Troubleshooting Stereo Audio Circuits

The stereo amplifier is a two-channel audio system for music reproduction from input to several connected speakers or line output jacks. The left and right audio channels are involved in a stereophonic reproduction system. The early stereo channels were found in the AM/FM/MPX receivers and phonographs. Just about every audio product today has a stereo system, including the deluxe TV chassis.

TYPICAL STEREO BLOCK DIAGRAM

The early audio stereo system found in the small radio receiver or cassette player might consist of an AF, driver and push-pull output transistors within the separate stereo channels. Next came the AF or driver transistor with a dual-IC output stage. Today, the small stereo amplifier might consist of only one large IC component for the entire audio system.

Several transistors are found in each left and right channels of high-powered amplifiers. The output circuits often consist of the AF, driver and output transistors in a directly-coupled push-pull arrangement. The AF transistors can be capacity-coupled to the next stage while the driver transistors are directly-coupled to the power output transistors. A directly-coupled transistor has a collector terminal of the preceding stage tied directly to the base terminal of the next audio transistor. Often, the AF transistor is a PNP while the directly-coupled transistor is an NPN type. You might find both driver transistors in each channel as an NPN transistor. PNP and NPN power transistors are found in the early output circuits.

The medium-powered stereo circuits found in the boom-box, CD player and TV, might consist of a buffer or AF transistor tied to a volume control IC. This IC controls the volume and amplifies the weak audio signal. The volume control IC varies the volume in both channels with only one volume control ( FIG. 2). A mute transistor tied to the volume control IC provides muting of the audio channels. An electrolytic capacitor couples the controlled-signal to a dual-stereo power output IC. The stereo speakers are coupled to the sound output IC with large electrolytic capacitors (470-1000 uF).

FIG. 2. A typical TV audio transistor and IC stereo block diagram.

The deluxe stereo channels found in the large screen TV might have several stages of stereo channels. Within the RCA CTC157H chassis, the stereo system starts at the stereo de-modular (IC17O1) and is coupled-directly to separate Summer transistors (Q1703 and Q1704). The NPN Summer transistor output is coupled to the Matrix IC (U1702). An audio switch (U1402) is coupled through a resistor to the right and left output terminals of the Matrix IC (FIG. 3).

FIG. 3. The block diagram of a stereo audio circuit in an RCA CTC157H chassis.

The audio IC switch (U1402) provides switching of the TV audio, auxiliary and mute functions. A control logic circuit controls the muting of both channels in the IC switch from a mute signal of the microprocessor. The volume control IC (U1801) controls the volume applied to the sound output IC (U1900). A signal from the AJU controller controls the volume in U1801.

The stereo volume control IC (U1801) couples the audio with a 1 uF electrolytic capacitors to the sound output IC (U1900). Two 220 uF electrolytics connect the 32 ohm speakers to the sound output IC. In the CTC157C and R chassis, two external Hi-Fi output jacks are provided that can be connected to a high-powered stereo amplifier system. Separate audio AF transistors are found after the volume control IC to the Hi-Fi output jacks.

THE PREAMP STEREO AUDIO CIRCUITS

Like most preamp, buffer or AF audio circuits, a separate transistor stage is found in each stereo circuit. The base of the preamp transistor might be coupled with a 1 uF to 10 uF electrolytic coupling capacitor. Most of the buffer transistors are NPN types with the audio taken from the emitter circuits instead of the collector terminals ( FIG. 4). A 22 uF capacitor couples the audio from Q851 and Q852 to a stereo or mono selector switch. Two separate 47 uF electrolytics connect the buffer signal to pins 6 and 8 of the volume control IC.

FIG. 4. The left and right buffer transistor output is taken from the emitter instead of the collector terminal.

The preamp or buffer signal can be signal traced with the audio signal generator or scope. Compare the audio signal at the base and emitter terminals of Q851 and Q852. The amplitude of the audio signal should be quite close with a normal stereo channel. Check Q851 for open conditions with a normal signal upon the base terminal and very little audio at the emitter terminal.

A distorted right channel can be caused by a leaky buffer transistor or electrolytic coupling capacitor. Suspect poor soldered terminals of buffer transistors for intermittent audio channel. Monitor the output signal with the scope or external amp for intermittent reception. Check C851, C853, C824, and Q851 for a weak or lower audio signal applied to pin 8 of volume control IC802.

