In the alignment and troubleshooting of a television receiver, a number of factors must be considered. The test equipment employed should be suitable for this type of work. All pertinent data relating to the television receiver to be aligned should be available for reference, and the manufacturer's alignment procedure must be followed closely.
TEST EQUIPMENT REQUIRED
At the outset it should be pointed out that no set of rules as to the number and exact specifications of the instruments required can be given here. Instead an effort will be made to give the most common type of equipment necessary in the average practice and also to explain how this equipment is used to accomplish its assigned function. (See Table 16-1.)
Table 16-1 . Test Methods for Troubleshooting TV Receivers
The intelligent servicing of a television receiver requires, first, a knowledge of the operation of the receiver and, second, a knowledge of the normal waveforms to be expected in the circuits of the particular equipment. The pieces of test equipment necessary for alignment and effective troubleshooting usually found in the average television service shop are as follows:
1. Oscilloscope
2. Sweep generator
3. Signal generator
4. Digital or vacuum-type voltmeter
5. Miscellaneous test equipment
The Oscilloscope
Few instruments are of greater utility in television receiver testing than the oscilloscope. A cathode-ray oscilloscope is necessary when alignment is performed with a sweep oscillator. It can also be employed with a modulated signal generator to align wave traps and other narrow-band circuits. It is also used in the sync and sweep circuits of the television receiver to observe the fidelity of the various waveforms and ascertain their presence at various points. The amplifiers, particularly the vertical amplifier, determine the utility of any particular oscilloscope in television receiver testing. A typical oscilloscope is illustrated in Fig. 16-1.
The best criterion of satisfactory operation is the faithful reproduction of a square pulse at a repetition rate of from 100 to 15,000 hertz/sec. Defined in terms of frequency response, which does not take into account phase discrepancies, this is roughly uniform response from 30 to 150,000 hertz.
Fig. 16-1. Typical triggered-sweep oscilloscope.
The synchronizing of the oscilloscope to that of the waveform being observed is accomplished through a switching arrangement from two or three possible sources: external, internal, or 60 hertz. Each switch position connects the sweep generator in the oscilloscope to the respective sources. The external position allows injection of sync through a BNC connector on the front of the instrument, which allows any pulse (whether actually derived from the trace under observation or not) of the correct frequency to synchronize the trace. It is thereby possible, through the use of a phase-shifting network, to vary the portion of the pattern at which the trace begins and so change its position on the screen. A 60-hertz sync is usually internationally provided because a larger portion of the traces encountered are some multiple or sub multiple of this frequency. Internally, the signal synchronizes itself since a portion of the signal is extracted from the vertical amplifier and injected into the sweep generator.
A complete treatise on the use of the oscilloscope is beyond the scope of this Section. However, some few basic operational procedures have been included, which may serve as a guide to the serviceman who has not used the instrument extensively.
Controls--The following controls are normally found on the oscilloscope:
1. Focus
2. Intensity
3. Vertical centering
4. Horizontal centering
5. Sync-selector switch
6. Volts/div
7. Sec/div
8. Triggering
The meaning of these labels is doubtless self-evident. Proficiency in their use, particularly in connection with 6 to 8, comes only with practice. Perhaps the greatest difficulty that will be experienced, aside from the proper interpretation of the reproduced trace, will be obtaining stability of the trace-a function of the three aforementioned controls.
Perhaps the most outstanding cause of failure to consistently obtain a stable pattern lies in the use of excessive synchronizing potentials, which may cause irregular synchronization and the loss of a portion of the desired sweep trace. In adjustment, it is improper to vaguely set the timing (sweep-repetition rate) controls and to attempt final stabilization by greatly advancing the sync control. Instead, the sec/div should be first adjusted to a moderate value and the picture stabilized by adjusting control.
Should insufficient sync then be had, as evidenced by inability to "stop" the picture, then the triggering control may be advanced slightly.
