By Bert Huneault, CET
The technician who is called to the scene whenever there is an electronics problem brings skills generally associated with the medical profession and criminal investigation. With sleeves rolled up, this combination healing artist and investigative expert delves into something "sick," taking its temperature with a test probe, treating it and, ultimately, making it (radio, television, VCR...) well again. Before the "patient" is discharged, however, the technician has used other, more sophisticated test equipment to find clues, relying upon theory, experience and continuously updated professional knowledge to interpret those clues that lead to open resistors, leaky capacitors, defective transistors or whatever is preying on the vulnerable electronics device.
This multiple choice quiz gives you the opportunity to pull up your own sleeves, put your basic know ledge to the test by analyzing a circuit, interpret a bunch of clues and see how many culprits you can identify correctly.
All questions refer to the accompanying schematic diagram: a simple, solid-state audio amplifier circuit of the type commonly found in radios, TV sets, tape recorders and phonographs. The answers with explanations are given at the end of the quiz. Score five points for each correct answer. No cheating now!
Note: All voltages shown on this schematic are normal dc voltages, measured in relation to common ground.
------- AUDIO AMPLIFIER CIRCUIT
-------- AUDIO AMPLIFIER CIRCUIT Note: All voltages shown on this schematic
are normal dc voltages, measured in relation to common ground.
For your convenience in answering these questions, this schematic is printed a second time.
Circuit analysis
This first section tests your knowledge of circuit operation and component functions, in preparation for the troubleshooting section that begins with question 11.
1. What is the function of capacitor C3?
A.) provides tone compensation B.) prevents ac negative feedback C.) prevents do negative feedback D.) reduces amplitude distortion
2. How much collector current flows in transistor Q1?
A.) 1mA
B.) 2mA
C.) 4.7mA
D.) 9.7mA
3. What is the purpose of C2?
A.) eliminates power supply ripple
B.) provides treble boost
C.) increases the input impedance of Q1
D.) decouples Q1 circuitry from the Vcc supply
4.
5.
6.
7.
8.
What is the function of resistor R10?
A.) stabilizes the Q2 bias B.) produces negative feedback C.) reduces amplitude distortion D.) all of the above What is the function of R3?
A.) stabilizes the Q1 base bias
B.) increases the input impedance of Q1
C.) provides bass boost
D.) increases the beta of Q1 Output transistors Q3 and Q4 are hooked up in a:
A.) OCL circuit.
B.) OTL circuit.
C.) push-pull circuit.
D.) both (B) and (C).
What is the function of C9?
A.) impedance matching B.) coupling capacitor C.) tone compensation D.) none of the above What is the function of diodes D1 and D2?
A.) bias stabilization for the output transistors
B.) audio signal rectification
C.) voltage doubler circuit
D.) none of the above
9. What is the function of the R7-R9 resistor network connected between the Q2 base and the emitter circuit of the output transistors?
A.) provides ac negative feed back to improve fidelity B.) increases power gain C.) protects the output transistors from thermal runaway D.) all of the above
10. What is the function of C6?
A.) decoupling capacitor
B.) bootstrap coupling capacitor
C.) tone compensation
D.) provides ac negative feedback 17.
Troubleshooting In all the following questions, assume that a suitable audio source, such as a record player, is plugged into input jack J1, and the volume 18.
control is turned up.
11. What symptom will most likely result from an open R2?
A.) no sound output B.) sound weaker than normal C.) distorted sound D.) squeals and whistles (oscillation) 12. Which of the following defects is most likely to damage the output transistors? A.) an open C6
B.) a shorted C7 C.) an open speaker voice coil 19.
D.) a shorted C9 13. Decoupling capacitor C2 be comes shorted. The resulting symptom is most likely:
A.) distorted sound.
B.) no sound output.
C.) squeals and whistles (oscillation).
D.) hum in the sound.
14. C8 opens. There will be:
A.) no sound.
B.) distorted sound.
C.) hum in the sound.
D.) loss of treble; bass okay.
15. R12 opens. The most likely symptom is:
A.) slightly distorted sound.
B.) reduced volume.
C.) no sound.
D.) smoke emerging from the equipment.
16. No sound comes out of the 20.
speaker. A signal tracer reveals that audio is present at the base of Q1 but not at its collector. Q1's collector volt age is + 16.4V and its emitter voltage is 0V. The most likely defect is:
A.) C3 shorted.
B.) C3 open.
C.) R3 open.
D.) R2 open.
No sound comes out of the speaker. All do voltages are normal in the audio output circuit. An oscilloscope shows no signals at the collectors of Q3 and Q4, but normal signals at the Q3 and Q4 bases.
A.) C8 is open.
B.) Transistors Q3 and Q4 are defective.
C.) R14 and R15 are open.
D.) Diodes D1 and D2 are open.
The sound is distorted, and the midpoint do voltage at the R14-R15 junction is only 5V, although Q1's collector voltage remains about normal. This indicates that:
A.) Q4 is turned off, for some reason.
B.) Q4 is conducting too heavily, for some reason.
C.) the Vcc power supply voltage is below normal.
D.) Q3 is conducting too heavily, for some reason.
