The dividing line between utility-type and hi-fi audio amplifiers is not sharply drawn, since no industry standards have been established. However, an amplifier in a low-priced record player or tape recorder will not be classified as a hi-fi unit by any technician or audio buff. On the other hand, almost any amplifier in an elaborate combo will be classified as a hi-fi unit. These examples leave an extensive "gray" area, in which classification is chiefly a matter of personal opinion. It is helpful to start our analysis of audio-amplifier servicing with a coverage of this intermediate type of equipment, because its comparative simplicity facilitates analysis and detailing of troubleshooting procedures. We are concerned with two broad categories of design, viz. solid-state and tube-type amplifiers. That is, the analytical approaches to trouble localization are not necessarily the same in these basic categories, inasmuch as design principles may depart from a transistor-for tube plan.
Common trouble symptoms caused by defects in the intermediate types of audio amplifiers are as follows:
1. No audio output.
2. Subnormal output level.
3. Excessive distortion at rated output level.
4. Excessive distortion at all output levels.
5. Noise and/or hum interference in sound.
6. Intermittent operation.
GENERAL DISCUSSION
Tube-type utility amplifiers may employ the "cord wood" assembly plan exemplified in Fig. 4-1. This is a traditional and conventional component layout that is comparatively easy to troubleshoot, although it lacks the sophistication of more modern physical designs. The more elaborate designs of utility amplifiers in the conventional group feature ordered layout of smaller components on terminal boards, as seen in Fig. 4-2. Although this arrangement is slightly more difficult to trace, each component will be identified in the amplifier service data for ready reference. The next step in sophistication entails packaged printed circuits, as shown in Fig. 4-3. For example, an RC coupling circuit may be fabricated in this type of package. Since individual components are not always externally accessible, analysis must be made on the basis of input-output relationships.
Some tube-type utility amplifiers employ terminal boards and plated ( or printed) circuits, as shown in Fig. 4-4. This type of circuitry is somewhat more difficult to trace; however, component identification is easy if we make reference to the amplifier service data. Only a few hybrid types of audio amplifiers (using transistor drivers and tube output stages) have been marketed. Solid-state utility amplifiers employ transistors, and semiconductor diodes may also be included. Printed-circuit boards are universally employed, and we will occasionally find integrated circuits in low-level stages. An integrated circuit differs from a packaged printed circuit in that an IC contains devices (specially fabricated transistors). Since the individual components are externally inaccessible, troubleshooting must be done on the basis of terminal-voltage measurements, and input-output relationships.
Amplifier Classification
Bench work involves various categories of audio amplifiers, and many sub-classifications. The chief basic arrangements include:
Hi-Fi Stereo Servicing Guide
Fig. 4-1. Utility amplifier employing the "cordwood" type of
component layout.
Fig. 4-2. Resistor terminal board in a tube-type amplifier.
Fig. 4-3. Typical packaged printed circuit.
1. Resistance-capacitance coupled stages.
2. Transformer-coupled stages.
3. Hybrid RC and transformer coupling.
4. Direct-coupled stages.
5. Output-transformerless (OTL) arrangements.
Fig. 4-4. Boards used in amplifiers.
(A) Capacitor terminal board indicated by arrow.
(B) Bottom view of plated circuit.
Fig. 4-5. RC coupling network between two triode amplifiers.
Fig. 4-6. Typical RC-coupled amplifier using pentodes.
Any of the foregoing types of coupling may be used with transistors or with tubes, and may be employed in single-ended or double-ended (push-pull) stages. Parallel connection of tubes or transistors may be used in output stages to provide a required power level. Triode, tetrode, pentode, or beam-power tubes are utilized. Plate output is most commonly used, although we will also find cathode output employed when substantial driving power is required, or when a very low impedance load must be accommodated as in OTL amplifiers. In solid-state amplifiers, the common-emitter configuration is in very wide use, although the common-collector ( emitter follower) configuration is used when substantial driving power is required, or when the low load impedance of an OTL amplifier is to be driven. The common-base configuration is occasionally utilized when an input device with very low impedance must drive a high-impedance load.