In a JC Penny AM/FM/MPX stereo receiver, a continuously low level noise was heard in the left channel. Sometimes the noise was very loud. The noise could still be heard with the volume control turned down, indicating trouble occurring after the volume control. Start at the volume control and signal trace the noise from volume control to the left speaker terminals. The noise was heard at the base terminal of preamp transistor Q402 and not at the volume control ( FIG. 5). Replacing C404 (1 uF) electrolytic coupling capacitor cured the low noise sound in the left channel.

FIG. 5. C404 was noisy in a J.C. Penney preamp audio circuit.

IN THE DRIVERS SEAT

The driver circuits in a transistor power amplifier stage might consist of a pre-driver and a driver transistor. In the IC stereo driver circuits, the driver circuits might be included in a dual-driver IC or combined in one large power output IC. The pre-driver transistor can be directly-coupled to the driver output stage. The regular driver transistor is connected directly to the output transistors.

When Q202 becomes leaky, the voltage will change on Q203 and Q204, since they are directly-coupled together. A change of voltage upon Q203 can change the base voltage upon the power output transistor ( FIG. 6). Signal trace the audio signal to the base of each transistor to determine where the audio stops, becomes weak or intermittent. Take an in-circuit voltage test upon each transistor and record them upon the schematic. Now check the forward bias of each transistor from emitter to the base terminal. The normal silicon transistor should have a 0.6 volt measurement between the two terminals.

FIG. 6. The pre-driver signal is directly-coupled to two output transistors with audio signal path.

Take critical resistance measurements from each base and emitter terminals of driver transistors to common ground and compare to the normal stereo channel. Remove one end of a suspected resistor from the circuit and take another measurement ( FIG. 7). Make a in-circuit test of each suspected transistor with transistor tester or diode-test of DMM. Sometimes a leaky transistor can be located with accurate voltage and resistance measurements to common ground.

Check the audio signal in and out of a driver IC circuit with the scope or external audio amplifier. Compare the input and output signal with the normal channel. If the signal is normal at the input terminal and weak, distorted or intermittent at the output terminal, suspect a defective preamp IC or connected components. Take critical voltage measurements upon each IC terminal and compare with the normal channel. Suspect a defective IC or improper voltage source when both channels are weak or distorted in a dual-preamp IC.

TRANSISTOR OUTPUT STEREO CIRCUITS

The transistor output stereo circuits might consist of two transistors directly-coupled to the driver transistors. The collector terminal of the NPN output transistor might be supplied from a higher voltage source (20 to 70 volts). A PNP matched output transistor collector terminal is at ground or a negative potential. The two emitter terminals are wired together through very low ohm emitter resistors. A large electrolytic capacitor couples the high- powered audio to a set of speakers. Both left and right stereo output circuits are identical ( FIG. 8).

When one of the audio output transistors appears open or leaky, the connected transistor can be damaged. Often, when one transistor becomes leaky the other transistor might be open. Sometimes a directly-coupled driver transistor becomes leaky or shorted can dam age both output transistors. Check the bias resistors for overheated and burned areas with a leaky or shorted output transistor. You might find the emitter bias resistor cracked into. Replace the damaged emitter resistor with a 2 or 5 watt replacement. When one output transistor is found to be leaky or open, replace both output transistors. If one is found to be open or leaky, you may end up replacing the driver and both output transistors.

FIG. 7. Test the suspected resistor within and out of the audio circuits.

FIG. 8. A typical driver circuit directly-coupled to two output transistors audio signal path.

TYPICAL DUAL-OUTPUT STEREO IC CIRCUITS

The dual-IC output stereo circuit within the RCA CTC1 57 TV chassis amplifies both audio channels. The left channel audio from the volume control IC (U1800) is coupled through

C1901 to pin 8 of sound output IC (U1900). Likewise, the right channel audio is fed into pin 4 by a 1 uF electrolytic capacitor C1900. The amplified power output signal is taken from pins 1 and 13 and fed to a 32 ohm PM speaker in each audio channel ( FIG. 9).

FIG. 9. The dual-power output IC (U1900) found in the RCA CTC1 57 TV chassis.

The +26 voltage supply source (Vcc) is connected to pin 12 of U1900. Pin terminal 14 is at ground potential. Both speakers can be muted with a signal from the mute switch (Q3107) at pin 7. Some stereo TV sound circuits are also muted at the volume control IC.

