling is employed, check entire system for any signs of leakage and for electrical leakage.

21. Check pressure of any gas-filled capacitors.


(In addition to daily and weekly items)

1. Make detailed inspection of all transmitter components using what ever tests seem advisable.

2. Clean and inspect tube-socket contacts and tube pins.

3. Clean or replace air filter. Brush dirt from blower impellers, canvas boots, etc.

4. Clean and adjust all relay contacts. Clean pole faces on contactors.

Replace badly worn contacts.

5. Oil the blower motors (carefully) .

6. Operate all spare vacuum tubes for a minimum of two hours under normal operating conditions. Clean up any gassy tubes as described in a later section on large power tubes.

7. Operate all spare mercury-vapor rectifiers normally, after first applying filament voltage only for a minimum of 30 minutes. Store the tubes upright.

8. Inspect all variable-inductor contacts for tension, signs of overheating, and dirt. Clean and adjust as required. Carbon tetrachloride or crocus cloth may be used for cleaning. Do not use emery cloth.

9. Check for proper operation of time delays, notching relays, and any automatic-control systems.

10. Clean audio-equipment (console, etc.) attenuator and low-level switching contacts with cleaner; wipe off excess.

11. Check tubes in station monitoring equipment, such as frequency monitor, modulation monitor, etc.

12. Clean switches in monitoring equipment with cleaner.


(In addition to the preceding)

1. Lubricate tuning motors and inspect for ease of rotation.

2. Check all indicating meters. Check ac filament voltmeters with an accurate dynamometer-type meter.

3. Check all connections and terminals for tightness.

4. Inspect any flexible cables to door connections.

5. Inspect and lubricate if necessary any flexible drive cables.

6. Inspect, clean, and service (if necessary) all switches (voltmeter selector switches, push-button switches, control switches, etc.) .

7. Clean transmission-line insulators, and take up slack if open-wire lines are used.

8. Check oil circuit breakers, if used, for sufficient oil and loose or defective parts.

E. SEMI-ANNUALLY (In addition to the preceding)

1. Test transformer oil for breakdown, and filter it if necessary. (This is done by the power company.)

2. Check protective overload relays or circuit breakers for correct operation.

A. Ac overload relays may be checked by shorting the high-voltage transformer secondary.

B. Dc overload relays may be checked by shorting the dc through the relay in the circuit protected by the relay.

3. In fm installations, check the accuracy of the modulation monitor as described below.

Measuring Frequency Deviation by the Bessel-Zero Method

An fm wave may be resolved into its carrier and sideband-frequency components, the amplitudes of which vary as Bessel functions of the modulation index. The modulation index is defined as the ratio of the peak deviation from the center frequency to the frequency of the modulating signal:

AF m=

where, m is the modulation index, OF is the peak deviation in Hz, f is the modulating frequency in Hz.

At certain values of m, the carrier amplitude becomes zero; that is, all the energy is transmitted in the sidebands. Fig. 14-30 shows relative carrier amplitude as a function of modulation index. Table 14-2 gives the modulation indexes necessary to produce successive carrier nulls. If the modulating frequency (f) is properly chosen, one of the carrier-null points can be made to correspond to the desired maximum deviation. For example, the peak deviation for an SCA subcarrier is ±7.5 kHz. For the first null, the modulating frequency (f) must be:

mF 7'5 Hz 3 125 kHz 2.4

Obviously, for the main carrier with a ¿F of ±75 kHz, the modulating frequency must be 31.25 kHz to use the first null. Since this modulating frequency is outside the passband of the audio system, the second null must be used, and the modulating frequency becomes f = 75 kHz/5.52 = 13.586 kHz, which is practical. However, the alternate method described later may be used.

Fig. 14-30. Variation of carrier amplitude with modulation index.

Table 14-2. Modulation Indexes Required for Carrier Null


1. Tune a communications receiver (using the narrowest IF selectivity possible to avoid beats with other sideband frequencies) to the un-modulated carrier (Fig. 14-31).

2. Peak the bfo in the receiver to the unmodulated carrier.

3. Assume the main carrier deviation (75 kHz) is to be measured.

Apply 13.586 kHz to the transmitter from the audio signal genera tor; set the output of the generator to zero.

4. Increase the signal-generator amplitude slowly from zero. The side-band frequencies and their beat notes are produced in the headphones.

As the amplitude is increased, note in particular the gradual attenuation of the beat note produced by the carrier. The first null exists when this beat first disappears.

5. Further increase of the input signal amplitude will cause the beat to reappear. When the second null occurs, the modulation monitor should indicate 100-percent modulation (accuracy is normally within 5 percent by this method). Otherwise, a suitable VU meter placed across the modulator-input terminals may be calibrated in percentage modulation.

Alternate Procedure--This procedure uses the fact that frequency-multi plier stages increase both the center frequency and the frequency deviation by the same factor. For example, a transmitter incorporating a reactance tube may have its oscillator frequency increased 18 times before its application to the power-output stage.

1. Couple a small portion of the modulated-oscillator output to the receiver (dash line in Fig. 14-31).

2. Tune the receiver to the oscillator frequency. If the transmitter has a typical 18-times multiplication factor, this frequency is the assigned carrier frequency divided by 18, which is somewhere between 5 and 6 MHz.

3. The peak deviation desired at the modulator stage is 75 kHz divided by 18, or 4.166 kHz. Therefore adjust the signal generator for 4166/ 2.4 = 1736 Hz. (The accuracy of measurement depends on the oscillator calibration accuracy and the operator technique in setting the dial.)

Fig. 14-31. Test setup for measurement of frequency deviation by the Bessel-zero method.

4. Gradually increase the input signal from zero until the first null occurs. This amplitude represents 100-percent modulation.


In the preceding material, some general facts about preventive maintenance were presented together with schedules showing when the different operations should be performed. Inasmuch as some of these operations deal with apparatus that can easily be damaged unless proper care is exercised, certain procedures should be followed so that no damage results from the periodic inspections and so that if repairs to the apparatus are necessary, they can be properly made. Remember that the information in the following pages is general, and it may happen that some manufacturers recommend specific procedures for their products. Of course, these procedures should be followed.

The reasons why preventive-maintenance operations are followed are obvious. It might be desirable, however, for the technicians who are responsible for this maintenance work to remember that the procedures discussed in the following pages are designed with the following objectives in mind:

1. Combat the detrimental effects of dirt, dust, moisture, water, and the ravages of weather on the equipment.

2. Keep the equipment in condition to insure uninterrupted operation for the longest possible period of time.

3. Maintain the equipment so that it always operates at maximum efficiency.

4. Prolong the useful life of the equipment.

The actual work performed during the application of the preventive-maintenance schedule items can be divided into six types of operations.

Throughout this section, the lettering system for the six operations is as follows.

Feel (F)--The feel operation is extensively used to check rotating machinery (such as blower motors, drive motors, and generators) for over heated bearings. Feeling may indicate the need for lubrication or the existence of some other type of defect. The normal operating temperature is that which will permit placing the bare hand in contact with the motor-bearing cover for 5 seconds without discomfort. The feel operation also is applied to items other than rotating machinery; the feel operation for these is explained in the discussion of each specific item. It is important that the feel operation be performed as soon as possible after shutdown and always before any other maintenance.

Inspect (I)--Inspection is probably the most important of all the preventive-maintenance operations. If more than one technician is available to do this work, choose the most observant, since careful observation is necessary to find defects in the functioning of moving parts and other abnormal conditions. To carry out the inspection operation most effectively, make every effort to become thoroughly familiar with normal operating conditions and to learn to recognize and identify abnormal conditions immediately.

Inspection consists of carefully observing all parts in the equipment.

Notice characteristics such as color, placement, and state of cleanliness.

Inspect for the following conditions:

1. Overheating, as indicated by discoloration, blistering, or bulging of the part or surface of the container; leakage of insulating compounds; and oxidation of metal-contact surfaces.

2. Placement, by observing that all leads and cabling are in their original positions.

3. Cleanliness, by carefully examining all recesses in the units for accumulation of dust, especially between connecting terminals. Parts, connections, and joints should be free of dust, corrosion, and other foreign matter. In tropical and high-humidity locations, look for fungus and mildew.

4. Tightness, by testing any connection or mounting which appears to be loose by slightly pulling on the wire or feeling the lug or terminal screw.

Tighten (T)--Any movement of the equipment caused by transportation or by vibrations from moving machinery may result in loose connections which are likely to impair the operation. The importance of firm mountings and connections cannot be overemphasized; however, never tighten screws, bolts, and nuts unless it is definitely known that they are loose. Fittings that are tightened beyond the pressure for which they were designed will be damaged or broken. When tightening, always be certain to use the correct type and size of tool.

Clean (C)--When the schedule calls for a cleaning operation, it does not mean that every item which bears that identifying letter must be cleaned each time it is inspected. Clean parts only when inspection shows it necessary. The cleaning operation performed on each part is described later.

Adjust (A)--Adjustments are made only when necessary to restore normal operating conditions. Specific types of adjustments are described later.

Lubricate (L)--Lubrication means the addition of oil or grease to form a film between two surfaces that slide against each other, in order to pre vent mechanical wear from friction. Generally, lubrication is performed only on motors and bearings.

NOTE: When a part is suspected of impending failure, even after protective maintenance operations have been performed, immediately notify the person in charge, who will see that the condition is corrected by repair or replacement before a breakdown occurs.

Suggested List of Tools Necessary for Relay and Commutator Maintenance

Several items listed on the preventive-maintenance schedule require work of a special and somewhat delicate nature. This work includes cleaning and repairing relay contacts, cleaning plugs and receptacles, polishing commutators, and adjusting motor and generator brushes. To do the work properly, special supplies and specially constructed tools are necessary. A suggested list is given in Table 14-3.

Construction of Relay and Commutator Tools

Crocus-cloth, canvas-cloth, and sandpaper sticks are constructed in the following manner:

1. First prepare a length of wood 33/ inches long, 3/8 inch wide, and 1/16 inch or less thick (Fig. 14-32A) . Cut one piece of crocus cloth 2 1/2 inches long and 1 inch wide.

2. Fold the crocus cloth as in Fig. 14-32A and cement it to the stick.

Note that both sides of the stick are covered. Place the stick in a vise, press it, and wait until the cement hardens. Cut off the crocus cloth that extends past the edge of the stick.

3. Obtain three pieces of wood that measure 8 inches long, 1 inch wide, and approximately 1/4 inch thick. Cut one piece of crocus cloth, one piece of No. 0000 sandpaper, and one piece of canvas cloth, each 51/4 inches long and 1 inch wide.

4. Fold the long, narrow pieces of crocus cloth, sandpaper, and canvas cloth prepared in step 3 as shown in Fig. 14-32B and cement one of them to each of the three sticks. Note that in this case the fold is over one end of the stick rather than over the side. Place the stick in a vise, press, and wait until the cement hardens.