It should be remembered that, for a single cycle of reproduction, the oscilloscope sweep-repetition rate must be equal in frequency to that of the waveform being observed. For two cycles of reproduction, it must be one-half. Most waveforms presented in service manuals and in manufacturers' literature show two cycles of the signal.
Necessary Characteristics--The characteristics of a test oscilloscope must of necessity be considered prior to its use for alignment and troubleshooting of a television receiver. Some of these characteristics will now be discussed.
Frequency Response: Generally commercial oscilloscopes, de pending upon the particular design, have a low-frequency limit somewhere between 10 and 100 hertz and a high-frequency limit of between 100,000 hertz and 5 MHz. If the vertical-interval test signal (VITS) is to be displayed, triggered sweep is required in addition to 4.5-MHz response. (See Fig. 16-2.) It is sometimes erroneously contended that since video frequencies in excess of 4 MHz are encountered, the oscilloscope must be responsive to this limit. Stress is placed upon the wide passband of some instruments in promotion literature. In laboratory usage, there are instruments extending as high as 60 MHz that are advantageous in specialized applications. At the same time, there are instruments giving practical satisfactory performance with response extending only to 15 MHz.
It should be realized that in general testing and servicing of monochrome receivers, it is of no advantage to observe the higher-frequency component of the video signal. These are the impulses that reproduce the fine detail of the image and that can only be interpreted from the face of the picture tube itself; their presence in the oscilloscope reproduction of the video signal means nothing.
Fig. 16-2. Display of front porch, sync lip, back porch, and burst.
Secondly, limited high-frequency response will cause a rounding-off of the sync pulses; therefore the question is primarily how high it will be necessary to go in order to satisfactorily reproduce them. In design and laboratory applications, it is often necessary that exactly true visualization of the wavefront be obtained, and that harmonics in the order of 120 or more be present. Receiver testing, however, permits some distortion and, furthermore, the high-frequency response of the sync circuits themselves are quite law. A typical circuit may use a plate-load resistor of 68,000 ohms and have a lumped capacity to ground of 50 µF, which gives a high-frequency cutoff of:
f 2 pi RC 6.28 x 68,000 x 50 x 10 ,2 - 46, 800 hertz
From this it follows that it would be of no particular advantage to use an oscilloscope with a frequency range extending to 5 MHz in such an application.
High-frequency response in the vertical-deflection amplifier might well be, for general testing and servicing, essentially flat to 100 kHz. Beyond this, the added cost is not justified by increased utility.
In the servicing of color receivers, it is sometimes necessary to observe signals at the color subcarrier frequency of 3.58 MHz. An oscilloscope response of 15 MHz is necessary.
Low-frequency response is often neglected in considering the oscilloscope, and yet it may be such that distortion is introduced.
With poor low-frequency response resulting in a large amount of distortion, it is naturally impossible to judge the circuit being tested, and therefore the reproduced pulse values are of no value.
Input Cable: The input cable should always be of as low capacity as possible. Ratings are in a given number of microfarads per foot of length. The impedance of the cable is of no consequence when working at the signal frequencies encountered. It is to be anticipated that high impedance and low capacity are inseparable; but in any given cable of 100-ohms impedance, various capacities are represented. The choice of a cable should there fore be on the basis of capacity only and should not be excessively long.
Oscilloscope Loading: Quite often, it is desirable to expand the horizontal sweep so that it extends far beyond the limits of the screen. This calls for an amplifier capable of developing sweep potentials greatly in excess of that necessary to the 3 to 5 in. of active screen. Then there is the likelihood of overload and compression of the sweep at either or both ends. This may or may not be apparent by mere observation of the trace, but it may be determined definitely by placing a signal upon the vertical plates.
(For a simple test, this signal could be simply placing a finger on the vertical-input terminal.) The horizontal amplifier should be such that this overloading condition is not apparent with horizontal sweep at its maximum and the horizontal positioning control rotated to extreme limits.