Go directly to the audio output transistors or IC for a distorted speaker. Suspect the dual- power output IC for improper voltage source when both speakers are distorted. Scope the audio signal in and out of the output IC. If a scope is not handy, check the audio at pin 4 and 8 of IC1900. Now check the output signal from the power IC to the PM speakers.

A defective power output IC can produce a weak, distorted, and intermittent audio. Replace the leaky dual-output IC, when one channel is distorted and the other channel is normal. The output IC is often leaky with a low voltage power source. Remove pin terminal 12 from the PCB with solder wick and iron. Flick the pin terminal with the small screw driver blade to make sure the terminal is free and no longer connected to the PC wiring.

Take a resistance check from pin 12 to the common ground. The power IC is leaky if the resistance is below 100 ohms. Now measure the +26 voltage supply at the pc wiring. If the voltage returns to normal or a few volts higher, IC1900 is leaky or shorted. Before removing and replacing the suspected leaky IC, check all components tied to the pin terminals of IC1900.

WEAK STEREO RIGHT CHANNEL

The weak stereo channel can be caused by a defective AF, driver or output transistors. The open AF, driver or buffer audio transistor can cause a weak audio signal. A defective driver or audio output IC can cause a weak right or left channel. Suspect an open electrolytic or bypass capacitor for a weak audio symptom. Check for a leaky or open bypass capacitor for weak sound on pins connected to the power output IC. Shunt the speaker coupling electrolytic capacitor between speaker and power IC for a weak stereo channel. Check for a defective electrolytic capacitor connected to the volume control for weak sound.

The unusual weak sound problem might be caused by a leaky or open diode off of the volume control pin 37 of the analog interface IC (AIU). The AIU IC (U3300) controls the OSD (on-screen-display), brightness, tint, color, contrast, and audio circuits in the RCA CTC157 chassis.

FIG. 10. The unusual sound symptom was caused by a leaky diode (D3306) in the AIU IC3300 volume control circuit.

LEAKY

Check for weak sound with an open electrolytic coupling capacitor within the MPX de coder IC. Don’t overlook improper voltage source feeding the sound circuits. An open voltage regulator transistor or low voltage from the power supply can cause a weak sound problem. Shunt each electrolytic in the voltage source feeding the sound circuits for weak reception.

DISTORTED LEFT STEREO CHANNEL

The weak volume and distorted symptom can be caused by defective electrolytic in the power output IC circuits. Distorted audio can result from a leaky driver or power output transistor. The left channel might be distorted with a leaky dual-power IC. Besides leaky capacitors, suspect a change in resistance for distortion within the driver transistor circuits. Remove one end of the suspected resistor from the circuit and test for an increase in resistance; this is especially true of resistors with large ohm values. After several hours of operation, the audio became distorted with a defective power switch in a Magnavox stereo receiver.

Check for poor soldered connections upon AF or driver transistors for distortion in either channel. The audio was distorted in an RCA CTC156 chassis with a leaky U1900 power output IC. A leaky SIF IC can cause distorted audio. Touch up the discriminator, quadrature or detector coil for weak and garbled sound. Suspect poor IF transformer connections for distorted audio. Check for cracked SMD resistors in the sound circuits for distortion. Suspect a video/Chroma/SIF/IC for distorted audio. Improper voltage or low negative voltage to the power output transistors can cause distortion.

INTERMITTENT STEREO SOUND

Intermittent electrolytic-coupling capacitors or poor capacitor connections can cause intermittent audio. Check for poor board connections on sound ICs or PCB. Place a little pressure on the body of the IC with an insulated tool and notice if the sound cuts up and down. Suspect a defective audio transistor after the sound quits in one hour or so of operation. Spray transistor and IC components with several coats of coolant to make the sound pop on and off.

Intermittent sound can result from a defective preamp or power output IC. Monitor the audio signal in and out of the suspected channel. Don’t overlook a defective level control for intermittent audio.

Distorted audio can be caused by broken resistors or an increase in resistance of bias resistors. Check for a defective AF or driver transistor when the sound blares out at high volume. Bad soldered connections on small resistors in the audio voltage sources can cause intermittent sound. Check for poor soldered griplets, feed-through wires or bars for intermittent audio. The defective mute transistors can also cause intermittent sound in the speakers.

