The circuitry of auto radios differs from that of other types of radios in several ways. The most important differences, from the standpoint of troubleshooting, are in the power sup ply and in the audio output stages when transistors are used.
VIBRATOR-TYPE POWER SUPPLIES
The source of power in automobiles is usually 6 or 12 volts DC. Thus, it is necessary to convert this low DC voltage into AC in order to step it up to the relatively high voltage required for the receiver. After it has been stepped up to about 600 volts, the AC is rectified, producing about 250 to 300 volts DC. A typical vibrator circuit for converting the low DC voltage to AC is shown in Fig. 11-1. The vibrator consists of a spring loaded reed which oscillates between two contacts like a buzzer.
The action of this reed reverses the DC current through the primary of the transformer with each cycle of the vibration, as illustrated in Figs. 11-2A and 11-2B. A study of these figures will reveal how AC can be generated in the secondary by the switching action of the vibrator.
Fig. 11-1. A typical auto-radio vibrator circuit.
In Fig. 11-2A, the full 12 volts of battery voltage is placed across the top half of the transformer secondary, with current flowing in the direction indicated. In Fig. 11-2B, the battery voltage is placed across the bottom half of the secondary.
The rectifier and filter circuit is the same full-wave circuit we have already studied. It is interesting to note, however, that the ripple frequency is about 300 cycles instead of the 60 cycles used in earlier discussions of power supplies.
Buffer Capacitor C16 is a very important part of the circuit which frequently causes breakdowns leading to burned-out vibrators and even burned-out power transformers in auto radios. It is called the buffer capacitor, and its purpose is to reduce the sparking at the vibrator contacts. The capacitor is connected in the secondary circuit because its reactance is reflected into the primary in proportion to the square of the turns ratio of the trans former. Thus, with a 1:100 step-up from primary to secondary, the reflected reactance of C16 will appear as only 1/10,000 of its original value. In this manner, a much smaller capacitor can be used across the secondary than would be required to produce the same low reactance if it were connected directly across the vibrator contacts.
The value of the buffer capacitor is critical and depends on the characteristics of the transformer. It is good practice to replace this capacitor on every repair job because it fails frequently, and such failure may result in considerable damage.
But the technician must be aware of the extra high breakdown voltage required for this unit. Replacements should have at least a 1600V rating.
As with any inductive circuit, the inductance of the trans former tends to keep current flowing in the same direction after the switch contacts in the vibrator have opened, and this gives rise to a large spark across the vibrator contacts. The voltage that appears across the points is made even larger by the effect of the square wave which appears in the inductance when the points are opened suddenly. For example, in Fig. 11-2A, when the vibrator points open suddenly, the magnetic field around the top part of the primary will collapse. In so doing, there will be a large current generated in the top part of the circuit. R18 provides a path for this current, completing the circuit around the top loop when the vibrator points are open. R19 serves the same purpose on the other half of the vibrator cycle. These two resistors are often overheated in normal use and should be replaced as a precaution whenever an auto radio is disassembled, even though the symptom may have nothing to do with the vibrator circuit.
The two coils, L6 and L7, and capacitors C17 and C18 (Fig. 11-1) are parts of a filter network to prevent pulses from the automobile ignition system from entering the receiver.
The unit marked M5 is an unusual type of capacitor which is found only in auto radios. It consists of a metal plate, usually copper, mounted flat against the radio chassis, but insulated from it by a thin piece of mica. By using this construction, it is possible to form a small but very efficient capacitor, which is essential in the reduction of noise pulses from the engine.
This capacitor seldom fails, but it is mentioned here because it may appear that the input lead from the battery is soldered directly to the chassis, since the mica insulation is practically invisible.
Fig. 11-2. Simplified versions of Fig. 11-1.
Cold-Cathode Rectifiers
Fig. 11-3. A cold-cathode rectifier circuit.
Fig. 11-3 shows another vibrator power supply which uses a capacitor, C16, across the primary, in addition to the regular buffer in the secondary. Another difference will be noted in the type of rectifier tube used. At one time, nearly all auto radios used this cold-cathode type of rectifier that does not require filament voltage. The tube is filled with argon gas, which ionizes when sufficient voltage is placed across the elements. Hence the name, cold cathode.
Failure of the OZ4 is perhaps the most common breakdown in this type of auto radio. The tube fails in mysterious ways that confuse the technician who is not familiar with its peculiarities. Often, when the voltage across the secondary of the transformer is slightly low, or the tube is aged, it will fail to ionize immediately. The operator will find that by switching the receiver on and off a few times it will suddenly come to life and operate normally until the next time the receiver is al lowed to cool. This failure of the OZ4 is more troublesome in the winter or in cases where the receiver is not used often.
At other times, ionization of the OZ4 may be intermittent, so that the receiver operates in short bursts. The effect is very confusing if one is not familiar with this type of trouble, be cause speech, for example, is broken into successive short segments hardly more than a word or two in length.
Checking the OZ4 in a tube checker does not seem to be reliable. Good tubes occasionally test bad, and tubes which will not ionize properly in the receiver may check good when operated with the light loads in the tube checker.