There is no sound out of the speaker. Signal tracing reveals a normal signal at Q1's collector, but no signal at Q2s collector. Dc voltages are nor mal. The most likely culprit is:
A.) a defective Q2.
B.) an open R10.
C.) an open C5.
D.) an open R7.
The sound suffers noticeable amplitude distortion.
A.) C7 is shorted.
B.) C3 is open.
C.) C5 is leaky.
D.) Transistor Q3 is open.
Answers
1. B. With a value of 15µF, electrolytic capacitor C3 is large enough to bypass emitter resistor R4 effectively at virtually all audio frequencies, thus preventing R4 from producing ac negative feed back (degeneration). Of course, R4 does produce do negative feed back, for bias stabilization (with or without C3).
2. B. Collector current can be determined two different ways, thanks to Ohm's law (I = V/R) and the do voltages shown on the schematic. One way is to take the potential difference across collector load resistor R5 (16.3-9.7=6.6V) and divide it by the resistance of R5 (3.3k); this yields a current of 2mA. The second method is based on the fact that collector current is equal to emitter current, for all practical purposes; applying Ohm's law to the emitter circuit (0.44V across 22052) also gives a current of 2mA.
3. D. C2 provides an ac ground for the do supply feeding the collector and base circuits of Q1.
Thus, together with R6, C2 forms a decoupling filter that prevents unwanted mutual coupling be tween stages via the common impedance of the power supply.
4. D. R10 is an emitter bias resistor that produces do negative feedback, thus stabilizing the base-
emitter bias of Q2. Because it is not bypassed by a capacitor, R10 also introduces ac negative feed back (degeneration), which reduces amplitude distortion.
5. A. Together with R2, R3 forms a base bias voltage divider net work that determines the operating point (bias) of Q1. This is superior to simple fixed bias (which would feature R2 only) because the voltage divider stabilizes the base bias against the effects of temperature changes within Q1; and against the effects of different transistor parameters, should Q1 ever be replaced.
6. D. The audio output stage is a single ended, push-pull circuit featuring complementary transistors, and hooked up in an output transformerless (OTL) totem pole arrangement. Note that the presence of coupling capacitor C8 pre vents the circuit from being called output capacitor-less (OCL), but the absence of a push-pull input trans-former qualifies the circuit as in put transformerless (ITL) in addition to being OTL.
7. C. Connected across the speaker, C9 provides frequency selective bypassing, and thus tone compensation. At treble frequencies, the reactance of C9 becomes low enough partially to bypass the speaker, thus reducing the amplitude of the higher frequency signals. This is often done, especially when a small-size loudspeaker is featured. Small speakers are generally more efficient at treble than bass frequencies and therefore tend to produce a tinny sound.
Tone compensation prevents treble sounds from being too loud, thus restoring a more natural balance between bass and treble.
8. A. Diodes D1 and D2 provide temperature compensation and therefore bias stabilization for out put transistors Q3 and Q4. The voltage drop across each of these silicon diodes is one junction voltage--about 0.7V at room temperature. Therefore the potential difference between the two transistor bases is 1.4V. This gives each output transistor a base-emitter bi as voltage of 0.6V, resulting in class AB operation.
When the output transistors heat up, they tend to conduct more. But diodes Dl and D2 heat up also, because they are in close proximity to Q3 and Q4. As a result, their junction voltage decreases, effectively reducing the output transistor base-emitter bias, thus offsetting the initial temperature-related rise in collector current. These temperature compensating diodes are commonly found in totem pole power-out put circuits.
Note that the two diodes also couple the audio signal from the collector of driver transistor Q2 to the base of output transistor Q3.
The diodes do not rectify the signal because they are permanently forward biased, being part of the R10-Q2-D2-D1-R12-R11 voltage divider network connected be tween the positive do bus and ground. Thus, the signal from the driver simply passes right through the low resistance of the diodes, on the way to the Q3 base, and suffers only a slight attenuation in the process.
9. C. The driver stage is dc coupled to the power output circuitry.
In such circuits, transistors are prone to chain reactions wherein temperature changes in one transistor can affect the operating point (bias) of another transistor, possibly resulting in thermal run away. R9 and R7 form a do feed back loop between output emitters and driver base, and it is this self biasing arrangement that protects the output transistors against thermal runaway.
Note that the Q2 base bias doesn't come directly from the Vcc supply; instead, it is taken from the junction of the R14-R15 emitter resistors and applied through R9-R7 to the Q2 base.
Suppose that Q4 heats up, reducing its internal resistance and con ducting more heavily. This causes the midpoint voltage at the R14-R15 junction to drop below 8V. This reduced do voltage is coupled through R9-R7 to Q2, causing the driver's base voltage to drop below its normal 0.86V. With less forward bias, Q2 conducts less and its collector voltage rises above 7.3V. Because of do coupling, the base voltage of Q4 likewise be comes more positive, reducing the base-emitter bias of PNP transistor Q4. The resulting decrease in collector current compensates for the initial rise in Q4 conduction due to heat.
A similar analysis can be applied to the operation of the Q3 transistor. Incidentally, capacitor C4 puts the R7-R9 junction at ac ground potential, thus preventing ac feed back.