In the RCA CTC167 chassis, the intermittent sound would cut in and out of both channels. Both stereo channels were monitored at the Summer transistors (Q1703 and Q1704), to no avail. The sound became intermittent in the left channel upon pin 1 of the left matrix IC (U1702). Voltage measurements upon Q1702 turned up nothing. When taking critical resistance measurements in the matrix circuits, R1753(120K) had increase in value ( Fig. 11). These same stereo circuits are found in the latest RCA TV stereo chassis.

FIG. 11 R1753 increased in value causing intermittent sound in the RCA CTC167 matrix stereo system.

BOTH STEREO CHANNELS WEAK OR DISTORTED

When both stereo channels are defective, check components or power sources that are common to both stereo channels. For instance, a high voltage source might feed +157 volts to one of the sound output transistors within each left or right stereo channel. A low negative or positive voltage source to the output transistors can cause a weak or distorted channel. A defective voltage regulator transistor, low ohm isolation resistor or open decoupling electrolytic can cause weak arid distorted conditions.

A leaky dual-driver IC or power output IC can cause a weak and distorted audio symptom. Likewise, a defective dual-preamp IC can also cause sound problems in both audio circuits. The defective matrix IC can cause sound problems in both audio channels. A defective dual-audio switch IC or dual-volume control IC, in the latest TV audio circuits, can cause distorted and weak sound in both channels. Replace the dual-output IC when the left channel is noisy and fuzzy after 5 minutes of operation.

DEFECTIVE BASS AND TREBLE CIRCUITS

A dirty or worn volume, bass or treble control can cause intermittent and distorted sound within the audio circuits. The open bass or treble control has no action when rotated. The poor terminal or broken connections might cause a no treble or bass sound. Check for a shorted wiping blade inside the control for a no audio symptom.

The audio can be intermittent with a low hum in the sound with an open volume control. The sound might be noisy when the stereo receiver is just turned on and caused by a defective volume or treble control. The left channel might be dead with a shorted left channel volume control. Suspect a defective coupling capacitor for a weak or no audio symptom, when wired to the volume or treble control.

STEREO RECORDING CIRCUIT PROBLEMS

Most problems with stereo playback or recording circuits are dirty or worn function switch contacts. Spray cleaning fluid down inside the switching area. Rotate the function switch back and forth to help clean the silver contacts. Replace the original function switch if there are worn or broken contacts.

A no record symptom in the left channel can be caused by a defective tape head. Suspect a worn tape head when the cassette player operates in the play mode and only a rushing noise in record mode. The distorted right channel might result from a dirty or packed oxide upon the tape head. An open right channel tape head might result in no playback or recording.

The intermittent recording was caused by a defective bias control (10K ohms) in a Silver Marshall cassette player. Check the recording amp transistor or IC for improper voltage in record mode.

The poor or no recording can result from a defective bias oscillator circuit. Poor erasing of the previous recording can produce a jumbled or cross-talk recording. The bias oscillator might operate from 60 to 100 kHz frequency range to erase the previous recorded music and provide linear recordings. In the early bias erase circuits, a dc voltage was switched into the head circuit to erase the previous recording in the low-priced cassette players ( FIG. 12).

FIG. 12. Low dc voltage is switched into the inexpensive cassette player to erase the previous recording.

You might find one bias oscillator transistor circuit exciting both stereo erase and recording channels in the cassette player. The erase head is excited from the oscillator coil or trans former as the erase head is switched into record mode. Both stereo tape heads are excited by the bias oscillator through isolation resistors and coupling capacitors. The bias oscillator 11.4 supply voltage source is switched only in the record mode ( FIG. 13).

No recording is noted when Q501 becomes leaky or open. Improper supply voltage can cause poor recordings. Take critical voltage measurements upon Q501 and compare to the schematic. Check for broken or poor oscillator coil connections for intermittent recordings. Scope the left and right stereo tape head terminals for a waveform in record mode. Suspect a defective bias oscillator circuit with no waveform on the erase or stereo tape heads.

FIG. 13. A dc voltage is switched into the bias oscillator circuit to excite the stereo and erase tape heads.

STEREO SPEAKER PROBLEMS

Speaker damage can be caused by too much volume applied to the speaker, weather conditions, and a dc voltage applied to the voice coil. Excessive power applied to the speaker can blow out the voice coil and cause it to drag or hang from the cone area. Wet weather can cause the cone to warp in the auto speaker. The speaker cone can come loose upon the framework and produce a blatting noise (Fig. 14 The solid-state amplifier can be damaged with an open speaker connection.