Table 14-3. Suggested List of Special Maintenance Tools and Supplies

Quantity Item 1 Non-magnifying dental mirror 1 Cleaning brush, 2-inch 1 Canvas-cloth strip 1 Small relay crocus-cloth stick 1 Relay-contact burnishing tool 1 Fine-cut file 1 Brush seating stone 1 Commutator polishing stone 1 Canvas-cloth stick 1 Sandpaper-covered stick 1 Brush, cleaning, 1-inch 1 Carbon tetrachloride, quart 2 Cement, household, tube 1 Cloth, canvas, 2x4 feet 1 Cloth or canvas strip, 2x6-inch, cut from sheet 1 Cloth, lint-free, package 6 Crocus-cloth sheets 1 Crocus-cloth strip, 3/4x6-inch, cut from sheet 1 Lubricant, petroleum jelly, container 6 Sandpaper sheets, No. 0000 6 Sandpaper sheets, No.00 1 Sandpaper No. 0000, 3/4x6-inch, cut from sheet 1 Sandpaper strip, No. 00, 3/4x6-inch, cut from sheet 1 Stick, crocus-cloth, large 50 Tags, small marker


Use and Care of Tools

The proper care of tools is as necessary as the proper care of radio equipment. Any effort or time spent in caring for tools is worthwhile. Clean them when necessary, and always store them so that they are easily accessible. The following information will be helpful in using and caring for the tools listed.

Crocus-Cloth Stick--The crocus-cloth sticks are used to clean the contacts of relays in the radio equipment.

Large Commutator Sticks--Commutator sticks with coverings of sand paper or canvas are used for cleaning the commutators of electric motors and generators.

(A) Stick for cleaning relay contacts.

Suitable Width

(B) Stick for cleaning commutators.

Fig. 14-32. Method of preparing crocus-cloth, canvas-cloth, and sandpaper sticks for relays and commutators.

Commutator Dressing Stone--The dressing stone is used to dress a commutator in case of emergency.

Brush Seating Stone--The seating stone is used when a set of new brushes is installed in an alternator or exciter. Only a very limited application of the seating stone is required to seat the average set of brushes.

Electric Soldering Iron--The use of the soldering iron is generally known. Remember to keep the tip properly tinned and shaped.

Allen Wrenches--Allen wrenches are used to tighten or remove set screws on fan pulleys, motor pulleys, etc. Keep these small wrenches in the container provided. After use, wipe them off with an oily rag and return them to their proper place.

Diagonal-Cutting Pliers--Diagonal pliers are used to cut copper wire (no larger than No. 14) when working in small places. Do not cut iron wire with the diagonals.

Gas Pliers--Gas pliers are used to hold round tubing, round studs, or any other round metal objects that do not have screwdriver slots or flat sides for wrenches.

Long-Nose Pliers--Long-nose pliers are used to hold and dent small wires and to grip very small parts. They are generally used on delicate apparatus.

Adjustable-End Wrenches-Adjustable-end wrenches are designed to re move or hold bolts, studs, and nuts of various sizes. Keep the adjusting nut free from dirt and sand, and oil these wrenches frequently.

Nut-Driver Wrenches--Nut-driver wrenches are used to remove or in stall nuts of various sizes. Choose a wrench that fits the nut snugly.

Screwdrivers--Screwdrivers of different sizes are important tools and must be kept in good condition. Select the proper size for the job. Never force a screw; if undue resistance is felt, examine the threads for damage and replace the screw if necessary.

Shorting Bar--The shorting bar must be constructed at the station.

Obtain a piece of wood about 15 inches long and 1 inch thick. Fasten a piece of copper, brass rod, or tubing securely to one end of the stick in such a manner that the rod extends 12 inches beyond the end of the stick. Solder a piece of heavy, flexible wire about 18 inches long to the metal rod at the point where it is fastened to the stick. When using the shorting bar, always attach the free end of the wire to a good ground connection before making contact with the terminal to be grounded.


Suggested preventive-maintenance procedures for various types of equipment and components are described in the following paragraphs.

Vacuum Tubes

The purpose of tube maintenance is to prevent tube failures caused by loose or dirty connections and to maintain the tubes in a clean operating condition. Certain types of vacuum tubes, especially those used in high voltage circuits, operate at high temperatures. Careless contact with the bare hands or arms causes severe burns. Keep a pair of asbestos gloves handy. Otherwise, sufficient time must be allowed for the tubes to cool before handling.

Maintenance of vacuum tubes involves making minor adjustments and cleaning. Because of their high operating potentials, tubes used in high-voltage circuits require more frequent inspection and cleaning than tubes used in low-voltage circuits. Loose coupling at the terminals of high-voltage tubes will result in pitting and corrosion of the terminals. Loose connections cause poor electrical contact and lower the operational efficiency of the unit.

Apply maintenance to vacuum tubes only when necessary; too-frequent handling may result in damage to the tube terminals and connections. As a rule, vacuum tubes need little maintenance; therefore, when the program calls for maintenance, but inspection shows that the tubes do not require it, omit the operation. It is advisable, however, to clean the glass envelopes of the tubes and remove dust or dirt accumulations in the immediate area. The object of the maintenance program is to keep the tubes free from dirt, oil deposits, and corrosion.

For maintenance purposes, vacuum tubes are divided into two groups, transmitting-type tubes and receiving-type tubes. Maintenance procedures for vacuum tubes differ according to their types. Certain maintenance operations that must be performed on transmitting-type tubes may be omitted for receiving-type tubes. Transmitting-type tubes are those used in transmitters, modulators, and high-voltage rectifier units. Because of their physical construction, they require careful inspection and cleaning during maintenance.

Five procedures are required in the performance of maintenance of vacuum tubes: feel, inspect, tighten, clean, and adjust. The procedures involved depend on the type of tube being maintained. Transmitting tubes may require the application of all the procedures, although the procedures for receiving tubes are limited by the tube types.

The following procedures are employed for the maintenance of vacuum tubes. (CAUTION: Discharge all high-voltage capacitors before performing any maintenance operations. Avoid burns by allowing a sufficient time for tubes to cool before handling.) These operations should be performed 5 to 10 minutes after the power has been removed from the tubes.

Feel (F)--This operation should be applied only to high-voltage tubes, such as those used in transmitters, modulators, and high-voltage rectifier units. Feel the grid, plate, and filament terminals of the tubes for excessive heat. Practice will determine the temperature to be accepted as normal. For example, when two grid terminals are felt, one should not be warmer than the other. The development of excessive heat at the terminals indicates poor connections.

Inspect (1)--This maintenance operation is applicable to all types of vacuum tubes and should be performed after the tubes have had sufficient time to cool.

1. Inspect the glass or metal envelopes of tubes for accumulations of dust, dirt, and grease. Inspect the tube caps and connector clips for dirt and corrosion. Inspect the complete tube assembly and socket for dirt and corrosion. Check the tube caps to determine if any are loose.

For glass tubes, check the glass envelope to determine whether or not it has become loosened from the tube base. Replace the tubes that have loose grid caps or envelopes. If replacement is impossible, do not attempt to clean or handle the tube, but operate the tube as it is, pro viding that its operation is normal. Enter the tube condition in the log so that replacement can be made at the earliest possible time.

2. Examine the spring clips that connect the grid, plate, and filament caps for looseness. Also examine all leads connected to these clips for poorly soldered or loose connections. These leads should be free of frayed insulation and broken strands. When removing clips from loosened grid caps, extreme care must be exercised, particularly if corrosion exists. Never try to force or pry a grid clip from the grid cap of a tube since damage to the tube or grid cap may result. If the grid cap is loose and it is necessary to remove the grid clip, first loosen the tension of the clip by spreading it open; then gently remove (do not force) the clip from the tube cap.

3. Inspect the tubes to be sure they are secure in their sockets. Certain types of receiving tubes are mechanically fastened with tube spring locks; others have sockets that lock the tube in place. Inspect by turning the tube in a clockwise direction in its socket until it is locked in place. This type of socket is generally used for transmitting-type tubes. However, the firmness with which the tube is held in place depends on the tension of the terminals in the socket. These terminals are the spring type (contact springs) and must have sufficient tension to make good contact against the tube prongs. The tension can be tested by grasping the tube and turning it counterclockwise and then clockwise to its original position. If the tube seems to snap into place as it is turned, the spring tension of the socket terminals is firm enough; however, if the tension seems weak, the terminals may be tightened or adjusted as explained later in the tube maintenance procedure under Adjust.

4. Inspect all metal tubes for signs of corrosion and looseness of mounting. Many receiving-type tubes have keyways in the center of the tube bases. These keyways sometimes become broken, and this has a tendency to loosen the tube in its socket. Do not attempt to replace tubes having broken keyways unless it is absolutely necessary and it is possible to replace the tube correctly in its proper position. Inspect the sockets of metal tubes for cracks or breaks. Do not force metal tubes into their sockets. If they are hard to insert, examine the tube pins for signs of corrosion or solder deposits.

Tighten (T)--In this operation, take care not to overtighten tube sockets, tube clamps, and tube-socket insulators. Porcelain sockets and standoff insulators crack due to heat expansion if they are excessively tightened.

Also be careful when tightening the tube caps of high-voltage tubes. Use the proper screwdriver or tool; if the tool should slip it might fall against the glass envelope and ruin a good tube.

Tighten all tube connections, terminals, sockets, and standoff insulators that were found to be loose during the inspection procedure. When tightening tube sockets having standoff insulators, determine before tightening if the fiber washers between the socket and the standoff insulators are in tact. If these fiber spacers are cracked or missing, replace them before tightening the tube socket. Tightening the socket without these spacer washers breaks or cracks the porcelain tube socket.

Clean (C)--In the performance of this procedure, clean only where it is necessary. Do not remove tubes for cleaning purposes unless it is impossible to clean them in their original positions. If the tube must be removed, exercise caution. Do not attempt to clean the envelopes if they are located in an out-of-the-way place; in this case remove them for leaning. When tubes are removed for cleaning, replace them immediately afterward. Do not leave them where they can be broken.

1. Clean the entire tube with a clean, dry cloth if the glass envelope is excessively dirty. Then wipe the glass envelope with a cloth moistened with water and polish after cleaning with a clean, dry cloth. Do not wipe metal tubes with a cloth moistened with water, since this causes the metal body of the tube to rust. Use a cleaning agent if the tube is excessively dirty because of oil deposits. Generally, metal tubes having oil deposits on their envelopes can be cleaned successfully by polishing them dry with a clean, dry cloth. The oil film remaining on the metal body of the tube prevents rusting. To remove oiliness, corrosion, or rust from tube envelopes, moisten a clean cloth with cleaning agent and apply to the area affected until it is clean. Wipe the envelopes dry with a clean, dry cloth.

2. Clean the grid and plate caps, if necessary, with a piece of No. 0000 sandpaper or crocus cloth. Wrap the paper around the cap and gently run it along the surface. Excessive pressure is unnecessary; neither is it necessary to grip the cap tightly.

3. When the tube sockets are cleaned and the contacts are accessible, fine sandpaper may be used if there is corrosion on the contacts. Clean the contacts thoroughly after sandpapering. Clean the area surrounding the tube sockets with a brush and a clean, dry cloth; this prevents dust and dirt from being blown back on the tube envelopes when the unit is put back into operation.