Testing the Oscilloscope: If it is desired to test the vertical response of an instrument prior to purchase or to determine its true operation in application, this may be done readily by applying to the vertical input the output of a variable-frequency, square-wave generator. Provided that the generator output is a true square wave, any departure there from on the screen is indicative of faulty operation. Frequencies should be between 30 and 15,000 hertz.
Should there be no distortion whatsoever, the vertical amplifier response is acceptable. By distortion is meant that the reproduction should not depart from the square wave; that is, the leading and lagging edges should be fairly straight, the corners square, and the baseline straight.
Other Features: Other features of the oscilloscope are more or less optional. It may have a 3- or 5-in, screen, although those with 3-in, screens are usually somewhat limited in other necessary requisites due to economic considerations.
Some instruments have provisions for internal sync at 60 hertz and an associated phasing device permitting the pattern to move along the frequency or time axis. These may be provided readily, however, by external devices of simple construction. It is immaterial whether the tube deflection plates are brought out to external connections since they have little application in receiver testing.
Also, the so-called Z axis is of little consequence in television testing work. This is a connection, either directly or through an amplifier, to the control grid of the oscilloscope tube. Its function is to cut off the beam at some desired time; for example, one of the two traces obtained from sweep-generator operation may be eliminated by the application of a negative potential to this grid during the time of one of the traces. Most sweep generators now provide this blanking function internally.
After becoming thoroughly familiar with the operation of a conventional scope, the technician may wish to investigate the facilities provided by a triggered-sweep scope, shown in Fig. 16-3. For example, an expert operator can use this type of scope to pick a color burst out of a waveform and expand the burst display on the scope screen. This is a useful procedure when checking out a color-bar generator, for example. However, it must be emphasized that the most elaborate scope will be of little use to a technician who lacks an understanding of circuit action and waveform analysis. The ability to interpret waveform distortions can be gained only by persistent study and experience.
Fig. 16-3. Tektronix 2213 dual-trace, delayed-triggering oscilloscope.
Fig. 16-4. B&K model 415 sweep/marker generator.
Sweep Generator
In television testing and alignment, the requisites of the sweep generator are far more stringent than in other types of work owing to the higher frequencies and increased sweep width required. A typical sweep generator is shown in Fig. 16-4.
Center Frequency--The requisite for center frequency is that the generator should cover at least to the highest intermediate frequency of television receivers, that is, slightly under 50 MHz.
This is a minimum requirement, and preferably it should extend also to the RF ranges of 220 MHz.
Sweep Width--The sweep excursion must be somewhat in excess of the greatest bandwidth encountered in television, which is 6 MHz. A sweep width of not less than ±4 MHz (8 MHz overall) with ±8 or 10 MHz being a more desirable range. Should the sweep excursion be too narrow, then the reproduced traces will be excessively broad and may not include the adjacent channel frequencies at which traps are sometimes placed. As a consequence, the two reproduced traces will merge into one where the sweep is very inadequate. A comparatively elaborate sweep generator is shown in Fig. 16-5.
Fig. 16-5. Lab-type sweep generator.
Signal Generator
If the sweep generator does not incorporate internal marker circuits, it will be necessary to employ a separate signal generator (without modulation) as a means of identifying frequencies within the reproduced trace. As such, its calibration must be quite accurate. In fact, it is recommended that a crystal calibrator be used frequently since the calibration of most service-type signal generators changes from time to time. A crystal controlled calibrator is shown in Fig. 16-6.
Fig. 16-6. Sencore video analyzer.
In television servicing the frequency range of the generator should be great enough to include all television channels so that the generator can be used to supply test signals at the picture and sound-carrier frequency for each television channel on which receivers will have to be adjusted. The commercial FM broad cast ban of 88 to 108 MHz should also be included. If the frequency range of the generator is restricted, it may be possible to use harmonics of the output signal for the higher frequencies.
Actually, many signal generators use harmonics of a lower range to provide frequencies in the highest range.