Fig. 11-3 has some other interesting features. The use of the double-pole on-off switch makes it possible for the radio dial light to be connected to the dash-light switch in the auto. Thus, the radio dial light does not come on when the radio is operated during the day, when the dash lights are off. At night, how ever, when the dash lights are on, the dial light will glow if the radio is turned on.
This switch frequently fails in auto receivers which use a vibrator-type supply, because of the large current drawn and the inductive spark present when the switch is opened. When it is difficult to obtain an exact replacement for a double-pole switch, a single-pole unit can be used by connecting the dial light lead to the "A" battery lead. This makes the dial light come on whenever the radio is turned on, even though the dash lights may be off, but this does not seem to be a severe disadvantage. A high-quality switch should always be used because of the heavy current to which it will be subjected and to avoid repeating this time-consuming job in the near future.
Special Features of Auto-Radio Circuits
There are several differences in the circuitry of auto radios which should be mentioned. Fig. 11-4 is a typical example of the older models which used vacuum tubes in the audio output and had vibrator power supplies.
Tuning is accomplished by moving the metal cores of the RF, mixer, and oscillator coils. This method is often used in mobile receivers because the movable cores can be made less susceptible to vibration. Each coil has trimmers and padders for tracking adjustments.
Auto receivers always include an RF amplifier, which is necessary to improve the signal-to-noise ratio. The circuit shown uses a 12BA6 RF stage which is coupled to the mixer through the combination of C4, L3, and C5. Leakage of B+ through C4 presents a symptom that is peculiar to auto radios of this type and does not occur in home radios.
Fig. 11-4
The circuit of V 4, the 12BF6 detector, AVC, and AF amplifier, has several features which are different from the familiar circuit used in home-type receivers. Besides the tone control, the circuit includes delayed AVC. The AVC diode plate (pin 5 of the 12BF6) is capacitively coupled to the plate of the IF amplifier, so that the IF carrier voltage developed in the primary of the transformer appears at this plate. The schematic shows that the cathode of the diode is held at 8.5 volts positive by the voltage drops across resistors R13 and R12. Therefore, the diode does not conduct and no AVC is produced until the carrier voltage exceeds the 8.5-volt cathode bias.
When the carrier strength is great enough to overcome this bias, the plate of the diode is driven positive with respect to the cathode. Electrons flow to the plate and through R10 to ground, producing a negative voltage at the top of R10 that is in proportion to the signal strength.
It is important to note that the IF frequency used in many auto radios is 262 khz and not 455 khz, as in home receivers.
AUTOMATIC TUNING SIGNAL-SEEKING CIRCUITS
An older-type circuit for automatic tuning is shown in Fig. 11-5. Each section of the circuit will be discussed separately.
The Trigger Circuit
Notice that the two triodes are directly coupled-that is, no coupling capacitor is used between the grid of the second tube, V7B, and the plate of the first, V7 A. This means that the grid voltage on the second tube must always be the same value as the plate voltage on the first tube. To maintain proper grid to-cathode voltage on V7B, its cathode is connected to B+ through R30. With R24 connected to ground, a voltage divider is formed which holds the cathode of V7B at 145 volts positive.
The grid is at 135 volts positive, giving a net negative bias from grid to cathode of -10 volts.
Not only does the relay coil, M8, operate the switch contacts, but the lever also controls the locking mechanism on the spring loaded dial-drive apparatus. When the relay is not energized, the lever remains in the up position, as shown on the drawing, and keeps the dial locked in place. When V7B conducts through the relay, the lever pulls down, releasing the dial lock and also changing some important electrical connections.
With the lever in the up position, as it is when a station is locked in, the cathode of V7A is held at +145 volts because it is returned through R29 to the voltage divider as previously described. With the grid practically at zero volts this tube does not conduct; only a slight leakage current of 0.08 ma flows in R22 and R25. (See Fig. 11-6.)
The Operating Cycle
Fig. 11-5. An automatic-tuning circuit.
When the start switch, shown in Fig. 11-6, is closed momentarily, a large current flows through R23 and through the solenoid, pulling the lever down. This causes several changes in the circuitry:
1. The lock on the tuning gear train is released, and the dial begins to move due to the spring loading.
2. The cathode voltages of V7 A and V7B are changed be-cause R28 is now in parallel with R29 and R24, making a relatively low-resistance path to ground. Both cathode voltages drop to about + 10 volts, but only V7B conducts, because its grid is the one with the high positive voltage.
There is still enough bias on V7 A to keep its plate cur rent very small.
3. The other section of the start switch shorts out the audio signal, temporarily muting the receiver so that noises are reduced. By the time the switch is released, the receiver is supposed to be between stations and not much noise should be present. Later models have the muting switch a part of the relay contacts so that the receiver remains silent during the entire search.
4. When the switch is released, V7B will continue to con duct, holding the relay lever down until there is a change in the grid voltage of V7B. This grid voltage will not change until V7 A conducts heavily and its plate current lowers the plate voltage. This will not occur until a positive-going pulse appears at pin 2. This voltage will be the result of changes in the A VC voltage when the next station is reached. These changes are somewhat complex and are illustrated in Fig. 11-7.