FIG. 14. The voice coil and cone can be damaged with too much output power applied to the speaker.

Always check the voltage at the speaker terminals, when the voice coil is frozen upon the magnet pole. A leaky output transistor or IC can destroy the voice coil by placing a dc voltage at the speaker terminals. The defective output component can place a dc voltage upon the speaker terminals if a electrolytic capacitor is not connected between amp and speaker. The voice coil heats up with applied voltage and drags or remains frozen against the magnet pole. A balanced output circuit should have zero voltage where the speaker is directly-connected to the output transistors or ICs.

Check the suspected speaker with an ohmmeter test across the voice coil terminals ( FIG. 15). Next inspect the cone for breaks or holes. Make sure the spider or cone is not loose upon the metal frame. Sometimes the cone or spider can repaired by re-gluing it into position. Place both thumbs upon the opposite sides of the cone and move the cone up and down. If the cone rubs or does not move, replace the speaker. Small holes punched into the speaker cone can be repaired with speaker or contact cement.

SERVICING AM/FM/MPX STEREO RECEIVER CIRCUITS

The left audio signal of an AM/FM/MPX receiver or cassette player can be signal-traced from the MPX IC2, through the radio-cassette switch S-4-1, Q13 and Q15, volume control, dual-power output IC3 to the left channel speaker. Signal trace the audio signal with the scope or external audio amplifier ( FIG. 16).

Determine if either the radio or cassette player are functioning. If the FM MPX receiver is not working and the cassette player is normal, the audio problem lies in the FM MPX IC2 or front-end circuits. When the radio circuits are normal and the cassette player sound is weak or distorted, check the tape head and preamp circuits ahead of S4-1 and S4-2.

A weak left channel is caused by a defective component in the left audio circuits. Check the audio signal at the left channel volume control and compare the signal at the right channel volume control. The greatest thing about the audio stereo circuits is that you can compare signal strengths against the defective and normal channel from the input to the output circuits. You can use either the test cassette in the cassette player or audio signal as the source in signal tracing.

FIG. 15. Check the voice coil with the low ohm range of DMM.

Start at the base terminal of the preamp (Q13) and compare the signal with Q14, when the audio is weak at the left volume control. You know the defective component is in the preamp circuits. Place the signal probe to the base terminal of the left preamp 015 and compare the signal at the base of Q16. Likewise check both sides of the electrolytic capacitor for a weak left channel. When the signal appears to be weak, you have located the defective circuit.

For instance, if the signal becomes weak at the output collector terminal of Q15 and the base signal was normal, suspect Q15. Take critical voltage measurements upon Q13 and Q15. Now compare these measurements with the right channel. Check both transistors with in-circuit transistor tests or a diode-junction DMM test. Take critical resistance tests on all Q15 terminals to common ground, if the defective component is not already located. Remember, weak audio signals can be caused by defective transistors, ICs, electrolytic coupling capacitors, and bias resistors.

SERVICING TV STEREO OUTPUT CIRCUITS

The TV stereo output circuits might consist of transistors, ICs or both. Only three output transistors are found in the audio output circuits of an Emerson MS1980R model ( FIG. 17). Each stereo channel has identical driver and sound output amplifier circuits. Notice the sound circuits have a much higher dc supply source then most solid-state sound out put circuits.

FIG. 16. Signal trace the audio signal and take critical voltage measurements to locate the defective stage.

FIG. 17. The left channel audio driver and output circuits in the Emerson MS198OR TV chassis.

C386 couples the audio from a tone control IC (IC371) to the base terminal of driver transistor Q356. A direct-resistor couples the audio at the collector terminal of Q356 to both base terminals of the sound amps (Q352 and Q354). The audio output is coupled from the emitter resistors to C394 (100 uF). A step-down transformer couples the power output to a 8 ohm PM speaker. The stereo headphone jack switches in the speaker or headphones when the plug is inserted into J351.

Service the TV sound circuits as any audio output stage. Signal trace the audio into the output circuits and at the speaker terminals. Check each transistor with in-circuit tests. A weak output circuit might be caused by an open driver transistor or electrolytic coupling capacitor. The dead output circuit can be caused by the open driver transistor, coupling capacitor, sound output transistors, or speaker coupling capacitor.

A leaky coupling capacitor, driver and sound output transistors can cause distortion in the speakers. Take critical voltage measurements on each transistor. Notice that these transistors operate from a much higher voltage source.