Adjust (A)--When performing this operation, care must be taken to place all leads and terminals as close as possible to their original positions.

1. Adjust all leads and tube connections. Check to determine if the leads are resting on the glass envelope of the high-voltage tubes; if they are, redress the leads so that the proper spacing is obtained. Examine all leads connecting to the tube caps. These should not be so tight that they barely reach the caps of the tubes. If this condition is found, redress these leads so that enough "play" is obtained. Adjust all the grid clamps so that the proper tension is obtained. To increase the tension of tube clamps, close the spring clamps slightly with a pair of long-nose pliers until the proper tension is obtained. Do not flatten the clamps.

2. The tube sockets for transmitting-type tubes should be adjusted if the tube is found loose in its mounting. The terminals of these sockets are spring-tensioned so that they may be adjusted to increase the pres sure against the tube pins. To adjust these contacts, simply bend them toward the center of the socket until the correct tension is obtained.

Do not apply too much pressure to the spring contacts; they may be broken from their mountings in the porcelain socket.

3. Any difficulty in removing or inserting metal tubes can usually be remedied easily. Remove the metal tube and examine the tube pins to determine if solder or corrosion has accumulated on them. Remove the solder deposits with a penknife; then polish the pins with fine sandpaper. Do not use a soldering iron to remove solder deposits; this makes them worse-the solder is built up on the pins rather than re moved. To remove corrosion, use fine sandpaper, but never use it unless it is absolutely necessary. Saturate a small piece of cleaning cloth with a light lubricating oil or petroleum jelly and wipe the tube pins. Remove the excess oil from the pins by wiping them almost dry with a clean, dry cloth. If these procedures are followed, no difficulty should be experienced in removing or reinserting metal tubes. Do not force metal tubes into their sockets. Do not pry or wiggle them loose, since this damages the prongs of the socket and results in the intermittent operation of the unit employing them.

Fig. 14-33. Method of checking condition of mercury-vapor rectifier tube.

Special Instructions for Transmitting--Type Tubes In addition to the preceding, high-power transmitter tubes and mercury vapor rectifiers require special consideration as follows:

Mercury-Vapor Rectifier--As soon as possible after receipt of a new batch of mercury-vapor rectifiers, they should be placed in the tube sockets of the transmitter without the anode lead connected. Filament voltage should be applied and maintained for at least 30 minutes to distribute the mercury properly. These tubes should then be placed upright in a rugged container and protected from any jarring or tipping that would splatter the mercury. Should this occur, they must again be seasoned before the application of plate voltage. Applying anode voltage to an unseasoned mercury vapor rectifier will cause severe arc-backs in the tube.

Unless proper precautions are taken, a major portion of lost airtime will be due to faulty rectifiers. These tubes should be observed whenever possible during each operating day. A good mercury-vapor rectifier is characterized by a healthy, clear-blue glow. A greenish-yellow color usually indicates a faulty tube or one which will soon cause trouble.

Due to the importance of foreseeing such trouble and due to the lack of familiarity of the average operator with testing methods for this type of tube, the reader should become familiar with the maintenance procedure illustrated by Fig. 14-33. The cathode-ray oscilloscope provides a convenient check. An isolation transformer of at least 300 volt-amperes rating should be used together with a series current-limiting resistor of 50 ohms as shown. The mercury-vapor rectifier tube is left in its regular socket with its regular plate-cap connection removed. The secondary of the isolation transformer is then connected in series with the resistor to the rectifier plate, and the other lead is connected to the filament center tap. The vertical-deflection plates of the oscilloscope are connected directly across the tube in the same manner. With the scope self-synchronized with the 60-Hz power line and power applied to the filament of the tube being checked, the scope pattern will show both the nonconducting half of the ac cycle and the conduction half which gives the dc potential. The sharp peak at the start of conduction reveals the condition of the tube under operating conditions. A good tube will fire at between 10 and 20 volts, as indicated by the amplitude of this peak on a calibrated screen. A tube approaching the end of its useful life will require a higher firing voltage and will break into conduction later in the conducting interval. When this breakdown peak reaches from 30 to 40 volts, the tube must be tested at more frequent intervals, preferably once a week. When the firing peak reaches close to 50 volts, the tube must be replaced with a new rectifier. Operators following this procedure will greatly minimize off-the-air time caused by rectifier arc-backs and otherwise defective tubes.

Always remember that mercury-vapor rectifiers must have their filaments operated at normal voltage for a minimum of 30 minutes and then be stored upright to prevent the mercury from splashing back on the envelope and elements. Tubes which have been accidentally jarred must again be pre heated before application of the anode potential.

CAUTION: This type of simple check is possible only for gas tubes ( such as the 866 and 8008) in which the filament directly faces the plate. Certain types of tubes ( such as the 673) employ an indirect (cathode) heater which provides an extra element between filament and plate. This projection is similar in action to a grid, since it is connected to one side of the filament and is either positive or negative with respect to the plate, de pending on which way the filament transformer is connected. The best way to field-test this type of tube is described in an RCA bulletin, Pulse Method of Testing Hot-Cathode Gas Tubes, Application Note AN-157. If this Bulletin is not included in your instruction book, write to RCA Electronic Components, Harrison, N.J. 07029 and request a copy. The circuitry is more involved, but it is worth your time.

Large Power Tubes--Larger-type power tubes, such as those used in modulator and final stages in a transmitter of 5 kW or more, also require special treatment on receipt from the factory and at 4- to 6-month intervals thereafter. They should be placed in the transmitter and only filament voltage applied for 30 minutes. Low plate voltage should then be applied for about 15 minutes. This processing materially aids in preventing gas formation within the tube-a common occurrence if such measures are not followed.

Occasionally a tube will develop a small amount of gas while in storage.

The RCA recommendations for their 892R tube are as follows. With the tubes in the power amplifier, apply a low plate voltage without modulation.

After a few minutes, apply 1000-Hz tone modulation, gradually increasing the percentage of modulation. If no gas flashes occur after 15 minutes of full tone operation, remove the modulating signal, apply high power, and then repeat the process.

If gas flashes occur during the process, go back to the low-power position with no modulation and repeat. Allow the tube to run for a consider able length of time with a low percentage of modulation, and then repeat the foregoing procedure. A tube that develops a small amount of gas in modulator service may usually be cleaned up by operating it in the power amplifier.

Forced-Air Systems

In recent years, the elimination of the water-cooling system has been accomplished for transmitters having as much as a 50-kW rating by the development of forced-air cooling systems. The control circuits for these systems are greatly simplified, consisting primarily of an air-interlock damper, which prevents the application of filament and plate voltages until the normal air-flow pressure is present, and a blower-motor "keep-alive" relay, which is a time-delay relay that keeps the blower motors functioning 4 to 7 minutes after the filament voltage is removed.

The maintenance of forced-air systems is simpler than that of water systems, but it is just as important for trouble-free operation. The canvas air ducts should be cleaned about once a month by removing them, turning them inside out, and using a vacuum cleaner to remove the accumulated dirt. While these ducts are removed, a cloth may be used to clean between the fins of the tube, especially against the tube anode. Care must be taken not to damage the mercury air-flow switches mounted on the blower housing. These switches prevent the application of filament and plate voltages until the proper air flow is present. Both sides of the air-flow vanes (half-circle discs used to operate the mercury switch) should be wiped clean with a cloth or chamois, and a small wire brush may be used to clean the corners of the fan blades. A vacuum cleaner then may be used to pick up any dust from inside the bottom of the blower frames.

After this cleaning procedure, the blowers should be started to check the air-flow vanes for proper operation of the mercury switches. Then the canvas ducts should be replaced and the over-all operation checked.

Capacitors--Capacitors are vital components in transmitters. The following paragraphs outline procedures for their maintenance.

High-Voltage Capacitors--Because of their high operating potentials, high-voltage capacitors must be kept clean at all times to prevent losses and arcing. Dirt, oil deposits, or any other foreign matter must not be allowed to accumulate on the high-voltage terminals of these capacitors. All leads and terminal connections must be inspected periodically for signs of looseness and corrosion, and the porcelain insulators must be inspected for cracks and breaks.

CAUTION: To avoid severe electrical shock in case of bleeder failures, always discharge high-voltage capacitors before maintenance.

Low-Voltage Capacitors, Oil-Filled--Low-voltage, oil-filled capacitors require the same care as the high-voltage type, although the frequency of the maintenance operation is not so critical. The terminals and connections of these capacitors should be given the same careful inspection as those of the high-voltage types. The leads of these capacitors are usually not as rugged as those on the high-voltage capacitors, and they should be inspected more closely for poorly soldered connections.

Tubular Capacitors--These capacitors are of the low-voltage paper type and are generally used in low-voltage circuits for coupling and bypassing.

They should be inspected and cleaned whenever the chassis in which they are located is removed for maintenance. The only maintenance requirement for these capacitors is inspection of the tubular body of the capacitor for bulging, excessive swelling of the capacitors, and signs of wax leakage.

Inspect the terminal leads (pigtail type) of the capacitors for firmness of contact at their respective points of connection. Never use a cloth to clean this type of capacitor; this may result in damage to the surrounding circuits. Dirt and dust are brushed from the capacitor and surrounding area with a small, soft brush.

Mica Capacitors--Mica capacitors require very little maintenance other than being kept free from dust and oil. Two types of mica capacitors are generally used, the high-voltage and low-voltage types. The low-voltage types are inspected whenever maintenance is performed on the chassis of the unit in which they are located. The capacitors are inspected for body cracks caused by excessive heat, and their leads (pigtail type) are inspected for firmness of contact at their points of connection. The terminals of the high-voltage types must be inspected for tightness and corrosion, firmness of mounting, and body conditions. The body of this type of capacitor is made of a ceramic material, and care must be exercised when tightening the mountings. The bodies of the capacitors are easily kept clean with a dry, clean cloth. For satisfactory operation, the terminals must be free from dirt and corrosion at all times. Take care when tightening the terminals of these capacitors, as excessive pressure damages or cracks the ceramic case where the terminals are coupled to the body of the capacitor.

Trimmer Capacitors--In very damp climates, trimmer capacitors must be inspected often. Moisture, if allowed to accumulate on the plates of the capacitors, causes erratic operation of the unit using the capacitors. In certain cases where high voltage is used, serious damage to the capacitors results. A minute amount of moisture is all that is necessary to short-circuit the plates of the capacitor and cause abnormal operation. When such conditions are encountered, the capacitor must be thoroughly dried with a small portable heater. A cleaning cloth used to dry the plates of the capacitors may throw the plates out of alignment when the cloth is inserted be tween them. In cases where the plates of the capacitors are very closely spaced, use a magnifying glass to locate the exact position of the moisture beads existing between the plates. Due to the sheen of the capacitor plates, very minute particles of moisture cannot always be detected by the naked eye.

Capacitor Maintenance

The following suggested procedures may be followed in the maintenance of the various types of capacitors already described.

Feel (F)--Feel the terminals of the high-voltage filter capacitors. These should be fairly cool. Excessive heat probably indicates losses due to loose, dirty, or corroded terminal connections. Feel the sides of oil-filled and electrolytic capacitors. These should be cool or slightly warm. If they are very warm or hot, excessive internal leakage is indicated. Capacitors in this condition can fail at any time and should be reported for immediate re placement.