Fig. 11-6. Simplified version of Fig. 11-5.
Fig. 11-7. Simplified version of Fig. 11-5.
Addition of AVC and Trigger Voltage
It is important to note that two signals are used to trigger the grid of V7 A. When the tuner reaches a station, the IF carrier appearing at C9 is rectified, and Ii, the negative half of the RF cycle, flows through R12 and R13. But on the positive half of the RF carrier voltage, electrons flow through the A VC diode as noted on the schematic, and no voltage is developed across R12 and R13 by this current. Negative voltage for the AVC is taken from the tap between these resistors so that only half of the total rectified carrier is actually used for A VC. At the same time, another signal is taken from the secondary of L6 and applied to the AVC line through C1 7. This is RF voltage, not rectified, and is larger in amplitude since none of it is lost across R12. The tuning of the detector input trans former, L6, is peaked more sharply on the secondary side so that this voltage rises to its maximum only when the carrier is exactly in the middle of the bandpass. In contrast, the AVC voltage taken from the primary increases gradually. It begins as soon as the carrier appears at the edge of the IF bandpass and reaches a broad, flat maximum when it is tuned in the center. It then gradually decreases as the carrier passes out of the bandpass on the other edge.
It is because of the broad response of the AVC that it cannot be used alone to operate the signal seeker. Negative bias appears before the incoming carrier is centered in the IF band- pass, and this would cause the seeker to stop the tuning action before the station was correctly tuned in. This effect would be worse on the stronger local stations because a greater bias would be developed at the bandpass edges.
To prevent this difficulty, the seeker is designed so that it requires a positive pulse to stop the searching action. In this manner, negative AVC voltage is used as a bucking voltage to prevent triggering on strong stations before they are properly tuned in. At the bandpass edge, the negative A VC voltage keeps the trigger stage cut off, and the trigger does not operate until the AC signal from the secondary is applied. The AC will not appear until the carrier is centered in the IF channel of the receiver. Thus, the AVC automatically adjusts the DC level from which the triggering AC signal will start. The stronger the incoming carrier, the more negative will be the starting point.
The combination of the AC from the secondary of L6 and the negative AVC at the grid of V7A is shown in Fig. 11-7.
The upward excursions of the resultant waveform cause the grid to become less negative and eventually bring the tube out of cutoff. The AC signal is thus detected in the plate of the tube, and the average plate current increases. With no AC applied to the grid, there is no plate current, and plate voltage is maximum (135 volts). Any plate current reduces the plate voltage and permits C18 (shown in Figs. 11-5 and 11-6) to discharge, lowering the grid voltage of V7B. When the plate current of V7B is reduced, the relay lever is released, stopping the searching action and increasing the cathode voltages again so that both tubes are cut off. Once this state is reached, no signal on either grid can start the searching action; it must be started manually by closing switch M6.
In the cathode of the RF amplifier, shown in Figs. 11-5 and 11-7, is a pair of switch connections used to control the sensitivity of the signal seeker during the search so that it can be adjusted to stop only on strong local stations or on all stations.
Depending on how the operator sets the switch, either R31 or R32 is inserted between the cathode and ground when the relay is energized. When the tuner reaches the next station and the relay falls out, the cathode is grounded directly, increasing the sensitivity to maximum.
12-VOLT HYBRID RECEIVERS
The term "hybrid" means that both transistors and tubes are used in the same receiver. The last of the vibrator-powered receivers were made about 1956; after that, most automobiles were supplied with 12-volt batteries. Special tubes, like the ones in Fig. 11-8, which use plate voltages in the vicinity of 12 volts, were designed for use in these cars. Because of the large plate current which is necessary to develop the required audio power at 12 volts, a transistor is used in the audio output stage. In a few models, a transistor driver stage is used to drive a pair of output transistors in push-pull.
The circuitry of a hybrid model using a single DS501 transistor in the output is shown in Fig. 11-8, where the relatively simple power supply can be seen. Some filtering is still needed to remove ignition noise and to provide decoupling.
Newer Versions of the Trigger Circuit
The trigger circuit shown in Fig. 11-8 uses two stages that are AC-coupled through C20. The circuit discussed earlier in Fig. 11-5 used DC coupling. When the start switch is closed momentarily, current through the relay coil closes switch M5, connecting the trigger stages to B+. Unlike the early version, the cathodes of the 12AL8 are permanently grounded, and so a heavy current flows in the tetrode section of the tube and through the relay coil. This current holds the plate switch closed during the searching and provides a cathode voltage of 2.5 volts for both cathodes, because of the drop across R27.
With only -0.11 volt on its grid, the triode section of the 12AL8 is practically cut off by the cathode voltage. Accordingly, its plate voltage is high. The tetrode section will continue to conduct and hold the relay in until the plate voltage of the triode section drops, which will occur as soon as a positive pulse appears at the grid of the triode.
AVC taken from the primary of the IF transformer and rectified at pin 1 of the 12DV8 tube is combined with the secondary voltage to produce the stopping pulse in a manner similar to the earlier models. A VC delay is accomplished by returning the cathode of the 12DV8 to ground through R27.