For a no audio symptom, signal trace the TV audio circuits with the scope or external audio amp. Check for an open transistor, IC or coupling capacitor for no audio in the speakers. Suspect a defective transistor or IC when the audio stops after operating for several hours. The no sound symptom can occur with a defective AF or preamp transistor or IC. A leaky or shorted output IC can cause no sound in both stereo channels. A defective switch or volume control IC can cause no audio in one or both audio channels. Suspect a shorted power output IC that might shutdown the whole chassis.

The no sound symptom might result from a blown power line fuse. Check the speaker fuse for open conditions when one channel is dead and no hum in the speaker. Suspect a defective speaker relay or poor relay points for no audio in the speaker. Check for poor or bad soldered board connections on power output transistors and IC components. A cold soldered connection upon coupling and bypass capacitors can cause a no sound symptom.

Don’t overlook a missing power supply source for no sound in both channels. Check the voltage across the main or largest filter capacitor. Next check the supply voltage source for correct voltage. Small ohm resistors (1.2 to 2.7 ohms) in the power source may be cracked or open. Inspect low ohm resistor terminals for burned areas upon the PCB, indicating a poor soldered connection. Open bias resistors in the emitter circuit of the audio output transistor or upon a power output IC can cause a no sound symptom.

Check for shorted or open electrolytic capacitors that may be off of their terminal pins of the power output ICs, for no audio. Within a JVC C1950TV chassis a no sound symptom with popping noises, resulted in a defective IC9Ol. Most no sound problems result from shorted or leaky output transistors and ICs with a blown line fuse. Don’t overlook the audio program set up procedures in the latest TVs for a no sound symptom.

After a couple of years it’s possible to collect several different no sound problems in a certain TV chassis. In the RCA CTC145 chassis, on different occasions, a no sound symptom was caused by Q1202, Q1203, and the sound IF IC (U1001). Check bias resistors R1209 and R1210 with a shorted output transistor. A no sound problem can be caused by a defective C1207 (100 uF) and C1206 (680 uF) electrolytics. Also check R1 211, a 5.6 ohm resistor off of collector terminal Q1202 for open conditions. Sometimes this resistor will open with a shorted Q1202 audio output transistor.

FIG. 18. Check the following components for sound problems in the RCA CTC145 TV chassis.

REPAIRING THE CASSETTE STEREO CIRCUITS

The function switch of an AM/FM/MPX boom-box player, switches in the cassette stereo circuits instead of the radio circuits. The same audio circuits are used for both receiver and cassette player circuits. Of course, a separate play/record switch provides playback and recording features. The preamp or equalizer circuit might contain a transistor or IC tape head circuit.

The tape head is switched into the base circuit through an electrolytic capacitor in play mode. The weak tape head signal is amplified and switched into the tape/radio function switch. Likewise the recording circuits are switched to the tape head to record the micro phone or auxiliary music. At the same time, the bias oscillator circuits are switched in for better recordings.

Most problems found in the cassette player preamp circuits are related to the tape head. A dirty RIP switch can cause weak or intermittent stereo channels. The dirty tape head might produce a dead left channel and weak right channel during recording or playback modes. A loud rushing noise might be caused by an open tape head or wires torn from the tape head. Poor tape head connections can cause intermittent audio.

Insert a cassette and notice if either stereo channel is active. When the right channel is dead or weak, locate the right channel preamp transistor. Touch the right channel tape head terminals with the blade of a screwdriver and the volume wide open. You should hear a loud hum. If no hum, take critical voltage measurements upon Q12 and compare them with the left channel of 011. Check the 7.45 voltage source on R122 (47K ohms). Test 012 with in-circuit transistor tests. The cassette music can be traced from the tape head to function switch S4 with external amp or scope.

TROUBLESHOOTING CD PLAYER STEREO CIRCUITS

Start at the D/A converter when either stereo audio channel is weak, dead or intermittent.

Signal trace the audio with the scope or external audio amp at the L and R channels of D/A converter IC. Locate the D/A converter upon the CD player chassis with a schematic layout drawing ( FIG. 20).

For instance, if the right channel is weak compared to the left channel line output, start signal tracing at pin 8 of IC706. Suspect IC706 and corresponding circuits if the signal is weak on pin 8. When the signal is normal at pin 8, check the audio signal at pin 6 of IC702. Compare this audio signal with pin 2 of IC702 ( FIG. 2). Proceed through the audio stages and through the line output circuits until the weak right channel component is located. Take critical voltages upon each IC or transistors. Shunt electrolytic capacitors with the exact capacity and working voltage to locate the weak audio signal.