It is helpful to briefly consider the circuitry for the basic amplifier arrangements. Fig. 4-5 shows the simplest form of RC-coupled triode amplifier. The most common source of trouble (apart from defective tubes) is leakage in the coupling capacitor C0 , which upsets the grid bias on V2. In practice, cathode self-bias is generally used, and pentode tubes are often employed because of their comparatively high gain (see Fig. 4-6). This type of circuitry utilizes more capacitors, and hence is more likely to develop trouble symptoms due to capacitor defects. For ex ample, if Ck is open, the associated tube becomes degenerative, and the stage gain is impaired. Or, if Ck shorts, the grid-cathode bias is disturbed. In case Csg opens up, we will find audio signal on the screen grid; the stage gain is greatly reduced, and the circuit develops poor frequency response. On the other hand, if Csg becomes shorted, Rsg is likely to burn out; in any event, the stage gain becomes greatly subnormal.
Fig. 4-7. Transformer-coupled amplifier using triodes.
Trouble in a given circuit is sometimes caused by a defect in the preceding circuit. For example, in Fig. 4-6, a short-circuit or heavy-leakage condition in Cc will cause seriously abnormal screen and plate current in V2. It is quite likely that the plate load, screen-dropping, and cathode-bias resistors for V2 will be badly overheated or burned out during the process of destruction of V2. By the same token, this type of trouble can change the value of a circuit resistor, even if it is not burned out. In such a case, the amplifier starts working again when Cc is re- placed, but normal operation is not resumed. Let us briefly consider the factors that are involved:
1. When a cathode resistor is much too high in value, the grid-cathode bias is excessive; the stage gain is reduced, and distortion will occur on high-level signals if a sharp-cutoff pentode is utilized.
2. When a plate-load resistor is much too high in value, the stage gain is increased to some extent; however, the dynamic range of the stage is also reduced, and overload distortion occurs on high level signals. High-frequency response is also impaired.
3. When a screen-dropping resistor is much too high in value, the stage gain is reduced and overload distortion occurs on high-level signals.
A simple transformer-coupled configuration is shown in Fig. 4-7. We are finding fewer amplifiers with transformer interstage coupling than in former years, simply because transformers are comparatively costly devices. Although low-priced coupling transformers are available, their frequency range is restricted and they tend to develop appreciable distortion. The response of a transformer depends considerably on the preceding plate-output impedance and the succeeding grid-input impedance. Therefore, exact replacements are advisable to avoid impaired response. Common causes of trouble symptoms in this type of amplifier are:
1. Primary winding of transformer burned out, due to preceding circuit fault that produces an excessive current demand.
2. Leakage to core or between windings due to absorption of moisture or because of serious overheating.
3. Response will sometimes be impaired if the primary or secondary leads are reversed accidentally.
4. Distortion due to shorted cathode-bypass capacitor.
5. Low gain due to open cathode-bypass capacitor.
In older types of amplifiers, such as those used in public-address (PA) systems, we will find hybrid RC and transformer coupling, as exemplified in Fig. 4-8. The arrangement is characterized by single-ended stages throughout, and provision of adjustable negative feedback for the second and third stages. We will consider the operation and troubleshooting of negative-feedback circuits in greater detail later.
Fig. 4-8. An older model 6-watt PA amplifier of the hybrid RC- and transformer-coupled
type.
The basic troubleshooting approach in Fig. 4-8 can be outlined as follows:
1. Check tubes first, or try substituting new tubes.
2. If there is no audio output, use a scope to trace the signal through the amplifier.
3. After a defective stage is localized, measure DC voltages and compare the measured values with those specified.
4. In case of doubt, disconnect one end of a suspected capacitor and test it with a capacitor checker, or make a substitution test.
5. Make supplementary resistance measurements of controls, fixed resistors, and transformer windings to confirm these possibilities of trouble.