When a station carrier is in the center of the IF bandpass, AC appears at pin 8 of the trigger amplifier. This tube con ducts and its plate voltage drops to a low value because of the large resistance, R26. The negative-going pulse thus developed appears at pin 2 and reduces the plate current through the relay coil, causing M5 to open. This opens the plate and screen connections in the trigger section, and all conduction stops in both tubes. Once this state is reached, a signal on the grid cannot start the search again-it must be started manually by closing M6 momentarily.
Fig. 11-8. A hybrid auto radio.
The sensitivity switch is shown in the cathode of the RF amplifier, where a pair of relay contacts open the cathode connection to ground during search. This leaves the only re turn for the cathode through the route selected at Ml, and sensitivity is reduced during search by the resistors which can be put in series with the cathode lead. The receiver then responds only to strong stations. This is exactly the same method as used in the earlier models.
Improved Muting It will be noted that when the relay is in the search position, the sensitivity contacts of M5 are used to ground the top of the volume control, thus muting the receiver during search.
This is an improvement over the old method because it pro vides continuous muting until a station is tuned in.
Transistorized Trigger Circuits
Fig. 11-9 shows a transistorized version of a trigger circuit used in an all-transistor receiver. NPN transistors are used throughout. This means that a negative base with respect to emitter will cause cutoff and a positive base will cause conduction, with the electrons moving through the emitter and out the collector.
Fig. 11-9. A transistorized trigger circuit.
Two voltages are applied through C1 and C2 to the base and emitter of the DS47 trigger amplifier. One of these voltages is taken from the primary of the last IF transformer and rectified to produce current I1 when a signal is present. The other voltage is taken from the secondary and is not rectified. This is similar to the method used in the vacuum-tube models to obtain a bucking voltage, except that the AVC voltage is applied to the emitter and the RF signal is applied to the base.
The search is started by momentarily closing the start switch, which causes a heavy current to flow through the relay, pulling the relay arm down and releasing the gear train. The DS47 is biased near cutoff by bleeder current through R1 and R2, and the additional current drawn by the DS46 output transistor drives it farther into cutoff by increasing the positive voltage on the emitter.
With the first stage cut off, there is no drop across R3, and the second stage has maximum positive voltage on its base (current I2 is not present). In this condition, it conducts heavily, producing a positive voltage at the top of R5. This voltage appears at the base of the output stage to keep it con ducting after the start switch is opened.
When the tuner reaches a station, the signal from the transformer primary is rectified and current I_1 flows. This increases the positive potential at the top of R4 slightly and prevents the DS47 from coming into conduction immediately when the signal through C2 reaches the base.
This RF voltage across C2 from the secondary comes to a sharp peak when the station is properly centered in the IF bandpass. This results in sufficient signal on the base of the DS47 to cause it to conduct. The resulting conduction on the positive halves of the signal causes current I2 to flow in R3, lowering the positive base voltage on the second stage. With the conduction of the second stage so reduced, there is less positive voltage on its emitter resistor, and the base of the output stage becomes less positive.
A slight drop in the current through the output stage permits the relay spring to return the lever to the lock position, stopping the gear train at the precise instant when the station is centered in the bandpass. This decrease in current also opens S_2, which kills the first two stages and leaves zero base voltage on the output stage.
The circuit is serviced by following the same general rules established for the tube versions but considering the delicate balance of small voltages on which operation depends. Leak age, bias-network failures, and effects of heat must be taken into consideration here, whereas these considerations are not important with the vacuum-tube counterpart of this circuit.
TRANSISTOR OUTPUT STAGES
In auto radios which use only the 12 volts DC supplied by the car battery as a power supply, the audio output stage must be transistorized. Since the radio must deliver about 10 watts of power, about 1 amp of current is required at 12 volts-a vacuum tube capable of such current would be very large and difficult to mount in an auto radio. Transistors capable of 1 amp on peaks at 12 volts are very convenient in size and can be easily mounted on the chassis.
A heat sink is used for mounting the transistor, and some extra circuitry is necessary for biasing and overload protection. Power transistors must be protected from thermal run away, a condition which results from an increase in collector current with heat. As the transistor gradually heats, the leak age current increases, producing more heat. This, in turn, produces more leakage. Finally, the collector current becomes excessive, and the transistor is ruined.
Overload protection is usually provided in the form of a 0.47-ohm fusible resistor (R25 in Fig. 11-8 and R32 in 11-10). This combination resistor-fuse is often used in auto-radio power stages, and its value is critical. In most repair jobs which require servicing for the No Signal symptom, this resistor will be open, indicating that the transistor has been operating at excessive current loads. The resistor must be replaced with one of the exact value, and the transistor must be checked for leakage as described in Section 9-6.
Even when a power transistor seems to check OK, it should still be regarded with suspicion because there is no way to predict its behavior when it is heated. But some other components in the circuit could also cause the fusible resistor to burn out. In Fig. 11-10 these are:
1. Collector bypass capacitor C1.
2. Emitter bypass capacitor C23.
3. Base bypass capacitor C22.
4. Input transformer T1.
5. Base resistor R31.
Fig. 11-10. Transistor output stage of an auto radio.