Inspect (I)--Inspect the general condition of all capacitors, regardless of type. Inspect for broken, frayed, or loose terminals, leads, and connections. Inspect the condition of the terminals of the high-voltage capacitors. Check these for dirt, corrosion, and looseness. Inspect the bodies of the capacitors for signs of excessive bulging and oil leakage. Inspect the plates of the tuning capacitors for dirt and corrosion. Check all capacitor shafts, bushings, bearings, and couplings for looseness or binding.

Tighten (T)--Tighten all loose terminals, connections, and terminal leads on all types of capacitors. Tighten all capacitor mountings and stand off insulators. Tighten all loose shaft couplings and bushings.

Clean (C)--Special attention should be given to all high-voltage capacitors to insure that they not only are kept clean but are free from moisture.

Thoroughly clean the insulators, terminals, and leads of high-voltage capacitors. When they are extremely damp due to high humidity, these capacitors frequently must be wiped dry with a clean, absorbent cloth to prevent arc-overs and breakdown of insulation. Remove the terminals that appear to be either corroded or dirty; also remove those causing power losses due to high-resistance connections. Clean them with crocus cloth which is either dry or moistened with cleaning fluid. After cleaning the terminals, polish them dry with a clean, dry cloth. Replace all the connections after cleaning, making certain that good electrical contact is obtained. Low-voltage capacitors require little attention. However, all insulated bushings and supports should be kept clean and free from foreign matter.

Adjust (A)--Adjust all leads if necessary. This requires the redressing of leads which may have been dislocated during the maintenance procedure.

If capacitor leads are stretched too tightly, redress or replace them to obtain the correct placement.

Resistors--Resistors may be divided for maintenance purposes into two groups:

those resistors easily detachable and known as ferrule-type resistors and those with soldered terminals and known as pigtail-type resistors. CAUTION: Do not touch power resistors immediately after the power has been removed. They are usually hot, and severe burns can result from contact with them.

Feel (F)-The springiness of ferrule clips may be ascertained when the ferrule. type resistor is removed. Insufficient pull at the clip may be an indication of a loose connection and poor electrical contact.

Inspect (I)-It is important to inspect all types of resistors for blistering or discoloration, since these are indications of overheating. Inspect the leads, dips, and metallic ends of the resistors and their adjacent connections for corrosion, dirt, dust, looseness, and broken strands in the connecting wires; also inspect the firmness of mounting.

Tighten (T)-Tighten all resistor mountings and connections found to be loose. If the tension at the end clips has decreased, it is common practice to press the clip ends together by hand or with a pair of pliers. The hand method is preferred because the pliers may bend the clip or damage the contact surface.

Clean (C)-Dirty or corroded connections of ferrule-type resistors can be cleaned by using a brush or cloth dipped in cleaning fluid. If the condition persists, use crocus cloth moistened with cleaning fluid. It may be necessary to sandpaper the resistors lightly with fine-grade sandpaper, such as No. 0000. Always wipe them clean with a dry cloth before replacing them.

Vitreous resistors connected across high voltage should be kept clean at all times to prevent leakage or flashovers between terminals. They can be wiped clean with a dry cloth or a cloth moistened with cleaning fluid. If cleaning fluid is used, polish the resistors with a dry, clean cloth.

Pigtail-Type Resistors--Maintenance of pigtail-type resistors is limited to an inspection of the soldered connections. These connections may break if the soldering is faulty or if the resistors are located in a place subject to vibration. The recommended practice is to slide a small insulated stick lightly over the connections and to inspect them visually for solidness. If connections are noticeably weak or loose, resolder them immediately. Discolored or chipped resistors indicate possible overloads. Although replacement is recommended, resistors in this condition may last indefinitely. The pigtail-type connections should be dusted with a brush or with an air blower.


A fuse consists of a strip of fusible metal inserted in an electrical circuit.

If the current increases beyond a safe value, the metal melts, thus interrupting the current. Fuses vary in size and rating depending on the circuits in which they are used. Two types of fuses are used: renewable and nonrenewable. The first type is designed so that the fuse link, or element contained within the fuse cartridge, may be removed and replaced when blown. The second type, however, is constructed so that the fuse element is permanently sealed within the fuse housing. When a fuse blows, an attempt must be made to determine the reason for its failure and to make corrections before a new fuse is installed.

Renewable Type--The renewable-type fuse assembly consists of a housing or cartridge of insulating material with a threaded metal cap ( ferrule) at each end. As a precaution against damage, the fuse element, or link, is placed inside the cartridge, or housing, and is held in position by the two end caps, or ferrules. When a fuse is placed in service, the two ends of the fuse cartridge are slid into spring contacts mounted on the fuse block. This places the fuse in the circuit to be protected.


Type-When nonrenewable fuses are blown, they must be discarded. Certain types of nonrenewable fuses are removed by unscrewing and withdrawing the cap screws holding them in place. When they are removed, the fuse and cap screw are separated by pulling them apart. The glass fuses are easily removed for inspection. Care must be taken to see that the fuse ends and holding clips are kept clean and tight. If they are not, overheating will result and make replacement necessary.

Inspect (I)-Inspect the fuse caps for evidence of overheating and corrosion. Inspect the fuse clips for dirt, loose connections, and proper tension.

Tighten (T)-Tighten the end caps, the fuse clips, and connections to the clips on replaceable fuses if they are loose. The tension of the fuse clips may be increased by pressing the sides closer together. Fuse caps should be hand tightened only. Excessive tightening results in difficulty in removing them.

Clean (C)-Clean all fuse ends and fuse clips with fine sandpaper when needed; wipe with a clean cloth after cleaning. If it becomes necessary to use a file to remove deep pits in the clips, fuse ends, or contacts, always finish with fine sandpaper in order to leave a smooth contact surface. As a final step, wipe the surface clean with a clean, dry cloth.

Bushings and Insulators Bushings and insulators are extremely important elements in electrical circuits, especially when located in high-voltage circuits where insulation breakdown is most common. Most of the high-voltage insulators are constructed of ceramic material with highly glazed surfaces. Exercise extreme care when working near these insulators, because they are easily chipped or broken.

Inspect (I)--Thoroughly inspect all high-voltage insulators and bushings for moisture, dust, and other accumulated foreign matter. Unless they are both clean and dry, leakage or arc-overs will occur and damage them permanently. Check for chipped surfaces, hairline cracks, carbonized arc-over paths, and other surface defects that may make the insulator unserviceable. Insulators in this condition should be reported to the person in charge for replacement.

Tighten (T)--Feedthrough bushings and standoff and other insulators should be tightened if they have loose mountings or supports. Tighten these insulators with care because the gaskets absorb only a small amount of pressure before permitting the insulator to break.

Clean (C)-Cleaning operations are similar to those outlined for tubes.

Use a clean cloth (dampened with cleaning fluid if necessary) to remove dust, dirt, or other foreign matter. Always polish with a dry, absorbent cloth after cleaning.


The various types of relays may be classified as follows: overload relays, time-delay relays, and magnetic contactors. Relays require a certain amount of preventive maintenance, which must never be performed except when absolutely necessary. Certain types are completely encased in dustproof and moistureproof cases. These require little maintenance other than a periodic inspection.

To service the relay contacts, several types of tools are necessary. Each of these has a special function, as described in the following paragraphs.

Burnishing Tool--This tool is used on relays that have extremely hard contacts; it is not a file. A contact should not be burnished unless it is found to be pitted or oxidized, and then not burnished more than necessary to re store a clean, smooth surface. The original shape of the contact must be retained.

Small Fine-Cut File--This file is to be used only on the larger contacts when they have become badly burned or pitted and a replacement is not available. This tool is not to be used on silver-plated contacts or on the contacts of telephone-type relays. Do not use the file more than necessary to remove the pit. The original shape of the contact must be preserved.

After filing, No. 0000 sandpaper may be applied to the contact and followed by crocus cloth to obtain a smooth finish on the contact surface. A clean, dry cloth is used for the final polishing.

No. 0000 Sandpaper Stick--This tool is made in the same way as the crocus-cloth stick, except that sandpaper is used instead of crocus cloth.

The use of sandpaper is limited, as is the use on the fine-cut file, to the treatment of badly burned or pitted contacts on the larger relays. Sand paper is not used on silver-plated contacts, except under extreme circum stances; when it is used it should be followed by crocus cloth. All contacts should be polished after sanding with a clean, dry cloth.

Crocus Cloth--This maintenance aid is available in two forms-as a tool and as a strip of material. It serves a twofold purpose: It may be used to remove corrosion from all relay contacts, or it may be applied to the contacts following the use of the fine-cut file and No. 0000 sandpaper.

Neither the file nor sandpaper leaves a finish smooth enough for proper relay operation. Use crocus cloth to polish the surface of the contact. The choice between the stick and the piece of cloth depends on the accessibility of the contacts. If the location of the relay and the position of the contacts permit the use of the crocus-cloth stick, it should be used; otherwise, the strip of crocus cloth must be used. The crocus cloth and tool are used as illustrated in Figs. 14-34 and 14-35. In both cases, the maintenance aid is inserted between the contacts and is drawn through them while the contacts are pressed together with the fingers.

Maintenance of relays requires that they be inspected periodically and preventive-maintenance measures performed if necessary. The inspection procedure requires that the terminals be inspected for looseness, dirt, and corrosion. Contacts may have become loosened because of the jarring of the equipment during shipment. The contacts may become dirty or corroded due to climatic conditions. Relay contacts must never be sandpapered or filed unless this procedure is absolutely necessary for the normal operation of the relay unit. A relay is considered normal if:

1. The relay assembly is free from dirt, dust, and other foreign matter.

2. The contacts are not burned, pitted, or corroded.

3. The contacts are properly aligned and correctly spaced.

Fig. 14-34. Use of crocus-cloth strip for polishing relay contacts.

4. The contact springs are in good condition.

5. The moving parts travel freely and function in a satisfactory manner.

The solenoids of plunger-type relays must be free from obstructions.

6. The connections to the relay are tight.

7. The wire insulation is not frayed or torn.

8. The relay assembly is securely mounted.

9. The coil shows no sign of overheating.

Fingers Pressing Contacts Together Tool Between Contacts

Fig. 14-35. Method of cleaning hard-alloy relay contacts.

A relay is considered abnormal if it fails to meet any of the above-mentioned requirements. The following are the procedures used in the maintenance of relay units.

Inspect (I)--Using the check list given above, inspect the relays to determine abnormal conditions. If the contacts are not readily accessible, they may be examined with the aid of a flashlight and mirror. Many of the relays can be inspected and cleaned without being removed from their mountings or without being taken apart. Mechanical action of the relays should be checked to make certain that the moving and stationary contacts come together in a definite manner and that they are directly in line with each other. The armature or plunger mechanism should move freely with out binding or dragging. Be careful during inspection not to damage or misalign the relay mechanism. Relays that require the removal of the cover for complete inspection may be found enclosed in glass, Bakelite, or metal cases. Relays must never be taken apart unless it is absolutely necessary.