Direct-coupled amplifier configurations have been used to a limited extent in older designs, but are widely used in the newer models. Therefore, we will encounter many more de-coupled circuits in solid state amplifiers than in tube-type amplifiers. A basic example of a de-coupled amplifier using triodes is shown in Fig. 4-9. The advantage of this design is that it can amplify very low bass signals with negligible distortion and with comparatively simple circuitry. A voltage divider is generally used to supply the necessary DC operating voltages for the amplifier from a regulated power supply. The B+ supply voltage is applied across the voltage-divider resistor Ra, which is tapped at suitable points. Capacitor Cd is used to bypass any ac voltage variations that might appear across Rd. The audio-input voltage is amplified by V1 and V2, and appears as the audio-output voltage at the plate of V2 in normal operation. The plate-load resistor Ru also serves as the grid resistor for V2, inasmuch as the voltage drop across it is applied to the grid of V2. R1, 2 serves as a plate-load resistor for V2.
Fig. 4-9. Typical direct-coupled amplifier using triodes.
Note that the cathode of V1 in Fig. 4-9 is connected to point A, and the plate is connected to point B on the voltage divider. Point B is positive with respect to point A, which makes the plate positive with respect to its cathode. This voltage relationship permits V1 to conduct. The voltage developed from point A to ground serves as bias voltage for V1. The plate of V2 must be positive with respect to its cathode for conduction to take place. Moreover, the grid voltage of V2 must not be positive with respect to its cathode.
The plate current of V1, which flows through Rr,1 , produces a considerable voltage drop across this resistor. In turn, the voltage at the plate of V1 and at the grid of V2 is less positive than at point B on the voltage divider. Tap D is located at a point on resistor Rd such that the magnitude of the positive voltage on the grid of V2 is lower than that of the positive voltage on the cathode of V2. Therefore, the grid of V2 is actually less positive ( or is effectively negative) with respect to the cathode of V2 in nor mal operation. The voltage between points C and D is the plate voltage for V2.
When trouble symptoms are encountered in DC coupled stages of a tube-type amplifier, the most likely causes are as follows:
1. A tube may have developed grid current be cause of grid emission or traces of gas. A substitution test is preferred.
2. Regulation of the power supply may be unsatisfactory; a common cause is aging of a voltage regulator tube.
3. The bypass capacitor for the voltage divider may be leaky, or may have opened up.
4. A fixed resistor (particularly of the composition type) will occasionally develop thermal instability.
5. Corroded contacts or imperfect connections are especially troublesome in de-coupled stages.
Prior to the advent of transistors, output-transformerless (OTL) amplifiers were in the minority, due to the extensive trade-off that was involved be tween output power and efficiency. However, some of these early OTL amplifiers are still in use, and will be briefly considered. One of the most familiar types is the Futterman amplifier shown in Fig. 4-10.
Its chief advantage is that it dispenses with an out put transformer, which is a costly component if it is designed to provide reasonably good audio quality.
In this example, three Type-6082 dual-triode power tubes are utilized in the output stage. A double ended power supply is employed to provide both B+ and B- voltages. The 6SN7 is driven by a paraphase inverter stage, and in turn drives the 6082s, which operate in a push-pull/parallel configuration. Since the speaker is connected at the midpoint of a series tube arrangement, a basic cascode configuration is employed in which the tubes serve as plate and cathode loads for each other. Note that 40 dB of negative feedback is provided from the voice coil to the cathode of the phase-splitter driver tube. With 1 volt rms input, this amplifier normally provides 25 watts of audio power into a 16-ohm voice coil with only 0.3 percent harmonic distortion.
Fig. 4-10. The Futterman OTL amplifier.
The chief troubles encountered in OTL amplifiers are caused by the same component defects that were noted previously for RC-coupled amplifiers. It is helpful at this point to briefly note the various classes of operation employed in audio amplifiers. They are:
1. Class-A operation.
2. Class-B operation.
3. Class-AB operation.
4. Class-AB1 operation.
5. Class-AB2 operation.
Fig. 4-11. Visualization of Class A operation. (A) Load line. (B) Circuit.