Signal tracing through the receiver circuit may be carried out with this generator as in conventional receivers, working back toward the antenna, stage by stage, to localize a defective stage.
When employing the 400-hertz output in the audio and video stages and the modulated RF signal in the IF and RF stages, the indication of normal operation is a steady tone from the loud speaker in the sound channel while horizontal bars are produced on the screen of the picture tube for signals in the picture channel.
The audio test signal is also useful when non-image methods of testing scanning linearity must be employed to produce horizontal bars in the picture for checking vertical linearity. A test-signal frequency of about 157.5 hertz can be used to check horizontal linearity.
Aside from frequency range and stability of calibration, there are two other points to be considered: First the output should be about 1 volt at a maximum, and attenuation down to almost zero, or about 1 µV, should be provided. Leakage is checked by connection to a sensitive receiver operating at full gain and then reducing the generator attenuator. The signal should then reduce to an imperceptible level. Second, the oscillator should be quite stable when used as a marker. Any factor that leads to instability of the oscillator within the signal generator leads to unsatisfactory results.
Marker Systems--Specifically, a marker system consists of an accurately calibrated signal source, which can be internal or external to the sweep generator. This calibrated source may take the form of a crystal oscillator of various frequencies, which can be switched in and out, permitting the calibration of the curve; or it can be a continuously variable accurately calibrated signal generator.
There are numerous marker generators on the market. When properly designed, it is a precision instrument, the function of which is to produce highly accurate marker pips to show specific frequency locations on a tuned-circuit-response curve on the oscilloscope.
The need for an accurately calibrated marker system will be seen by the fact that any response waveform, as reproduced on the oscilloscope screen, is only approximately calibrated. The center of such a trace represents the center frequency at which the sweep generator has been set. Without a marker system, extremities of the sweep can be approximated only by adding and subtracting from the center frequency the maximum frequency deviation for which the sweep generator has been adjusted.
Fig. 16-7. Beckman 310 digital voltmeter.
MISCELLANEOUS TEST EQUIPMENT
Following are additional items necessary for television-receiver maintenance and troubleshooting:
Tube Tester--The tube tester should be preferably of the dynamic type, checking mutual conductance (Gm) of all types used, including the miniature types.
Digital Voltmeter--A digital voltmeter (DVM), reading volts, ohms, milliamperes, etc., should be of the high-impedance type (Fig. 16-7)-20,000 ohms/volt or better-and may be used also in reading high voltage by applying a multiplier. There are available high-voltage cables and test prods that have built-in multipliers to extend the range of the meter to any reasonable value. (See Fig. 16-8.)
Fig. 16-8. High-voltage probe.
CRT Testers--There are many CRT testers on the market.
These give a simple "good-bad" indication based on amount of cathode emission. Many of these instruments also provide a means of "rejuvenating" the CRT to extend its useful life.
Monochrome Picture Tubes--Monochrome picture tubes have acceleration potentials approaching 16,000 volts, which should be the minimum requirement in high-voltage meters. Color receivers will use up to 30 kV. It must be borne in mind that even 50-µA drain (at full scale in the meter) may drop the high voltage appreciably. This may be ascertained by observing change in picture size. If the picture materially increases in size as the reading is being taken, some allowance must be made for reduction under test. The only true measure, in such a case, is by an electrostatic de voltmeter, an instrument usually restricted to the laboratory due to cost factors.
Variable-Voltage Transformer--A variable-voltage transformer is necessary for proper voltage control. Picture size and brilliance are dependent upon line voltage, and changes are more apparent in some receivers than in others. Also, it has been noted that the line voltage in most service areas is variable over wide limits. Therefore, in adjusting the receiver, there should be some means of simulating the actual operating conditions encountered in the field. The transformer should be at least of 5 ampere size and should be the isolating type with no connection between primary and secondary. The use of an isolating transformer eliminates the shock hazard during servicing of ac-dc chassis, which have one side of the line connected directly to the chassis.
Small Tools--Small tools, including alignment tools, are essentially the same for any type of receiver, whether AM, FM, or television.