Exercise care if they must be taken apart for maintenance purposes. When disassembling relays, tag all leads as they are removed. This insures that the leads are returned to their proper terminals after the maintenance procedure is completed.

Tighten (T)--Tighten all mounting screws found to be loose, but do not apply enough force to damage the screw or to break the part that it holds. Do not start screws with their threads crossed. If a screw does not turn easily, remove it and start again. Relay coils can be tightened by inserting, if possible, a small wooden or paper wedge between the coil and the core of the relay. This prevents chatter of the relay. Tighten any and all loose connections. Also tighten the mounting of the relay assembly if it is loose. When replacing glass or Bakelite covers over relay cases, take care not to overtighten the screw cap holding the cover over the relay.

Clean (C)--Clean the exterior of the relay with a dry cloth. If it is very dirty, clean with a cloth or brush dipped in cleaning fluid; then wipe the surface with a dry cloth. If loose connections are found, they should be inspected. Remove and clean connections that inspection reveals are either dirty or corroded.

The relay service aid is a narrow piece of folded cloth or canvas. It serves a twofold purpose: it is suitable for polishing a clean surface, and it is used as a follow-up to a crocus cloth. It is also intended to remove the grains that come off the crocus cloth and adhere to the contact surface. The cloth is used as shown in Fig. 14-34.

Hard Contacts--Hard-alloy contacts are cleaned by drawing a strip of clean wrapping paper between them while they are held together. It may be necessary in some cases to moisten the paper with cleaning fluid. Corroded, burned, or pitted contacts must be cleaned with the crocus-cloth strip or the burnishing tool as shown in Fig. 14-35.

Solid-Silver Contacts--Dirty solid-silver contacts are easily cleaned with a brush dipped in cleaning fluid. After they are cleaned, the contacts are polished with a clean, dry cloth. Note that the brown discoloration that is found on silver and silver-plated relay contacts is silver oxide and is a good conductor. It should be left alone unless the contacts must be cleaned for some other reason. It may be removed at any time by a cloth moistened in cleaning fluid.

Dress corroded contacts first with crocus cloth, using either the stick or the strip of crocus material. When all of the corrosion has been removed, wipe with a clean cloth moistened in cleaning fluid and polish with a piece of folded cloth. Make certain that the shape of the contacts has not been altered from the original.

Burned or pitted contacts may be resurfaced, if necessary, with No. 0000 sandpaper, making certain that the original shape of the contacts is not changed. Next, smooth the surface of the contacts with crocus cloth until a high polish is obtained. Wipe thoroughly with a clean cloth to remove the abrasive remaining on the contacts. When contacts are very badly burned or pitted and a replacement is not available, use a small fine-cut file and No. 0000 sandpaper.

Silver-Plated Contacts--Dirty silver-plated contacts are cleaned with a cloth or brush dipped in cleaning fluid. After they are cleaned, the contacts are polished with a dry cloth.

Dress corroded contacts first with crocus cloth, using either the stick or strip of crocus material. The work must be done very carefully so as not to remove an excessive amount of silver plating. When all of the corrosion has been removed, polish with a clean, dry cloth. Make certain that the shape of the contacts has not been altered.

Dress burned or pitted contacts with crocus cloth until the burned or pitted spots are removed. This may require an appreciable amount of time and energy, but it is preferable to using a file or sandpaper. If the crocus cloth does not remove the burns or the pits, use the sandpaper tool very carefully. When sandpaper is used, it must be followed with crocus cloth to polish the contacts, and then with a cloth moistened in cleaning fluid.

The contacts are then polished with a clean, dry cloth.

Never use highly abrasive materials, such as emery cloth, coarse sand paper, or Carborundum paper for servicing relay contacts, since damage to the contacts will result.

Adjust (A)--Adjust relay contacts after cleaning if necessary. The contacts should close properly when the plunger is hand operated. Adjust the relay springs if necessary, but do not tamper with them unless it is absolutely necessary. These springs are factory adjusted and maintain a certain given tension; they rarely get out of adjustment. If the spring tension must be changed, exercise care when doing so. The adjustment of current-control relays is usually accomplished by turning calibrated knobs to the desired setting or by turning a knurled adjustment sleeve which has a calibrated scale mounted adjacent to it. The adjustments should not be changed from their original factory setting except in cases of emergency. Overload relays must never be adjusted unless the person in charge has been notified and has sanctioned the adjustment.

Shapes of Relay Contacts--Relay contacts have varied shapes (Fig. 14-36) depending on their size and application. In some instances, both contacts are flat; in others, one contact is convex although its mate is flat.

The original shape of a contact must be retained during cleaning. If burning or pitting has distorted the contact so that it must be reshaped, the original shape must be restored. It is essential that the maintenance personnel familiarize themselves with all details of the relays by examining them while the relays are in good condition.

(A) Flat. (B) Convex.

Fig. 14-36. Relay-contact shapes.

Sparks and "Key Thump" Suppression

In some composite equipment (equipment not commercially designed), the engineer encounters "key clicks" or excessive sparking of relay or switch contacts. If this occurs, capacitor-resistor suppressor circuits placed in shunt with the contacts will aid materially in reducing the problem and increasing the life of the contacts.

Exact values of capacitance and resistance to use are best determined by trial. The resistance should be as high and the capacitance as low as is effective for the circuit in question. When the controlled current is pulsating or alternating, start with a capacitance of about 0.04 µF and a series resistance of 1000 ohms. If these values are not effective, gradually increase the capacitance and decrease the resistance for each capacitance until the desired suppression is achieved. When sparking occurs across contacts in a noninductive dc circuit, start the test runs with 0.5 µF capacitance and 200 ohms resistance. The optimum values will not normally be beyond these limits.


For the purpose of maintenance, switches may be classified into two general groups: those with contacts that are readily accessible, and those with contacts that are completely encased. The basic maintenance operations of inspection, cleaning, adjusting, and lubrication are applicable only to the first group. Because of the enclosed construction of the second group, no maintenance can be applied except to make a mechanical test of their operation.

Accessible-Contact Switches--This group consists of knife-blade switches, start-stop push-button switches, and high-voltage shorting bars. With the exception of the shorting bars, all of these switches consist of blades that are mechanically inserted into spring contacts.

Inspect (I) all the terminal connections of each individual switch for tightness and cleanliness. Check the mounting of the switch for firmness.

Operate the mechanism of the switch and see if the parts move freely.

Observe the stationary spring contacts to determine if they have lost tension and if they are making good electrical contact.

Tighten (T) all loose mountings and connections properly. If inspection shows that the fixed contacts have lost tension, tighten them with the fingers or pliers. Tighten every loose connection or terminal.

If inspection shows that any terminal, connection, or section of the switch is dry, dusty, corroded, or pitted, clean (c) the part with a dry, clean cloth. If the condition is more serious, moisten the cloth with cleaning fluid and rub vigorously. Surfaces which have been touched with the bare hands must be thoroughly cleaned with a cloth moistened in cleaning fluid and then polished with a clean cloth. The points of contact with the moving blade are naturally those which most often show signs of wear.

Examine these points very carefully to insure that both sides of each blade, as well as the contact surfaces of the clips, are spotlessly clean at all times.

Crocus cloth moistened with cleaning fluid usually produces this condition; however, if this is not sufficient, No. 0000 or No. 000 sandpaper may be used. Always polish clean after the sandpapering operation.

Adjust (A) if necessary. Some of the switches have a tendency to fall out of alignment because of loosening of the pivot. In most cases, tightening the screw on the axis of motion corrects this condition.

Lubricate (L) when necessary. If binding is noted during inspection of the operation of the switch, apply a drop of instrument oil with a toothpick to the point of motion or rotation. Do not allow oil to run into the electrical contacts, since a film of oil may cause serious damage or a poor contact.

Lubrication of switches is not recommended unless serious binding is noticed.

Non-accessible-Contact Switches--Under this heading are included all the remaining switches not discussed above. Interlock switches, toggle switches, meter-protective push buttons, and selector switches have been designed so that it is impossible to get at the contacts without breaking the switch assemblies. The only maintenance possible is to check the operation of the switch and, if something abnormal is detected, to notify the person in charge immediately so that a spare may be obtained and a replacement made as soon as possible. Do not lubricate these switches under any circumstances.

Generators and Motors

Certain preventive-maintenance procedures must be applied to motors and generators if proper functioning and dependable performance are to be obtained. There are three principal causes that contribute to faulty operation of this type of equipment: accumulation of dirt, dust, or other foreign matter on the windings and moving parts of the equipment; lack of sufficient lubrication on bearings and other moving parts; and improper adjustments or damaged parts. Given proper care, motors and generators provide long and efficient service. In addition to the techniques given in the following paragraphs, additional maintenance instructions covering specific motors or generators will be found in the manufacturer's instruction books.

The maintenance techniques that follow apply to the motors and generators used at the transmitter for standby power or in field use.

Feel (F)--The bearing and the housings can be tested by feeling them to determine overheated conditions. An accepted test, except in very hot climates, is to hold the bare hand in contact with the bearing or housing for a period of at least 5 seconds. If the temperature can be tolerated for this length of time, the bearing temperature may be considered normal.

Overheating may indicate lack of sufficient lubrication, a damaged bearing surface, or, in rare situations, an accumulation of dirt in the field windings.

Inspect (I)--Each motor and generator exterior, and any other visible parts, must be inspected for dirt and signs of mechanical looseness or defects. Wherever wires are exposed, see that all connections are tight and in good condition and that the insulation is not frayed. Inspect the motor ends for excess oil and the mounting for loose bolts. Wherever possible and practicable, feel the pulleys, belts, and mechanical couplings to insure that the proper tension or tightness is present. (Naturally, this must not be done while the machine is in motion.)

Tighten (T)--Any mounting, connection, or part found to be loose must be properly tightened. If any internal part such as a commutator segment or an armature coil is loose, notify the person in charge, and repair the part immediately or replace it at the first opportunity. Operation under these conditions will cause considerable damage in a very short period of time.

Clean (C)--Carefully wipe the exterior, base, and mountings of each motor or generator with an oiled cloth in order to leave a thin, protective film of oil on the surfaces. If available, use an air blower or hand bellows to blow the dust and dirt out if inspection shows that the windings are dusty or dirty.

If inspection of the commutator and brushes shows that cleaning is necessary, the accepted cleaning practice is as follows. Lift or remove the most accessible brush assembly, and press a piece of canvas cloth folded to the exact width of the commutator against the commutator; then run the motor for about 1 minute, exerting the necessary pressure. If the condition persists because the commutator has been burned or pitted, use a piece of fine sandpaper (No. 0000) , preferably mounted on the commutator cleaning stick. While exerting the necessary pressure, rotate the motor for approximately 1 minute. Stop the motor and wipe around the commutator bars with a clean cloth. It may be necessary to polish the commutator with a piece of canvas, as explained previously. Identical maintenance procedures apply to slip rings.

Transformers and Choke Coils

Some transformers are enclosed in metal housings; others are not. How ever, similar maintenance techniques are applicable to all of them.