(C) Equivalent circuit. (D) Coupling circuit CR, added.
Fig. 4-12. Class-B operation on dynamic characteristic. (A) Class-AB, operation.
Fig. 4-13. Push-pull triode amplifier circuit.
Fig. 4-14. Class-AB operation on dynamic characteristic.
Class-A operation is shown in Fig. 4-11. The tube is normally biased so that there is current at all times. Point B is called the operating point, and is determined by the value of grid voltage ( DC bias voltage). The output plate waveform is practically undistorted in normal operation. Of course, over drive will cause a departure from linear operation and will result in distortion. Note that when the coupling circuit in Fig. 4-llD is included, the effective plate load is decreased with respect to an audio signal. Therefore, the signal development occurs on the ac load line shown in Fig. 4-11A. That is, the load for an ac signal is only half as great as the load for the DC plate circuit in this example.
Class-B operation is shown in Fig. 4-12. The tube is normally biased at or near cutoff. Thus, plate cur rent flows during the positive half of the input grid signal, but stops flowing during the negative half cycle. In turn, the output plate-current waveform is seriously distorted. If the amplitude of the grid driving signal is increased so that it extends beyond point F, power output is increased; but if the drive extends beyond point G, grid current will flow and the plate-current waveform will be clipped, resulting in increased distortion. To minimize distortion in Class-B amplifiers, a stage commonly employs two tubes operating in push-pull as shown in Fig. 4-13.
Thereby, the distortion produced by one tube is largely nullified by the other tube. Class-B amplifiers are occasionally designed to draw a little grid current at maximum rated power output. In such a case, the driver stage is designed to supply the required driving power.
Class-AB operation is pictured in Fig. 4-14. Note that the subclasses of this mode are designated AB1 and AB2. This type of amplifier operates in the region between Class A and Class B. Plate current flows more than one-half, but for less than the entire cycle in normal operation. A subscript 1 denotes that grid current does not flow at maximum rated power out- put. A subscript 2 denotes that grid current flows at maximum rated power output. Since the output waveform from a Class-AB tube is necessarily distorted, an audio stage commonly employs a pair of tubes operating in push-pull, as in a Class-B amplifier. The advantages of a Class-AB amplifier are its greater power output, compared with a Class-A amplifier, and its lower distortion, compared with a Class-B amplifier.
Fig. 4-15. Dynamic transfer characteristic curves for different plate loads
of a triode.
Fig. 4-16. Dynamic characteristic of two tubes in push-pull Class-A operation.
Fig. 4-17. Operation of push-pull, Class-B amplifier biased exactly at
plate-current cutoff.
Table 4-1. Amplifier Characteristics
Fig. 4-18. Output stage with phase-inverter action.
It should not be supposed that Class-A operation is basically distortionless. A triode provides less distortion than a pentode, and a high value of plate load resistance minimizes residual distortion. How ever, as seen in Fig. 4-15, the dynamic transfer characteristic of a triode is never absolutely linear. There fore, even triodes in the Class-A mode must be operated in push-pull to obtain minimum distortion, particularly at high audio levels (see Fig. 4-16). Next, in Class-B operation, we will obtain a certain amount of crossover distortion when the tubes are biased exactly to plate-cutoff, as shown in Fig. 4-17.
Crossover distortion is eliminated, of course, by operating the tubes in Class AB. The amount of distortion that is produced by Class-AB2 operation depends chiefly on the ability of the preceding driver stage to supply the grid-current demand. Beginners will find the data tabulated in Table 4-1 informative.
We will find various types of phase inverters in commercial audio amplifiers. One of the simplest forms was shown in Fig. 4-10; this configuration exploits plate output and cathode output from the same tube. The inverter stage provides negligible gain, and operates as an electronic transformer to develop a double-ended output from a single-ended input.