If none of these seem to be the cause of too much cur rent through R32, then the transistor must be replaced, regardless of its apparently healthy condition. The new transistor must be installed using silicon grease between the shell and the heat sink. Care must be taken to replace all gaskets or insulators used on the heat sink. The outer shell, which is the collector of the power transistor, is tightened very securely to the heat sink, which is then insulated from the chassis to avoid shorting the collector to the chassis.
When a transistor has been replaced, it will be necessary to readjust the bias potentiometer, R3 in Fig. 11-10 (or R2 in Fig. 11-8). The instructions given on the schematic for this adjustment assume that the input voltage is exactly as called for and that the correct speaker is connected. Always allow the transistor to warm up for about 10 minutes before making this adjustment.
Fig. 11-11. Fig. 11-10 redrawn in the proper form for calculations.
One more precaution should be mentioned in connection with high-power transistor output stages. That is the importance of keeping the proper load on the collector at all time.
This means that the receiver should not be operated without a speaker connected, and that a speaker having a resistance very close to the original should always be used. These stages are operated near their maximum capabilities, and the values of components are critical.
Current and Voltage Analysis of Transistor Output Stage
When the output circuit of Fig. 11-10 is analyzed by Ohm's law, the critical values of currents and voltages present can be seen. In Fig. 11-11 the circuit is redrawn in the standard form which is always used for calculations. The battery volt age is shown as 11.8 volts because this is the value to which the base is actually returned in the original circuit.
In the original schematic, the voltages given at the base and emitter terminals are the voltages measured from these points to ground. In Fig. 11-11 the ground symbol is shown at the top and is connected to the negative terminal of the battery, and these two voltages are shown from the base and emitter terminals to ground. The true emitter voltage is the voltage from the emitter to the positive side of the battery; it turns out to be 0.6 volt, after subtracting 11.2 from 11.8.
The true base voltage is found in the same way, and it can be seen on Fig. 11-11 as 0.8 volt. The forward base bias is the difference between the emitter and base voltages:
V be = 0.2V.
The base current, l_b, flows through the secondary of the input transformer, T1, and since its value is not given, the resistance of this winding is assumed to be 1 ohm.
If a reasonable value of f3 is assumed, we can calculate the emitter, base and collector currents as follows: l_e = _ 4 6 7 = 1.28 amp or 1280 ma
l_e= le+ lb Since f3 = ~:, then le = h /3
Substituting, l_e= lb /3 + lb le = lb (/3 + 1)
Assuming f3 = 29, (/3 + 1) = 30 I le b - (/3 + 1)
1280 lb = 30
= 42.6 ma
le= le - lb= 1237.4 ma
The values in the biasing network can be found easily.
Assume To find R3, RT1 = 1 ohm ET1 = (lb) (RT1)
= 0.04V E 1 = 11 - 0.04 = 10.96V E2 = 11.8 - E1
= 11.8 - 10.96
= 0.84V lb + IDiv = IR3 and R30 = 42.6 ma + 84 ma = 126.6 ma
The sum of (R3 + R30) = 1
:I b DIV 10.96 126.6 ma = 86 ohms
R3 = 86- R30 = 86 - 50 = 36 ohms
The stability factor, S1, of a circuit is the ratio of the change in collector current to the change in leakage, lcb (see Section 8, Fig. 8-4): Since, A l_e = (S1) (A lcb), it is clear that S1 represents the factor by which A l,.b will be amplified in the collector current.
Thus, the lowest value of S1 is most desirable.
A very close approximation to the stability factor in a common-emitter circuit which has good f3 is, R1 R2 S1 = ( R 1 + R2) R. where R1 and R2 are the resistors in the base bias voltage divider.
Using the values R1 = 86 (that is, R3 + R30); R2 = 10 ohms (that is, R31); and R. = 0.47 ohm (that is, R32), (86) (10)
S1= (86 + 10) (.47) = 19
So it can be seen that a change in leakage current will appear 19 times greater in the collector current. With germanium transistors, the leakage doubles for each 10°C of temperature increase.
If the temperature rises 50°C during operation, the leakage current will double itself five times, and the resulting change in collector current can be calculated as follows: Assuming a minimum leakage of l_cb = 0.5 ma, Leakage after 50° rise in temp= 0.5 x 25
= 0.5 x 64 = 32 ma with S1 = 19, A le= (S1) (A lcb)
= (19) (32 ma)
= 608ma
This means that after the temperature of the transistor has increased 50°C the collector current will be 608 ma higher than normal for the same set of operating voltages. At 10 volts Vee, this is an increase of 6 watts dissipation and could easily exceed the limits for which the transistor was designed.
The importance of adjusting R3, the bias pot, for the proper operating point can be easily seen. Too much bias will increase the temperature, with resulting changes in the collector current.
The importance of R32, the emitter resistor, can also be seen now. The stability is directly dependent on this resistance.