Inspect (I)--Carefully inspect each transformer and choke for general cleanliness, for tightness of mounting brackets and rivets, for solid terminal connections, and for secure connecting lugs. The presence of dust, dirt, and moisture between terminals of high-voltage transformers and chokes may cause flashovers. In general, overheating in wax- or tar-impregnated trans formers or coils is indicated by the presence of insulating compound on the outside or around the base of a transformer or coil. If this condition is encountered, immediately notify the person in charge.

Tighten (T )--Properly tighten mounting lugs, terminals, and rivets found to be loose.

Clean (C)--All metal-encased transformers can be easily cleaned by wiping the outer casings with a cloth moistened with cleaning fluid. Clean the casing and the immediate area surrounding the transformer base. Clean any connections that are dirty or corroded. This operation is especially important on high-voltage transformers and coils. It is very important that all transformer terminals and bushings be kept clean at all times and examined regularly.

Variable Transformers

Variable transformers, as a rule, are sturdily built and are protected so that very little maintenance other than regular inspection is required.

Inspect (I)--Carefully inspect the exterior for signs of dirt and rust.

Inspect the mounting of each variable transformer to determine if it is securely mounted. Inspect all connections for looseness, corrosion, and dirt.

Check the slip rings for signs of corrosion or dirt.

Clean (C)--The perforated casing of each variable transformer as well as the area surrounding the base must be cleaned regularly. If the slip rings need cleaning, disassemble the unit and clean them with a cloth moistened in cleaning fluid, and then polish with a clean, dry cloth. If the dirty condition persists, use crocus cloth and rub vigorously. Again polish with a clean cloth. Reassemble the unit; then reinstall it, reconnecting all terminals.

Lubricate (L)--If the shaft shows signs of binding or if it squeaks, apply a few drops of household oil to the front and rear bearings. Rotate the control shaft back and forth several times to insure an equal distribution of the lubricant in the front and rear bearings.

Rheostats and Potentiometers

Rheostats and potentiometers fall into two main groups for maintenance purposes: those which have the resistance winding and the sliding contact open and accessible, and those which, by construction, have their inner parts totally enclosed. In the latter group very little maintenance can be performed, since opening and removing the metal case may damage the unit.

Inspect (I)--The mechanical condition of each rheostat must be inspected regularly. The control knob should be tight on the shaft. Inspect the contact arm and resistor winding for cleanliness and good electrical contact. Check the rheostat assembly and mounting screws for firmness, the sliding arm for proper tension, and the insulating body of the rheostat for cracks, chipped places, and dirt.

Tighten (T)--Tighten carefully any part of the rheostat or potentiometer assembly found to be loose.

Clean (C)--The rheostat or potentiometer assembly is easily cleaned by using a soft brush and then polishing with a soft, clean cloth. If additional cleaning is needed, or if the windings show signs of corrosion or grease, the brush may be dipped in cleaning fluid and brushed over the winding and contacts. Use a clean cloth to remove the film that remains after the cleaning fluid has evaporated. If the contact point of the sliding arm is burned or pitted, it is a good practice to place a piece of folded crocus cloth be tween the contact and the winding and then to slide the arm over the crocus cloth a number of times. When cleaning the winding, do not exert excessive pressure, or damage will result.

Adjust (A)--If the tension of the sliding contact is insufficient, an adjustment can be made with long-nose pliers. A slight bending of the rotating piece in the proper direction restores the original tension.

Lubricate (L)--Apply lubrication only when necessary; that is, when binding or squeaking is noticed. One or two drops of instrument oil applied to the bearings with a toothpick is sufficient. Since the slightest flow of oil into the winding or the sliding-arm contact may cause serious damage, lubrication must be applied very carefully and only to the bearings. Wipe off all excess oil.

Terminal Boards and Connecting Panels

Little preventive maintenance is required on terminal boards and connecting panels. The following paragraphs contain some suggested procedures.

Inspect (I)--Carefully inspect terminal boards for cracks, breaks, dirt, loose connections, and loose mountings. Examine each connection for mechanical defects, dirt, corrosion, or breakage.

Tighten (T)--All loose terminals, screws, lugs, and mounting bolts should be tightened properly. Use the proper tools for the tightening procedure, and do not overtighten, or the assembly may crack or break.

Clean (C)--If a connection is corroded or rusty, it is necessary to disconnect it completely. Clean each part individually and thoroughly with cloth or crocus cloth moistened with cleaning fluid. All the contact surfaces should be immaculate for good electrical contact. Replace and tighten the connection after it has been thoroughly cleaned.

Air Filters--Air filters are placed in blowers and ventilating ducts to remove dust from the air before it is drawn into and circulated through the ventilating system. Some filters are impregnated with oil and some are filled with cut strands of glass to facilitate the filtering action. The following procedures cover their maintenance.

Inspect (I)--The filter should be inspected for any large accumulation of dirt and for lack of oil. Note whether the filter is mounted correctly and whether the retaining clips are in place. Improperly assembled filter elements allow unfiltered air to leak around the edges and thus permit dust to enter the ventilating system.

Tighten (T)--Tighten the retaining clips if they are loose, and readjust the filter in its mounting.

Clean (C)--Usually the filters are easily accessible and may be taken out after the removal of the cover plate. The general procedure is as follows.

Mark the outside of the filter before removing it from the air duct. Before washing it, tap its edges against the wall or on the ground to remove as much dirt as possible. Wash the filter in gasoline, using a brush to remove dirt from the steel wool. After the filter has been washed, place it face down on two supports. Allow the filter to drain and dry thoroughly before lubricating it.

Lubricate (L)--Lubricate or recharge the filter element by dipping it in a bath of oil. For temperatures above 20°F, use SAE-10 oil. Allow the filter to drain thoroughly, intake side down, before it is put into use. While the filter is draining, keep it away from places where sand or dirt is being blown through the air. Always replace a filter with its intake side facing the incoming air flow.


The cabinets that house the various components of the installation are generally constructed of sheet metal. Suggestions for their care follow.

Inspect (I)--The outside and inside of each cabinet must be inspected.

Check the door hinges (if any), the ventilator mountings, the panel screws, and the zero-setting of the meters. Examine the pilot-light covers for cracks and breaks. Occasionally remove the covers and see if the pilot-light bulbs are secure in their sockets. Inspect the control panels for loose knobs and switches.

Adjust (A)--Adjust the zero setting of meters if they are incorrect.

Follow the specific instructions given in the subsection on meters.

Clean (C)-Clean each cabinet, including the control panel, outside and inside with a clean, dry cloth. Also clean the meter glasses and control knobs with a clean, dry cloth.

Lubricate (L)--Door hinges and latches need little lubrication, but if inspection reveals that they are becoming dry, apply a small amount of instrument oil. Remove excess oil with a clean, dry cloth.


Meters require very little maintenance. They are extremely delicate instruments and must be handled very carefully, and, because they are precision instruments, they cannot be repaired in the field. A damaged meter should be replaced with a spare and the defective meter returned to the maker for repair and calibration.

Inspect (I)--Inspect the leads and connections to the meter. Check for loose, dirty, and corroded connections and for cracked or broken cases and meter glasses. Since the movement of a meter is extremely delicate, its ac curacy is seriously affected if the case or glass is broken and dirt and water filter through. If the climate is damp, it is only a matter of time until enough moisture seeps through a crack to ruin the meter movement.

Tighten (T)--Tighten all loose connections and screws. Any loose meter wires should be inspected for dirt or corrosion before they are tightened. The tightening of meter connections requires a special technique because careless handling can easily crack the meter case. To prevent break age, firmly hold the hexagonal nut under the connecting lug while the out side nut is tightened. This permits the tightening of the connection without increasing the pressure of the head of the stud against the inside of the meter case.

Clean (C)--Meter cases are usually made of hard, highly polished Bakelite and can be cleaned with a dry cloth. If cleaning is difficult, the cloth may be dampened with cleaning fluid. Dirty connections may be cleaned with a small, stiff brush dipped in cleaning fluid or with a small piece of cloth dipped in solvent. Remember that solvents do not remove all dirt from hard surfaces. Some of the dirt remains in a softened state and must be removed with a damp cloth. Corroded connections are cleaned by sanding them lightly with a very fine grade of sandpaper, such as No. 0000.

After they are cleaned, the connections are wiped carefully with a clean cloth.

Adjust (A)--Normally, all meters should indicate zero when the equipment is turned off. The procedure for setting a meter to zero is not difficult.

The tool required is a thin-bladed screwdriver. Before deciding that a meter needs adjusting, tap the meter case lightly with the tip of one finger. This helps the needle overcome the slight friction that sometimes exists at the pointer bearings and prevents an otherwise normal unit from coming to rest at zero. If an adjustment is needed, insert the tip of the screwdriver in the slotted screw head located below the meter glass, and slowly turn the adjusting screw until the pointer rests at zero. Observe the following pre cautions: View the meter face and pointer full on and not from either side.

Avoid turning the zero-adjust screw too far, since the meter pointer may be bent against the stop peg, or the spring may be damaged. Zero adjustments should not be made for several minutes after shutdown. Always re member that meters are delicate instruments.

Pilot Lights

Pilot lights are used to indicate that power has been applied to a circuit or that a circuit is ready for the application of power. They are easily re moved and replaced. The colored pilot-light covers must be removed care fully, lest they be dropped and broken. The maintenance of pilot lights presents no special difficulty, but the following instructions are given for general guidance.

Inspect (I)--Inspect the pilot-light assembly for broken or cracked pilot-light shields, loose bulbs, bulbs with loose bases, loose mounting screws, and loose, dirty, or corroded connections.

Tighten (T)--Tighten all mounting screws, and resolder any loose connections. If the connections are dirty or corroded, they should be cleaned before they are soldered. Loose bulbs should be tightly screwed into their bases. Broken or cracked pilot-light shields may sometimes be temporarily repaired by joining the broken or cracked pieces with a narrow piece of friction tape. Replace them as soon as possible; also replace broken or burned-out pilot-light bulbs as soon as possible. While the removal of a bulb may sometimes be difficult, the process is simplified by folding a small piece of friction tape over the top of the bulb and pressing it firmly from the two sides. After the tape is attached, the bulb can be unscrewed and re moved from the socket. The socket connections are, of course, inspected when the bulb is out. A new bulb can usually be replaced with the fingers, but if difficulty is experienced, use friction tape to grip the glass envelope of the bulb.

Clean (C)--The pilot-light shield, the base assembly, and the glass envelope of the light bulb can be cleaned with a clean, dry cloth. Clean the accumulated dust or dirt from the interior of the socket base with a small brush. Corroded socket contacts or connections can be cleaned with a piece of cloth or a brush dipped in cleaning fluid. The surfaces are then polished with a dry cloth. Clean contacts and connections are important.

Plugs and Receptacles

There are two main types of plugs and receptacles used to interconnect the various components. The first type of plug is used with a coaxial line and consists of a metal shell with a single pin in the center and insulated from the shell. When the plug is inserted into the receptacle, this pin is gripped firmly by a spring connector. There is a knurled metal ring around the plug that is screwed onto the corresponding threads on the receptacle.

The second type of plug is used for connecting multiconductor cables.