Phase inversion is often accomplished in the output stage; one widely used arrangement is shown in Fig. 4-18. The single-ended input is amplified by V1 and appears in opposite phase at the plate. A portion of this output signal is dropped across R4, and is coupled to the grid of V2; thereby V2 is driven in opposite phase to V1. Resistors R5 and R6 are parasitic-suppression resistors. These resistors are required to avoid parasitic oscillation at a very high frequency corresponding to lead resonances in the grid and plate circuits. Parasitic oscillation reduces the output of an amplifier, causes distortion, and sometimes damages the tubes.
The same basic method of phase inversion is often provided in the driver section for a push-pull output stage, as exemplified in Fig. 4-19. This arrangement is less troublesome with regard to parasitics. The configuration provides for either microphone or phono input, and the inputs may be mixed, if desired. Note that the bass tone control varies the proportions of bass and treble amplitudes applied to the grid of V2. The treble tone control operates on a similar principle, except that the setting of R1 determines the proportion of treble tones that are by passed to ground. Negative feedback is employed from the voice coil to the cathode and suppressor grid of V2 to provide improved frequency response. It is desirable to have a reasonable impedance match from the output stage to the voice coil, both to obtain maximum power transfer, and also to optimize the frequency response.
Most solid-state audio amplifiers in the utility category are designed on a transistor-for-tube basis, and are thus comparatively easy to service by analogy with tube-type amplifiers. Fig. 4-20 shows the basic comparative transistor and tube configurations.
One of the common trouble symptoms, particularly in a solid-state power stage, is thermal drift, which results in distortion and reduced power output, and can cause transistor damage in serious cases. A basic method of overcoming thermal drift is to use thermistor control of the emitter bias voltage, as shown in Fig. 4-21. The thermistor is usually a specified type of semiconductor diode. Proper bias control depends on the condition of the thermistor. Exact replacement is required in case of thermistor failure. In normal operation, greatly increased thermal stability is provided, as shown in Fig. 4-22.
Fig. 4-19. Public-address amplifier with phase-inverting driver stage.
We will also find amplifier stages that employ two junction diodes for compensation of temperature variation, as shown in Fig. 4-23. One of the diodes compensates for temperature variations of the emitter-base junction resistance; the other diode compensates for temperature variation of saturation current. Note that diode X2 is reverse-biased, whereas diode Xl is forward-biased. The improved thermal stability obtained in normal operation is shown in Fig. 4-24. As noted previously, direct coupling is in wide use-an example is shown in Fig. 4-25. This two-stage amplifier is arranged so that an increase in collector current caused by a temperature rise in transistor Q1 will reduce the forward bias on transistor Q2. Circuit operation is as follows:
1. Transistor Q1 (Fig. 4-25) is connected in the common-base configuration, and is inherently highly stable. However, there will be some variation in collector current with temperature change. If the temperature increases, the collector current increases in the direction of the arrow. A portion of this increment flows through R3 and produces a voltage drop with the polarity indicated. Another portion of the incremental current is through R2, and develops a voltage drop with the indicated polarity.
Fig. 4-20. Basic amplifier configurations using transistors or triode vacuum
tubes.
Fig. 4-21. Transistor amplifier with thermistor control of emitter bias
voltage.
Fig. 4-22. Collector current in stabilized and non-stabilized transistor
circuit.
Fig. 4-23. Diode compensation for effects of temperature variation in a
transistor circuit.
Fig. 4-24. Collector current with and without diode stabilization.
Fig. 4-25. Two-stage temperature-stabilized DC amplifier.
2. Now consider the base-emitter bias circuit of Q2. This bias is the sum of the voltages across R3, R2, and battery Ve. The voltage indicated across R3 aids the forward bias; the voltage across R2 opposes the forward bias. By pro portioning the values of R2 and R3 so that more voltage drops across R2, the resultant forward bias will be decreased. This action limits the tendency of the collector current in Q2 to in crease in response to an increase in temperature.