If the resistor were made 0.75 ohm instead of 0.47 ohm, the stability factor would be 12 instead of 19, and this would make ~ le in the above example 32 x 12 = 384 ma instead of a 608-ma increase in collector current with a 50°C rise in temperature.
Increasing the size of R32 would, of course, call for corresponding changes in the other resistors in the bias network in order to maintain the 0.2 volt Vbe which is required. We have neglected these changes, but when they are made, the improvement in stability is even more pronounced.*
Negative Feedback Through C2
In Fig. 11-8, a 1000-mfd capacitor, labeled C2, is connected between the emitter and the bottom end of the T1 secondary.
The purpose of this component is to provide an AC voltage bucking against the input voltage. The negative feedback so produced reduces the current in the transformer secondary and, in this manner, increases the input resistance to the stage.
Higher input resistance means more stable operation, less loading of the driver stage, and better gain.
Since C2 is connected to the emitter, the AC voltage through it is in phase with the incoming signal and opposes the base voltage. Since the emitter voltage is always .2 volt less than the base, C2 reduces the AC current in the secondary by keeping the potential nearly equal at both ends. If C2 is shorted, the base and emitter will be effectively shorted, and the for ward bias will disappear, rendering the stage with little or no collector current. If C2 is open, the input resistance of the stage will be lowered, and heavy loading of the preceding stage will occur, producing a loss of volume and possibly some distortion.
The stability formula we have been using is actually a shortcut approximation. It would seem that if R32 were made still larger (10 ohms, for example), S1 would be less than 1 ; actually this can never happen, because the minimum value possible when the full formula is used is S 1 = 1.
DIRECT-COUPLED TRANSISTOR STAGES
A trend toward elimination of the driver transformer seems to be developing in auto radios. The typical direct-drive circuit shown in Fig. 11-12 is representative of this design. One of the advantages of direct coupling is that it improves frequency response, particularly at low frequencies.
The collector of NPN transistor X4 is connected to 12 volts through R26. Resistor R25 develops 1.4 volts of base voltage from the slight current flowing from ground (negative) across the base junction to the collector (positive). Voltage at the emitter depends on the value of R2. Thus, the emitter-base bias of X4 and, consequently, its collector current, is set by R2.
Fig. 11-12. Direct-coupled output stages.
Collector current for X4 flows through R26, dropping the collector voltage to 6.4 volts. The base of X5, connected directly to the X4 collector, acquires the same potential. A divider net work in the power supply sets the bias at the emitter of X5.
The voltage difference between base and emitter constitutes bias for X5. Current through R28 causes the collector voltage to stabilize at 11 volts.
The direct-coupled base of X6 attains the same 11 volts.
Since X6 is a PNP transistor, the collector is connected to ground (negative) through a low-impedance choke, LS. For ward bias causes X6 to draw collector current. Fusible resistor R29 develops a slight drop (about 0.8 volt) to prevent thermal runaway in the output transistor. Since R2 controls the bias of the entire system, it is usually set for the correct collector voltage on X6-approximately 1.6 volts.
TEST POINT 1, CHART XII TROUBLESHOOTING THE TRANSISTOR OUTPUT STAGE
11-1
Failure in the output stage is the most common trouble in hybrid receivers. CHART XIII at the end of this section gives details of the servicing procedure. The only test which has not been described in earlier sections is the use of a speaker at TEST POINT 1.
At least one lead from the input transformer secondary should be disconnected to remove the winding from the circuit, and then an ordinary 4- to 6-ohm speaker can be connected across the winding while the receiver is operated. If signals are heard from the speaker, this proves that all previous stages are working, and testing is confined to the output stage only. If no signals are heard, the testing proceeds in the manner de scribed in Sections 3 and 4.
TEST POINT 2, CHART XII BASE-TO-EMITIER VOLTAGE
11-2
Measurement of the forward bias is selected as the first step in isolating the trouble. Although several other tests could be used, this voltage is a critical one, and the base and emitter terminals can be located easily. Measurement of the collector voltage does not help much for the following reasons:
1. Even with the emitter open, there is likely to be consider able leakage current present, and a voltmeter from collector to ground might give a reading which could mistakenly be assumed to indicate that some normal collector current is flowing.
2. If the collector were open due to a burned-out output transformer, there might be some current through the meter, which would give a confusing indication.
3. If the meter shows no voltage between collector and ground, not much information has been gained, since there are numerous ways in which this could occur shorted bypass capacitor C1 in Fig. 11-10, open junction in transistor, open bias pot causing cutoff bias on the transistor, etc.
TEST POINT 3, CHART XII RESISTANCE FROM COLLECTOR TO –B
11-3
This test has been used before in Section 9. If the resistance is normal ( about 10 ohms) , the speaker and the wiring associated with speaker plugs and cables should be suspected.
TEST POINT 4, CHART XII INCORRECT BASE-EMITTER VOLTAGE
11-4
The fusible resistor R32 in Fig. 11-10 is the most probable cause for incorrect base-emitter voltage and must be replaced with exactly the correct part. Under no circumstances should a receiver be operated with a jumper across this resistor or with an incorrect value of resistance connected.