Mirror-In servicing television receivers, a mirror is often used for observation of the picture screen while making adjustments at the rear of the chassis. To prevent breakage, this may be a metallic sheet, possibly a ferrotype plate, obtainable at any photographic supply house.
Detector Probe--In checking operation or alignment in a single IF or RF stage, detection must be had before application to the oscilloscope. The detector probe may contain a tube or crystal, as long as it does not seriously introduce capacity into the circuit.
COLOR TEST EQUIPMENT
All the aforementioned test-equipment items are applicable to the servicing of color receivers. A few special pieces of test equipment are intended specifically for color servicing.
White-Dot Generator--The white-dot generator produces white dots on the screen of the receiver to permit adjustment of CRT convergence. An alternative output is a crosshatch of white lines. These outputs are available as a video signal (to be injected into the video-amplifier circuit) or as an RF signal (to be injected into the antenna terminals). Many instruments of this type also produce color bars.
Color Bar Generator--The final check, after serving a color receiver, is to see that it reproduces colors correctly. The color bar generator produces on the screen a series of vertical bars of different colors. If the receiver reproduces these in the proper sequence (both saturation and hue), the color circuits are working properly. A typical color-bar generator is shown in Fig. 16-9.
Fig. 16-9. Digital IC color generator.
WAVEFORM AND VECTORSCOPE GRATICULES
A graticule is a ruled transparency placed over the CRT face to facilitate waveform analysis. The graticule may also provide filtering for better contrast under ambient lighting conditions. For example, scope graticules are often green transmission filters.
Some graticules are edge-lighted so that the rulings are clearly visible in low ambient lighting. Figure 16-10 shows a basic vectorscope graticule ruling. Only X and Y coordinates may be ruled on the graticule, or chroma values may be indicated along the vertical axis, as seen in Fig. 16-11.
Fig. 16-10. Graticule markings of a vectorscope.
Fig. 16-11. Chroma values displayed by a waveform monitor.
Fig. 16-12. Tektronix vectorscope.
When an NTSC color-bar signal is used, the vectorscope graticule is ruled as shown in Fig. 16-12. Phases are indicated for burst, yellow, red, magenta, blue, cyan, and green. The small circles are also spaced from the center of the graticule to show the normal relative amplitudes of the color signals. Note that the burst phase will not be indicated in a vectorgram display if the burst is blanked out prior to chroma demodulation. This depends on the design of the color-TV receiver. In some receivers, the burst passes through the chroma demodulators and is indicated in the pattern. When the burst is blanked prior to demodulation, the residue of the blanking pulse is displayed. However, this residue has an arbitrary phase and does not denote the burst phase as such.
Figure 16-13 shows a waveform monitor with an NTSC color bar signal displayed. This same display can be obtained on an oscilloscope.
SUMMARY
Test equipment necessary for alignment and effective trouble shooting the average television in the shop are the oscilloscope, sweep and signal generators, and a vacuum-tube or digital volt meter. The oscilloscope is used many times in many different operations, such as alignment, and in the sync and sweep circuits to observe various waveforms.
The frequency range of the generator should be great enough to include all television channels at the picture and sound carrier frequency. The commercial FM broadcast band of 88 to 108 MHz should also be included. White-dot and color-bar generators are instruments that must be included to service color television receivers.
Fig. 16-13. Tektronix waveform monitor.
Meters that read volts, ohms, and milliamperes should be the high-impedance type (20,000 ohms/volt) and may be used to read high voltage with the use of a multiplier. Dynamic-type tube testers should be used to check the mutual conductance of all tubes, including the miniature type.
QUIZ
1. What two instruments are necessary to align a television receiver?
2. What are the four basic instruments needed for trouble shooting and the alignment procedure?
3. What are the low- and high-frequency responses of commercial oscilloscopes?
4. What should the frequency range of a signal generator be?
5. What are the requirements for a good VTVM? A good DVM?