Trouble can be caused if an excessive emitter collector voltage should be applied when the normally forward-biased base-emitter circuit is reverse biased, due to internal oscillation that can destroy the transistor. This condition can occur in transformer-coupled amplifiers such as the example in Fig. 4-26. If the signal from the previous stage is suddenly stopped, or in case an excessively strong noise pulse occurs, the base-emitter circuit can be driven into a reverse-biased condition, and the collector current is thereby cut off rapidly. The magnetic field in transformer T2 collapses rapidly and produces a high emitter-collector voltage during the time that the base-emitter circuit is reverse biased. Because of this action, strong oscillations are set up, and this power must be dissipated in the transistor, which is likely to be destroyed.
Fig. 4-26. Junction diode used to prevent reverse bias in a base-emitter
circuit.
To prevent the possibility of transistor damage in the circuit of Fig. 4-26, a junction diode is connected between the base and emitter which prevents the base-emitter junction of the transistor from becoming reverse biased. We call this a shunt-limiting action. The voltage divider consisting of resistors R1 and R2 provides a voltage at the base which forward biases the base-emitter junction, and simultaneously reverse biases junction diode X1. In normal operation, X1 is effectively an open circuit. Now, if a strong surge voltage is applied from the secondary of T1, and if it is greater than the voltage across R1, the junction diode X1 will then become forward biased and will start to conduct. With X1 conducting, only a small voltage drop is produced across the diode, which prevents the base-emitter junction from becoming reverse biased. Note that capacitor C1 serves to bypass the ac signal around R1.
Fig. 4-27 shows a typical circuit for a two-stage direct-coupled amplifier. Note that C2 and R3 form a low-pass filter, which provides some attenuation of high audio frequencies. This action compensates for the lack of full low-frequency response by micro phone Ml. Transistor Q2 employs the common emitter configuration with degeneration in the emitter circuit. This increases the input impedance of Q2 to approximately 30,000 ohms. The collector voltage and emitter current of Q1 have low values to optimize the signal-to-noise ratio. Since most of the noise is contributed by the first stage (being amplified by subsequent stages), the collector voltage and emitter current of Q2 can have substantially greater values without greatly impairing the signal-to-noise ratio.
Resistor R1 provides bias stability, and is bypassed by C1. Resistor R5 provides bias stability and is by passed by C3. If C1 or C3 should become open, the amplifier gain would be greatly reduced.
Fig. 4-28. Typical two-stage preamplifier, providing high frequency compensation
for transducer.
Fig. 4-27. Typical two-stage direct-coupled preamplifier, providing low-frequency
compensation for transducer.
A transducer may have more or less high-frequency attenuation. In such a case, the amplifier will include a suitable equalizer network, like that shown in Fig. 4-28. The equalizer network (shown inside the dashed lines), attenuates the low frequencies more than it does the high frequencies. Capacitor C1 serves as a DC blocking capacitor, but it also tends to produce some low-frequency attenuation.
However, this action is compensated and the desired overall frequency response is provided by C2 and R3. Because of its higher reactance at the lower frequencies, C2 bypasses the high frequencies around R3, and thus provides greater amplification of higher frequencies. At low frequencies, C2 can be regarded as an open circuit. Because the low frequencies must pass through R3, part of their voltage is dropped across the resistor.
Next, let us note the basic way in which transistors are used as phase inverters in audio driver stages. Fig. 4-29 shows the configuration for a split load (paraphase) phase inverter employing transistor Q1 for driving a push-pull output stage (transistors Q2 and Q3). The path of output current from Q1 is through collector load resistor R3 and through emitter load resistor R2. Resistors R2 and R3 have equal values. R1 is used to establish the base-bias voltage of the inverter transistor. The collector and emitter outputs have equal amplitudes and are 180 degrees out of phase in normal operation. When high level signals are present, however, noticeable distortion can occur because the collector output impedance of Q1 is higher than the emitter output impedance. In other words, the collector branch has less current capability.