It is advisable to also replace the transistor at this time, and this will call for readjustment of the bias pot. The adjustment can be made by strictly following the directions given on the schematic. However, with certain universal replacement type transistors, it may be found that the adjustment is best done by measuring the base-to-emitter voltage and setting the pot for about 0.2 volt. Allow the transistor to warm up a bit before adjustments are made.
It is not good practice to make this adjustment of the bias pot "by ear," that is, by listening for the "best" sound from the speaker. With many circuits, an adjustment of the bias which gives the most volume exceeds the ratings of the transistor or alters the stability factor, S1, so that thermal runaway is more likely to occur.
Auto radios have special symptoms because of the transistor output circuit which is not used in other kinds of receivers, and because of failures in the signal-seeking circuits, which are also unique. Also, in older models, the vibrator power supply requires special test methods. In addition, these receivers also have all the other symptoms common to other kinds of radios, and the test procedures described in earlier sections apply to the RF, mixer, IF, detector, AVC, and audio stages of auto radios as well.
TROUBLESHOOTING THE TRIGGER CIRCUIT TEST POINT 1, CHART XIII CONTINUOUS SEEKING
11-5
This is the condition where, once started by depressing the manual starting switch, the tuner continues to recycle, tuning from one end of the broadcast band to the other, with out stopping at any stations. A rare form of this symptom occurs when the tuner begins to seek after it has been locked on a station for some time and without having the manual starting switch depressed. These troubles are usually due to dirty or defective switch or relay contacts, or a filament-to-cathode short in the trigger tube (in an older model) which could re duce the cathode voltage.
The first test, after checking the switch and tube, is accomplished by disconnecting the lead from the relay or starting switch which leads to the speaker-muting circuit, so that the speaker will not be muted during the search. Connect a VTVM to the grid of the input tube in the trigger circuit, and start the searching action by depressing the starting switch. One of the three results shown in CHART XIII should occur.
If no stations are heard in the speaker, it is probable that the receiver has a typical No-Signal condition, and reference should be made to Sections 3 and 4. One common cause of no signals in auto radios which does not occur in other kinds of receivers is an open cathode in the RF stage. Because of this common trouble, an ohmmeter check from cathode to ground is recommended before beginning the procedures for a No Signals condition.
Further Tests When Stations Are Heard but There Is No Change in the Grid Voltage
11-6
The receiver circuits are working, but the trigger volt age is not being developed. This limits the testing to a narrow area. First, the detector/ AVC tube should be checked, prefer ably by substitution. If this tube is not at fault, then each of the parts listed on the chart should be methodically removed and new ones tried.
A common cause of this condition is a failure in the output IF transformer. There may be enough signal to operate the detector, but not enough to develop a trigger voltage. Alignment of this transformer may be necessary, although it is not likely that it could become misaligned by itself. Nevertheless, it is good practice to check the alignment at this point. If the transformer does not seem to respond properly, it is probably defective and should be replaced. (The IF frequency should be noted from the schematic. It will usually be 262 khz, but there are exceptions.) One very important notation regarding the alignment of the secondary of the output IF transformer is that it must be tuned for minimum A VC voltage, and not maximum as is usually done. This fact is always noted in the alignment instructions furnished with the schematic. Proper operation of the seeker depends on accurate alignment.
Further Tests When Stations Are Heard in the Speaker and the Trigger Voltage Changes
11-7
In this case, there is good evidence that the entire receiving circuitry is working, and the failure is confined to the trigger stage itself. TEST POINT 2 calls for a voltage measurement on the plates in the trigger section. If a voltage is incorrect, the plate resistor and other components between the plate and B+ must be checked.
TEST POINT 3, CHART XIII NORMAL PLATE VOLTAGES IN THE TRIGGER
11-8
TEST POINT 3 is an ohmmeter check of the cathodes in the trigger stage. The tube which controls the relay may be conducting too heavily, due to a low-value resistor in the cathode, making it impossible to reduce the plate current enough to drop the relay contacts out. If the cathode resistor is larger than normal, it may be difficult to get a pulse on the grid of the input tube strong enough to cause conduction.
If the cathodes are found to be normal, the possibility of sticking relay contacts remains. The contacts should be cleaned and burnished.
11-9
CHART XIV indicates that the test procedure is based on the clue obtained by noting whether the speaker is muted when the starting switch is depressed. If the speaker is not muted, it is likely that the failure is in the switch itself.
When the speaker-muting takes place with operation of the starting switch, the relay solenoid should be checked first, after which the resistance of the cathodes to ground in the trigger stage should be measured.
TEST POINT 4, CHART XIV MEASURE CATHODE VOLTAGE WHILE SHORTING RELAY CONTACTS
11-10
The object of this test is to determine if the trigger output tube will conduct. In circuits like that of Fig. 11-5, shorting the relay contacts removes the positive voltage from the cathodes, and this should be observable on the voltmeter placed at the common cathode resistor. In other models, the relay contacts control the B+ to the trigger stages, as shown in Fig. 11-9. In this case, the output tube will begin to conduct when the contacts are closed, because the screen voltage is connected, resulting in a change in the cathode voltage.