10. B. Because the audio signal from the collector of the driver must pass through diodes D2 and D1 on the way to the base of Q3, it suffers a certain amount of attenuation due to the resistance of the diodes, as previously explained in the answer to question 8. This results in a somewhat unbalanced push-pull operation. Capacitor C6 takes care of correcting the situation by coupling the emitter follower's in-phase output signal from the R14-R15 junction back into the base of Q3, via resistor R12. R12 reduces this positive feedback signal down to just the right amplitude. In effect, then, this boost circuit causes Q3's signal to lift itself up by its own bootstraps, offsetting the attenuation caused by the diodes.
11. A. There is no sound at the amplifier's output because the pre-amp stage is inoperative. With the essential base bias resistor (R2) open, Q1's base voltage drops to zero and the transistor turns off, preventing the weak phono input signal from passing through to the driver and output stages.
12. D. Push-pull output transistors connected in the totem pole configuration are likely to over heat when the load is shorted. With the volume control turned up, the output transistors produce a large signal at the circuit midpoint (R14-R15 junction). If the speaker terminals are accidentally shorted, or if C9 shorts, this large signal voltage tries to supply (through C8) the excessively large ac cur rent demanded by the OSi load impedance. This is most likely to zap the output transistors in short order.
The moral of the story is this:
When working with OTL circuits, never short the speaker terminals, and never drastically reduce the output load impedance by connecting additional speakers in parallel with the original one; you're courting trouble if you do.
If you selected answer (C)--an open speaker--you're probably an old timer. Earlier audio amplifier circuits often featured a balanced push-pull circuit with an output transformer. In those circuits, it's an open speaker that spells trouble because the open load reflects a very high impedance back into the primary of the output transformer. With the volume turned up, the large swings in signal current through the primary winding result in excessive back-emf, easily exceeding the transistors' break down voltage ratings. The writer remembers arcing and even fires actually caused by this type of fault in vacuum tube amplifiers.
So, if you're an old timer, welcome to the club.
13. B. There's no sound because a shorted C2 kills the 16.3V do source supplying Q1. Note that excessive current through the shorted C2 is likely to cause R6 to over heat and burn out. Always look for discolored, swollen or cracked de-coupling resistors, when decoupling capacitors short.
14. A. No audio signal reaches the speaker because C8 is the coupling capacitor. That was easy, wasn't it? A technician's life is not filled with tough dogs only; easy ones come along once in a while!
15. C. The amplifier is dead be cause with R12 open, no do voltage is available for the collector of Q2 and the output transistor bases.
16. D. An open R2 causes Q1 to turn off, as mentioned in the answer to question 11. With no collector current flowing through R5, the load resistor doesn't drop any voltage, allowing Q1's collector voltage to rise to the full supply voltage. Also, emitter current stops flowing and the voltage across R4 drops to zero.
17. A. With C8 open, no output signal can reach the speaker, as mentioned earlier. If you thought that both output transistors are defective because no signals are present at their collectors, you fell into a trap. Q3 and Q4 are emitter followers; therefore signals are not supposed to appear at their collectors. Also, if Q3 and Q4 were defective, do voltages would not re main normal in the output circuit.
18. B. With R14 and R15, the internal resistances of Q3 and Q4 form a series voltage divider across the do power supply. The circuit's midpoint (R14-R15 junction) is a good test point to monitor when troubleshooting these OTL power amplifiers. Its do voltage should normally be equal to approximately half the power supply voltage. With the problem at hand, the midpoint voltage measures well below 8V, indicating that the voltage divider is unbalanced, the lower half of the circuit having much less resistance than the up per half. This implies that either Q4 is conducting more heavily than normal or that Q3 is conducting less than it should. Therefore (B) is the only correct answer. The culprit is probably a leaky Q4.
19. C. This is another easy one.
An open C5 is the only one of the four answers that explains the absence of signal at Q2's collector while all do voltages remain nor mal. Of course, a scope check at the base of Q2 would reveal no signal there either.
20. C. The dc voltage from the pre-amp's collector leaks through C5 and shifts the operating point (base bias) of Q2 towards saturation, causing clipping of signal peaks, hence distorted sound.
If you chose answer (D), you apparently overlooked an important point: Although a dead transistor can indeed produce distorted sound in some push-pull amplifiers, in this OTL circuit an open Q3 would cut off the do supply to Q4 (resulting in no sound at all), because Q3 and Q4 are stacked in series, totem pole fashion.
The moment of reckoning
This quiz was designed to test your knowledge of circuit operation and your understanding of troubleshooting techniques. It also gave you an opportunity to pit your wits against a number of defective circuits. How did you make out? Were most of the culprits identified correctly and most of the patients made well again? At five points per correct answer, tally up your score. Your report card is shown below.
Hoping that you managed to pick up one or two useful tidbits, here's wishing you Happy and Successful Troubleshooting.
POINTS GRADE
90-100 A
75-85 B
60-70 C
Under 60 F
VERDICT
Congratulations, Dr. Kildare! You're a promising technician. You need to hone your skills a bit. A Mike Hammer you are not!
Also see: Test your electronics knowledge