To avoid this source of distortion, the arrangement shown in Fig. 4-30 may be employed. Note the addition of resistor R4, which eliminates the unbalance in output impedance that is present in Fig. 4-29.
The values of resistors R2 and R4 in Fig. 4-30 are proportioned so that the signal source impedance for Q2 is the same as the signal source impedance for Q3.
Thereby, both branches have the same current capability, and distortion is minimized at high levels of operation. The signal loss across R4 is compensated by making R2 higher in value than R3. Because of the large negative-feedback voltage developed across R2, a rather strong signal is required to drive a one stage phase inverter. This disadvantage is overcome in practice by employing a two-stage phase inverter when appropriate.
Fig. 4-29. One-stage phase inverter.
Fig. 4-30. One-stage phase inverter circuit with equalized out put impedance.
Fig. 4-31. Two-stage phase inverter circuit using two common emitter configurations.
Fig. 4-31 shows the circuitry of a two-stage phase inverter consisting of two identical common-emitter configurations. Let us assume that an input signal drives the base of Q1 negative. Since a common emitter configuration produces a 180-degree phase reversal, the collector of Q1 goes positive. One portion of this positive signal is coupled to the base of Q2 through C2 and attenuating resistor R4. The other portion of the positive signal is coupled through C4 to one input circuit of a push-pull output stage.
The positive-going signal on the base of Q2 causes a negative-going signal at the collector of Q2. This negative signal is coupled through C5 to the other input circuit of the push-pull output stage. R1 pro vides base bias for Q1, and R5 provides base bias for Q2. Resistors R2 and R7 provide bias stabilization and are bypassed by C1 and C3, respectively.
ANALYSIS OF COMMON TROUBLE SYMPTOMS
An analysis of the common trouble symptoms listed earlier in this Section is presented in this section.
1. No Audio Output Possible causes of no audio output from an amplifier are as follows:
a. Open coupling capacitor (such as Cc in Fig. 4-5).
b. Shorted coupling capacitor (such as Cc in Fig. 4-6).
c. Burned-out winding, such as primary P in Fig. 4-7.
d. Open volume control, such as R1 in Fig. 4-8.
e. Speaker plug not fully inserted in socket (see Fig. 4-19).
f. Open or shorted transistor.
g. Shorted overload diode, such as X1 in Fig. 4-26.
h. Break in printed-circuit conductor.
To localize the defective stage, a scope may be used to trace the signal through the amplifier to the point of signal stoppage. Then, DC voltage measurements will usually permit the technician to pinpoint the defective component. Supplementary resistance measurements and continuity checks are also required in some situations.
2. Subnormal Output Level
When the audio output level is subnormal, the first step is to localize the weak stage in the amplifier.
This requires a practical knowledge of stage gains in the various configurations. For example, an approximate gain of unity in a paraphase inverter is normal, whereas this figure would be greatly subnormal for a common-emitter stage. A scope is very useful to compare the amplitudes of input and output wave forms at each stage, in preliminary localization procedure. Subnormal output may or may not be ac companied by waveform distortion. If distortion is present, useful clues may be provided; for example, clipping distortion points to an incorrect operating point, whereas strong noise spikes point to a defect in the first audio section, such as a virtually open coupling capacitor.
Possible causes of subnormal output level from an audio amplifier are:
a. Leaky coupling capacitor (such as Cc in Fig. 4-5).
b. Open screen-bypass capacitor (such as Csg in Fig. 4-6).
c. Open cathode-bypass capacitor (such as Ck in Fig. 4-7).
d. Shorted capacitor in negative-feedback loop ( such as C5 in Fig. 4-8) .
e. Open coupling capacitor (such as C2 in Fig. 4-18).
f. Shorted equalizing capacitor (such as C2 in Fig. 4-27).
g. Open emitter-bypass capacitor (such as C1 or C3 in Fig. 4-31).
h. Leaky transistor.
3. Excessive Distortion at Rated Output Level
Some types of amplifier defects will cause excessive distortion at the maximum rated output of the amplifier, whereas the distortion tends to disappear at lower output levels. This difference in response can provide clues concerning trouble possibilities when evaluated by an experienced technician.