The plug usually consists of a number of pins insulated from the shell and inserted into a corresponding number of female connectors in the receptacle, although in some cases the female connectors are in the plug, and the male connectors are in the receptacle. This type of plug usually has two small pins or buttons that are mounted on a spring inside the shell and protrude through the shell. When the shell is properly oriented and placed in the receptacle, one of these pins springs up through a hole in the receptacle, firmly locking the plug and receptacle together. When it becomes necessary to remove the plug, the other pin is simply depressed and the plug removed.

The connections between all plugs and their cables are made inside the plug shell. The cable conductor may be soldered to the pin, or there may be a screw holding the wire to the pin. Remove the shell if it is necessary to get at these connections for repair or inspection. Loosen the screws if there is a clamp holding the cable to the shell. In some cases, it is found that the shell and plug body are both threaded; in this case the shell may simply be unscrewed. Usually there are several screws holding the shell; these are re moved and the shell is pulled off.

Inspect (I)--Each of the following parts should be inspected for the items indicated.

1. The part of the cable that was inside the shell for dirt and cracked or burned insulation.

2. The conductor or conductors and their connection to the pins for broken wires, bad insulation, and dirty, corroded, broken, or loose connections.

3. The male or female connectors in the plug for looseness in the insulation, damage, dirt, or corrosion.

4. The plug body for damage to the insulation and for dirt or corrosion.

5. The shell for damage, such as dents or cracks, and dirt or corrosion.

6. The receptacle for damaged or corroded connectors, cracked insulation, and proper electrical connection between the connectors and the leads.

Tighten (T )--Perform the necessary operations to correct the following conditions.

1. Any looseness of the connectors in the insulation, if possible; if not, replace the plug.

2. Any loose electrical connections. Resolder if necessary.

Clean (C)--Perform the necessary cleaning operations on the following items.

1. The cable, using a cloth and cleaning fluid.

2. The connectors and connections, using a cloth and cleaning fluid. Use crocus cloth to remove corrosion.

3. The plug body and shell, using a cloth and cleaning fluid; use crocus cloth to remove corrosion.

4. The receptacle, using a cloth and cleaning fluid if necessary. Corrosion can be removed with crocus cloth.

Adjust (A)--Adjust the connectors for proper contact if they are the spring type.

Lubricate (L)--Lubricate the plug and receptacle with a thin coat of petroleum jelly if they are difficult to connect or remove. The type of plug with the threaded ring may especially require this treatment.


"We're off the air!" This is a declaration that invariably causes a state of panic for the newcomer to a transmitter operating job. In many in stances, he is alone with the responsibility of correcting the trouble as quickly as possible to avoid loss of revenue for his employer. The highest efficiency in correcting trouble will come with more experience at the particular installation. However, the operator who can visualize general circuit theory in relation to the particular circuits with which he is concerned will find a logical and natural sequence for locating the fault. The main requirement, quite naturally, is to become thoroughly acquainted with the circuits used. He should be able to draw a good general functional diagram of all circuits from memory, and be able to draw a block diagram of the operating sequence for starting relays and protective relays in the power-control circuits. It is obvious that confidence and peace of mind can be achieved only with a complete familiarity with all circuits and their relation to the overall performance of the transmitter.

There is one piece of equipment at the transmitter installation that should be the central focusing point for the operator's first attention when trouble occurs. This is the modulation monitor; it has an rf-input meter that reads a definite place on the scale for normal operation, and, of course, it has the percentage-modulation indicator. The purpose of observing this instrument will be evident in the following discussions.

At the first interruption of the program, or the occurrence of noise or distortion in the monitoring loudspeaker, the modulation monitor should be observed. Assume that the rf-input meter is at the normal point on the scale; this means that the trouble is not in the rf section, because any trouble there would cause some deviation in the rf input to the monitor.

The following is a procedure to use when the program suddenly stops:

1. If the rf-input meter shows normal and the modulation meter shows modulation taking place, the trouble is obviously in the monitoring line or amplifier, and the station is not off the air.

2. If the rf input is normal and no modulation is shown on the meter, the trouble is either in the audio section of the transmitter, in the line amplifier, in the program line from the studio to the transmitter, or at the studio.

3. Call studio control to ascertain the condition at that point. If everything there is normal, check the line by patching the line into the monitor amplifier or spare amplifier to see if the program is coming into the transmitter. If not, notify control to feed the program on the spare line, and call the local test board of the telephone company. If the program is coming in satisfactorily from the line, use a spare line amplifier to feed the transmitter. If the regular line amplifier is working normally, then the trouble obviously lies in the audio section of the transmitter. Usually any trouble there will be indicated by abnormal plate-current meter readings, and, of course, tube trouble is the most common source of program interruption.

The same procedure should be used when noise or distortion occurs; first check with the studio, then check the line, the line amplifier, and the audio section of the transmitter. If all speech-input tube currents are zero, then the trouble is in the associated power supply. Most likely, the trouble again is due to a tube, and the tube should be changed on indication of abnormal plate current. Next in line come bleeder resistors, taps from a bleeder sup ply, and line-to-plate circuits of tubes. Power-supply component parts usually show a visual indication of damage, such as smoking, unless they have opened.

If, at the first indication of trouble, a glance at the modulation monitor shows zero or low rf input, then the trouble lies in the rf stages of the transmitter. The operator must accordingly proceed to check for the trouble by observing all rf-circuit meter indications. Observation of plate-cur rent and grid-current meters aids in quickly determining which of the stages is faulty.

When the transmitter is shut down by relay operation in the control circuits, the cause of the failure can be traced quickly if the operator is familiar with the relay sequence and functions. Control circuits are divided into two functional purposes, those which control circuits to the primaries of power supplies and those with protective functions. Pilot lights are often associated with the various relays to show when they are open or closed.

As stated before, the sequence of operation should be committed to memory. The filament power supply, for example, will not operate until the cooling-motor contactors have functioned to supply the cooling medium (water or air) to the tubes. After the filament contactor has applied filament voltage, the plate-voltage contactor will not operate until the time-delay relay has functioned, etc.

Rectifier tubes of the mercury-vapor type usually arc backward several times before expiring. When arc-back indicators are used, the faulty tube may be observed quickly and changed immediately. Other troubles in high-voltage power supplies nearly always show signs of physical deterioration.

Short circuits which cause a quick tripping of overload relays are the most difficult troubles to locate. In some difficult troubles of this kind, over- load relays have been strapped out of the circuits, and limiting resistors put in the current fuse box to limit the amount of current. The circuits were then visually observed for arcing with doors open and interlock switches short-circuited. This is a dangerous procedure, however, and should be left to the more experienced operators. Any unusual procedure of this kind should be carried out by two or more men.

This all may be summarized into the most important factor: Be familiar with the transmitter, and know what indications would be for the most common sources of trouble, such as tubes and power supplies, in the various circuits.

Be Mentally Prepared

We have discussed the art of being mentally prepared for emergencies in the section on studio emergency techniques. Obviously, the same sort of preparedness is an important factor in mastering technical emergencies at the transmitter.

The first step, then, is to become familiar with the circuits. The second step is to become familiar with the locations of circuit components. Where is the main rectifier time-delay relay? Where are the interlock contactors? Where is the modulator bias relay? Take the complete (not simplified) schematic "behind the doors" of your transmitter, and visualize each component in the physical layout in relation to the schematic. In most modern commercial transmitters of vertical-chassis construction, the schematic numbers are stamped either directly on the parts or on the chassis adjacent to the particular component. For example, if a certain coupling capacitor is shown on the schematic as C102, this number will appear on the capacitor itself or on the chassis adjacent to it. In some cases, only the tube numbers (V1, V2, etc.) and transformer numbers, (T1, T2, etc.) are shown. The parts associated with a particular tube may be oriented easily by tracing their connections to the proper tube socket or transformer terminals. When the operator has become thoroughly familiar with the equipment and feels at ease with the first steps, he is ready for the all-important third step, getting mentally prepared for trouble.

One important example of mental prepardeness: Will reduced power help to stay on the air, either until the end of the day's schedule or until a sustaining program comes along to avoid loss of revenue? This will, of course, be determined by the nature of the trouble. Any capacitor, resistor, tube, transformer, insulator, or power lead that would undergo less strain because of a power reduction obviously might hold a while longer by this means. Also, in the case of tripping of overload relays in rapid succession when the source of the trouble is still not apparent, reducing the power should be the very first step in attempting to stay on the air. This is advantageous, too, when help may be needed in cases of the more serious type.

The station may be kept on the air until help arrives to carry out emergency operations.

Transmitter Emergency Procedure

The following example routines are applicable to any station, whether locally or remotely controlled.

1. The first duty of the transmitter operator is to minimize down time.

If you have a failure and are not able to get back on the air in a reasonable length of time ( within 1 or 2 minutes) , advise the studio operator of the condition. The studio operator will then stand by to make telephone calls to get in touch with the chief engineer or pro vide any other assistance the transmitter operator may request.

2. Operators not on duty and listening at home may call in to offer help, but they should not call the transmitter. They should call the studio operator, who will keep them advised.

3. In no event should more than 2 minutes elapse before advising the studio of a failure. Advise them sooner if you feel that there might be a chance of not getting back on in 2 minutes.

4. At the first sign of a possible failure, the filaments of the auxiliary transmitter should be turned on and preliminary adjustments made so that it can go on the air if the main transmitter fails.

NOTE: If trouble develops on the antenna side of the directional relays, where the antenna system is common to both transmitters, it will not help to change transmitters. It is quite possible, however, that components that will not hold on full power will hold on less power.

5. Make it an important habit to note the time of any interruption to the program, whether transmitter failure or lack of program from the studio. This will serve also as a double check on studio or network failures.

6. Try to be mentally prepared to meet an emergency. Inspect relays and lightning gaps often during an electrical storm. In this way, you may see part of the trouble developing and can replace them or have temporary clip leads ready to put in the circuit.

7. In case of time lost due to failure at the transmitter, report to the control room the time the program stopped and the time it started again.

8. Fill out a transmitter operating-room report form.


Mental preparedness at the transmitter is rigorously tested during electrical storms. In spite of the many provisions in modern transmitter installations to help protect the equipment from heavy lightning surges (ball lightning gaps, automatic carrier-off relays, etc.), the great majority of time lost in well-maintained stations is due to lightning.

There seems to be no rhyme or reason to some troubles that develop during storms. Regardless of critically spaced lightning gaps at the bases of the towers and both ends of transmission lines, lightning has been known to open or short interlock circuits, power supplies, speech-input equipment, etc. The shortest path to ground relative to the antenna circuit is definitely not a 100-percent rule for lightning.

It is true, however, that a majority of failures due to electrical storms occur in the antenna or directional phasing equipment. The trouble usually may be recognized immediately from blackened or smoking parts. When a line or antenna current meter is damaged by lightning, the face is nearly always so black as to be unreadable. Even when switch-blade shunts are kept on meters, lightning charges are apt to damage a transmission-line current meter by a heavy current arc from coil to magnet to ground. It is necessary to remove the leads from the meter to remove the short that re mains after the initial strike. Relays in the antenna circuits are another common source of failure when severe lightning surges occur. It is very important that every transmitter operator have various sizes of clip jumpers handy to strap around any such failure. If only the relay holding coil is opened, the relay may be blocked shut by some kind of prop or weight, depending on the type of relay and the method of mounting.