Further Tests When the Cathode Voltage Changes
In the circuit of Fig. 11-8, it is likely that the entire trigger stage is functioning properly electronically, but some mechanical defect, such as jammed gears, is preventing the tuner from searching. In the older models that used a 12AU7 in a circuit like those of Figs. 11-5 and 11-6, a change in cathode voltage does not prove that the output tube is conducting properly, so the tube should be checked and its plate voltage measured. Also, the resistance from grid to ground in the output tube should be checked to determine the condition of C18.
Further Tests When the Cathode Voltage Does Not Change
In the newer-model receivers, the cathode resistor should be suspected along with the relay contacts and the main source of B+, which may be absent. In older types, the relay contacts could cause the trouble. Another common cause is R28 in Fig. 11-6.
Seeker Stops on Strongest Station Only
11-11
This condition is commonly caused by a weak tube in the RF or IF sections of the receiver, by misalignment, or even by a defective antenna on the auto.
An ohmmeter check from the cathode of the RF stage to ground will indicate the condition of the resistors in the sensitivity selecting circuit. A resistor at R31 or R32 (Fig. 11-5) which has increased in value can reduce the sensitivity to a point where only a very strong carrier could stop the searching action. The other tests mentioned in CHART XIV are very simple and need no explanation.
Seeker Stops Between Stations
11-12
This kind of trouble does not occur very often, but when it does the technician may not know where to begin testing. First, it should be ascertained that there is no mechanical jamming which stops the tuner. The entire gear train and recycle mechanism should be cleaned and oiled.
Any defect which reduces the AVC voltage but does not weaken the incoming signal will tend to increase the stopping sensitivity of the trigger because the AC applied from the transformer secondary will have no bucking voltage. The slightest incoming signal could bring the first tube into conduction.
Of all the causes listed in CHART XIV, the most difficult to isolate is a defective IF transformer. As was described in other sections, a slight leakage sometimes develops through the transformer, which puts a tiny positive voltage on the secondary. When the secondary feeds the trigger stage, this leakage can stop the searching by causing conduction of the first tube in the trigger section. In a similar manner, a leakage of the capacitor feeding the AVC voltage from the primary to the A VC rectifier will put a small positive voltage on the grid of the trigger tube and stop the tuner when no station is present.
Dirty relay contacts are another common cause of stopping between stations. If the contacts open momentarily during the search, current ceases to flow through the relay coil, and the contacts will not be pulled closed again by the magnet.
They will remain held open by the relay spring until the next time the relay is energized by the starting switch.
--- Servicing Chart XII: Hybrid Auto Radio, No Signals.
Servicing Chart XIII: Auto Radio, Automatic Tuning-Continuous Seeking.
Servicing Chart XIV: Auto Radio, Automatic Tuning-Seeking Malfunction.
QUIZ
1. Look up the schematic of a vibrator-type auto radio. Draw a schematic of the complete power supply, and describe two common failures, giving the symptoms and the procedure you would use to isolate the defect.
2. Describe two special features of auto-radio circuitry which lead to symptoms that do not occur in home radios.
3. In Fig. 11-5:
a. What is the purpose of R31 and R32?
b. What would probably happen if R29 were open?
c. Explain the purpose of C18.
4. Describe at least three ways in which the circuit of V5 in Fig. 11-8 differs from the earlier versions.
5. A hybrid receiver, like the one in Fig. 11-8, has a No Signals symptom.
a. What test could you use to determine if the transistor stage had failed, or if the trouble were elsewhere?
b. Suppose your test proves that the output stage has failed and you make the following tests :
1. Base-to-emitter voltage is -0.2V.
2. V,. is 11.8V.
3. Resistance from collector to ground is 0.5 ohm. What parts would you suspect, how can they be tested?
6. In the signal seeker shown in Fig. 11-8, describe the symptoms which could result if R19 were open.
7. The circuit of Fig. 11-5 has the symptom of continuous seeking. The following facts have been established:
a. The tube, switch, relay contacts, etc., have been found to be OK.
b. The ground connection of M6 was opened, and when the bottom end of R23 was shorted to ground the seeker action began, but stations could only be heard briefly as the tuner passed by them.
c. The voltage from pin 2 of V7 A to ground was -4V, and it jumped to -0.5V when stations were heard.
What would you do next?
8. Describe the symptoms that would result if R3 in Fig. 11-9 were open.
9. Set up in chart form a series of tests and probable results which would lead to isolation of the fault in Question 8.
10. The transistorized seeker in Fig. 11-9 seeks continuously after the start switch is closed momentarily. The following tests have been made:
a. With muting disabled, stations are heard as the tuner passes them by.
b. The voltage from the base of the input stage to ground jumps strongly positive when a station is heard.
c. The collector voltage of the first stage drops from + 10.5 volts to +5 volts when a station is heard.
d. The collector voltage of the second stage is + 11.9V and changes only slightly when a station is heard.
e. When the base and emitter of the output stage are shorted, the seeker stops immediately and will not start again until the start switch is closed.
Assuming all transistors, switches, and relay contacts to be good, what parts would you suspect, and what tests would isolate the defects you are thinking of?