Possible causes of excessive distortion at rated output level are:
a. Subnormal supply voltage.
b. Leaky cathode-bypass capacitor (such as Ck in Fig. 4-6).
c. Leaky coupling capacitor (such as Cc in Fig. 4-6).
d. Incorrect type of replacement transformer ( such as T in Fig. 4-7) .
e. Open capacitor in negative-feedback loop (see C5 in Fig. 4-8).
f. Incorrect load-correction network used with a particular speaker ( see the optional load corrections in Fig. 4-10).
g. Use of incorrect output-impedance tap (see T2 in Fig. 4-19).
h. Marginal thermistor, such as RT in Fig. 4-21.
i. Leaky transistor.
j. Leaky emitter-bypass capacitor, such as C1 or C3 in Fig. 4-27.
4. Excessive Distortion at All Output Levels
When excessive distortion occurs at all output levels, look for a major component defect that can produce serious waveform distortion regardless of the signal amplitude. With a practical knowledge of circuit action, the experienced technician can "weed out" the possibilities by an analysis of the amplifier configuration. A scope is very useful to trace the signal and to determine the point at which distortion first occurs.
Possible causes of excessive distortion at all out put levels are as follows:
a. Component defect that "kills" the operation of one-half of a phase-inverter section; for example, Q2 might be open or shorted in Fig. 4-31.
b. Component defect that "kills" the operation of one-half of a push-pull amplifier section; for example, Q3 in Fig. 4-30 might be open or shorted.
c. Open coupling capacitor in driver section; C1 in Fig. 4-29 is a typical example.
d. Open equalizing capacitor, such as C2 in Fig. 4-28.
e. Shorted equalizing capacitor, such as C2 in Fig. 4-27.
f. Defective transducer; for example, the micro phone in a PA system might be defective, or the speaker might be defective. A substitution test is advisable.
5. Noise and/or Hum Interference in Sound Possible causes of noise and/or interference in the sound, due to defects in an audio amplifier are:
a. Noisy resistor in the first stage (most of the noise is contributed by the first stage).
b. Cold-soldered or unsoldered contact.
c. Cable plug not fully inserted into connector.
d. Noisy transistor.
e. Defective filter capacitors produce hum, which should not be confused with heater-cathode leakage in a tube.
f. Poorly grounded or ungrounded cable to the input of a high-gain amplifier will permit the entry and amplification of stray fields.
g. Defective transducer.
6. Intermittent Operation Intermittent conditions are unquestionably among the most vexing problems encountered at the service bench. Most "tough-dog" jobs fall into this category.
An intermittent may be mechanical, electrical, or thermal in nature. The time-honored procedure of jarring and tapping will sometimes turn up a mechanical intermittent. PC boards can be flexed gently, and the amplifier can be oriented in various positions to speed up the occurrence of certain types of mechanical intermittents. Electrical intermittents can occasionally be started by throwing the power switch on and off. Operation at undervoltage or over voltage may also be helpful; however, in the case of solid-state receivers, never check with high supply voltage, as the transistors may be damaged. It is sometimes helpful also to vary the input signal level up and dt>wn from zero to maximum rated power output. Thermal intermittents can sometimes be turned up by heating component pigtails with a soldering gun. However, use great caution when checking semiconductors for heat response. A cool ant spray will sometimes cause an intermittent to start or stop. The chassis can be operated at elevated or reduced temperature-however, to avoid the possibility of transistor damage, solid-state receivers should not be operated at excessively high temperature.
Possible causes of intermittent operation in audio amplifiers are:
a. Marginal open or short condition in a capacitor.
b. Unstable composition resistor or operating control.
c. Intermittent transistor.
d. Poor connection, such as a cold-solder joint.
e. Broken conductor inside an insulated lead.
f. Microscopic break in a PC board.
g. Defective function switch.
h. Corroded plug or connector.