Cracked insulator bowls on top of the tuning house will cause continued arcing when power is restored, resulting in tripping of overload relays.

Look for arcing inside the bowl.


Electrical storms are hazardous to a transmitter operator when work on the antenna system is necessary during the height of the storm.

Never touch anything under these conditions until the tower has been well grounded by a "hot stick" or other arrangement; hang the metal connection onto the tower lead as close to the actual tower base as possible. Fig. 14-37 illustrates such an arrangement. Grounding sticks of this type are an important and necessary item for any transmitter operating room or tuning house.

Control Circuits

Over a period of several years, an operator will be faced with the situation of quick tripping of overload relays, finally resulting in a complete shutdown of the transmitter. Modern transmitters employ an automatic re turn circuit so that three to five overloads must occur in rapid succession before the power is removed, requiring a notching relay to be reset by hand. Also, some form of visual indication is provided to show which general section is being overloaded, such as the power amplifier, modulator, etc. The exact cause of the overload, however, is often hard to locate unless visible arcing serves to indicate the source. Past history of the particular installation and observation of parts with voltage applied are helpful in most such instances.

Where arcing occurs, the sense of hearing usually is able to locate the approximate vicinity. On opening the doors, signs of arcing, such as burned spots on the frame immediately adjacent to a coil or capacitor corner, should be visible.

The first emergency measure is to reduce power. If the carrier will hold on reduced power, the chief engineer or supervisory personnel may then be consulted to determine the best possible course of action from then on.

When overloads must be traced down by strapping interlock circuits and opening the doors for visual observation with voltage applied, extreme care must be taken to avoid contact with high voltages.

To Antenna Tower

Fig. 14-37. Safety device for grounding high-voltage points.

Power-amplifier overloads may, of course, be caused by trouble in the antenna system. Any fault that would cause grounding of the transmission-line center conductor or antenna (unless shunt fed) would cause the final stage to overload and operate the overload relay. Such faults may usually be found by visual observation. Look for blackened meter faces, smoking capacitors, or a collapsed static-drain choke causing a tower to be grounded for rf and dc.

Field mice have been known to get into antenna tuning houses and meet their end by becoming a direct rf path to ground on the tuning-component chassis. When examining any part of the transmitter or antenna system for possible sources of overloads, look around for such possibilities as mice, bugs between plates of tuning capacitors, etc. The occurrence is more common than the newcomer might suspect.

Rectifier line overloads that often trip circuit breakers or blow line fuses are almost always caused by a defective mercury-vapor rectifier tube which arcs back. Modern transmitters employ arcback indicators which will give a visual check on any tube in which current has passed in the reverse direction. If tubes are visible from the front of the transmitter through the panel doors, a tube so afflicted will have its blue haze extinguished momentarily at the time of the arcback, or a distinct flash will be apparent within the envelope. If the rectifiers are not visible, the entire set of mercury-vapor tubes should be replaced with new ones. Remember that the new tubes must have been preheated for at least 30 minutes to assure that all mercury has been removed from the tube elements.

In some cases, the relay is at fault. If, for example, a holding coil should open, as sometimes happens, the simplest procedure is to prop or tie the relay shut manually. When this cannot be done conveniently, the proper terminal board numbers should be jumpered to complete the circuit, or the jumper may be used at the relay contacts in some instances.

An example of jumpering terminal-board connections to meet emergencies in control circuitry can be seen by reviewing Fig. 9-36, Section 9.

Assume that the filaments have been turned on by closing the main line switch. Power is now also applied to timing relay K1, which should cause green indicating light IS1 to light after the customary 30 seconds of time delay. This will occur if:

1. Relay K1 is in working order.

2. Door interlocks S1 and S2 are closed.

3. Green indicating light IS1 is in working order.

Now assume that after one minute the green light (indicating that high voltage can be applied) has not come on, although the doors are closed (closing S1 and S2) and it should do so. Pressing momentary-contact switch IS1 fails to apply plate voltage, so you can assume that either the time-delay relay has not operated or a door interlock switch is open. Note from Fig. 9-36 that you can jumper terminals 11-10 and 11-11 to complete the interlock circuit even with the doors open.

If this still does not allow application of plate voltage, you can jumper terminals 11-11 and 11-12 to bypass the time-delay contacts. In case of extreme emergency where it is necessary to get on the air in a hurry, and doubt exists as to which circuit is faulty, a jumper from terminal 11-10 to terminal 11-12 completes the entire circuit of timing relay and door inter locks.


The chief engineer normally assumes the entire responsibility for all technical matters concerning his installation. Exceptions are the following duties sometimes assumed by a consulting engineering firm.

1. Filing of the original engineering sections of forms submitted to the FCC on an initial application for a broadcast station.

2. Design and supervision of the initial adjustment of a directional antenna system as may be required by the FCC.

An engineering consultant is often kept on a retainer fee by the station management for the purpose of alerting the station to legal changes in engineering requirements and to assist the chief engineer in meeting the legal requirements for changes in the technical installation.

The type of work performed by the chief engineer can vary from the small station where he performs all actual maintenance and repair to the larger stations where his duties are mainly administrative. He normally handles scheduling for technical operations and important preventive maintenance and test measurements. He should have a current copy of the FCC Rules and Regulations available and at least be sufficiently familiar with their format to know where to look for reference in cases such as the following.

The engineer in charge of the FCC field office for the district in which the station is located must be notified when indicating instruments such as frequency monitors, modulation monitors, antenna meters ( such as the common-input meter for directional-antenna systems) , etc., are temporarily out of service during an emergency. A record of the telegram sent should be kept for reference. The same notification must be given on the restoration of such devices to service.

Notification must also be given when the power must be reduced below specified tolerances for some reason ( such as failure of the antenna trans mission line or other component under authorized power) . Check current rules for the maximum allowable time of operation on low power before notification is made.

Compliance with FCC requirements on tower painting and lighting, technical logs, and all other matters pertaining to the technical operation of the station is normally the responsibility of the chief engineer. He will see that the local FAA (Federal Aviation Administration) office is notified immediately when a flasher beacon at the top of the tower installation is inoperative. In some cases, notification is required if any other tower light is off, particularly when the location is on or near a main artery of air traffic. A record of the notification must be kept in the technical log, which should have a designated space for tower-light condition and time of observation, as well as the time of FAA notification when required.

The chief engineer should have on file an up-to-date equipment and component list from a sufficient number of sources so that he can immediately take under study any special requirement proposed by management.

In some stations, special problems occur almost daily, and the chief engineer must find the quickest and most practical solution at the lowest possible cost.

In the case of a new installation or expansion of an existing facility requiring new construction, the chief engineer must be able to direct the electrical and construction contractors in drawing up the initial technical plans. He must have a sufficient knowledge of heat dissipation to assist the air-conditioning engineer in properly installing an adequate system if it is required.

He should have available the latest revised FCC Tariff No. 198 giving regulations, minimum costs, and leasing times for various grades of service, etc., concerning channels for program transmission in connection with radio broadcasting. This enables him to be prepared to come up with a quick estimate on the technical costs of a special event and provides a basis for judging whether certain classes of lines should be ordered instead of using mobile equipment that is either owned by the station or available for lease. This information is available from the American Telephone & Telegraph Co. Also, it is a good idea for the chief engineer to become acquainted with the supervisor of the local AT&T toll test and telephone company test board and be familiar with the extent of prior notification necessary for the various classes of line service. This requirement varies somewhat with local conditions.

He should be sure that all emergency numbers, such as police, fire, power and light company emergency service, AT&T toll test, and telephone company test are plainly posted at the studio and transmitter operating positions. He should also be sure that fire extinguishers of the proper type for electronic equipment are installed and that they are recharged at the required intervals.

The chief engineer must be acquainted with the FCC requirements for proof-of-performance measurements, including the technical standards that must be met and the dates when the appropriate forms must be filed. For example, a license-renewal application must be filed at least 90 days prior to the license expiration date. A complete proof of performance must be run in the four-month period preceding the date of filing application. Thus, if a station license expires October 1, the renewal application must be filed not later than July 2. (NOTE: Always check current FCC Rules and Regulations.) The proof-of-performance runs must then be made in the period of March 3 to July 2. It is advisable to run such tests about three times every year at intervals such that the tests occur at the proper time in the renewal year. Remember that proof-of-performance data must be filed with the application for license to cover the construction permit (CP) for new or modified facilities.

The chief engineer must also be familiar with FCC requirements for checks on existing directional-antenna system patterns, the frequency with which such checks must be made, and the proper recording of all related information.

When preparing the technical portion of an application for renewal of the license, be sure that data entered in the form are in agreement with the log (where involved) and that such data indicate compliance with the FCC rules and the license specifications.

It is good practice for the chief engineer to check the readings of meters periodically under normal operating conditions. Compare these with the log, the present license, and the previous application for the license or license renewal. This helps to eliminate errors, to ascertain that the observed data are typical, and to determine if the variations from licensed values are within the limits permitted.

Carefully file all original copies of station-frequency checks made by the frequency-measuring service, and be sure that this information has been properly logged. Preserve all records of such items as meter calibration for any meter replacement in the final stage or antenna system with the full details required by the FCC.

All of the foregoing condenses to this: Know exactly what the FCC and the specific station license require, and see that the proper steps are taken to meet these requirements. Make equipment-performance measurements more often than required by the FCC. Measurements made on a regular basis and a complete record of steps taken to remedy any deficiency go a long way toward assuring the radio inspector that the station is in competent engineering hands.


Q14-1. Would you expect the studio VU meter and the transmitter modulation meter to "peak" the same on program waves at normal modulation?

Q14-2. Name the two basic methods of measuring antenna impedance.

Q14-3. How must transmitter output power be determined when field-intensity surveys are made? Q14-4. How is transmitter output power determined by the (A) direct method (B) indirect method? Q14-5. Must the limiter or agc amplifier installed in the normal transmission path be used in proof-of-performance measurements?

Q14-6. The stereophonic transmission standards state: "The stereophonic subcarrier shall be the second harmonic of the pilot subcarrier and shall cross the time axis with a positive slope simultaneously with each crossing of the time axis by the pilot subcarrier." Why is it necessary to standardize this time relationship?

Q14-7. The stereophonic standards also state: "At the instant when only a positive left signal is applied, the main channel modulation shall cause an upward deviation of the main carrier frequency, and the stereophonic subcarrier and its sidebands signal shall cross the time axis simultaneously and in the same direction." What stereo characteristic does this standard affect?

Q14-8. Review paragraphs L and M of the stereophonic standards (see Section 10-6, Section 10) . What stereo characteristic do these paragraphs of the standard affect?

Q14-9. In stereo transmission, do the terms "separation" and "cross talk" define the same characteristic?

Q14-10. When audio is weak or lost entirely at the transmitter, where is the first place to look?

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Updated: Wednesday, 2021-09-15 12:39 PST