FIG. 19. Start at the tape head and signal trace the audio with the scope or external audio amp in the cassette tape circuits.

FIG. 21. Signal trace the left and right audio channels on pins 8 and 21 of IC706.

A defective D/A converter IC has been noted to cause no audio, weak, intermittent, and distorted sound. Check the D/A IC for a background noise when the music is playing. Defective electrolytics have caused weak, dead, noisy, and distorted music. The defective Ram IC in the CD player can cause a high-pitched noise, distortion and a ticking noise in the sound when the disc is playing.

The defective line amplifier can produce a clicking noise, distortion after warm up, and intermittent static in the audio. Check for poor soldered pin connections for intermittent sound of the audio amp IC. Replace the IC amp for frying noises. Check the supply voltage source for low voltage with weak and distorted music symptoms. A defective power source voltage regulator can cause weak or no audio, intermittent audio, and a clicking relay with poor regulator transistor terminals in the CD player.

FIG. 20. Locate the suspected D/A converter IC on CD player PCB.

SERVICING THE AUTO STEREO CIRCUITS

In the early auto receivers, the auto front-end (AM/FM/MPX) circuits are switched into the stereo audio output circuits at the volume control. A cassette player IC preamp circuit amplifies the tape head circuits when the cassette is inserted into the radio. When the cassette is inserted, the receiver circuits are switched out with a function switch or with fixed diodes.

When the receiver circuits are functioning and no tape music can be heard, signal trace the tape head and preamp circuits. If both tape head and radio circuits are dead, weak or intermittent, signal trace the audio output circuits.

Check the audio at the input circuits at pin 6 of both audio output ICs (IC4 and IC5). Suspect IC4 when the left channel is weak or dead and the right channel is normal ( FIG. 22). Likewise, suspect IC5 when the right channel is noisy or distorted with a normal left channel. Check for low or improper voltage source when both channels are dead or weak. Check all components tied to the audio output ICs before removing and replacing the suspected IC.

Suspect an open tape head with a loud rushing noise in one channel. Check for a broken tape head wire for no sound, when the cassette player is normal in the play and reverse mode. Suspect a dry supply reel when the audio seems to flutter during playback. Check for an open electrolytic coupling capacitor for motorboating in one channel. A loud pop ping noise can be caused by poor soldered connections on the output IC. Replace IC4 for a low rushing noise in the left channel with the volume turned way down.

The left channel was dead in an older Motorola FM481AX car radio. The right audio channel was normal. Q9 was found leaky and replaced. The left channel was still dead. Several voltage measurements were off on Q7 and Q9 ( FIG. 23). The output transistor Q9 measured 3.5 volts on the collector and should have been 1.8 volts. Driver transistor 07 measured only 3 volts upon the collector terminal and should be around 11.4 volts. The emitter terminal of 07 was quite high (2.9v). A shorted electrolytic C7 (100 uF) was replaced and solved the dead left channel.

FIG. 22. The stereo cassette tape head circuit in the auto receiver.

After removing the suspected power output transistor and locating a correct replacement, test the new replacement before installation ( FIG. 24). Check the replacement with a transistor tester or the diode test of a DMM. If not, many hours can be wasted looking for other problems. Believe it or not, you can receive a new replacement transistor or component that is defective.

FIG. 23. Capacitor C7 destroyed 09 and produced improper voltages on Q7 and Q9.

REPLACEMENT OF POWER OUTPUT TRANSISTORS

Most audio output transistors are mounted upon a heat sink with a piece of insulation between transistor and metal heat sink; the heat sink might be the metal chassis ( FIG. 25). Apply a coat of silicone grease on both sides of the mica insulation. Since the emitter and base terminals are offset with aTO-43 type transistor, you cannot place it incorrectly within the socket.

FIG. 24. Check the new transistor replacement on transistor tester or diode-test of DMM.

Some heat sinks are mounted on top of the PC board. Here, the heat sink is insulated away from other components and does not require a piece of insulation between heat sink and transistor. Silicone grease should be placed upon the heat sink to provide greater heat dissipation from the transistor to the metal heat sink.

The metal chassis of the auto receiver might serve as a heat sink for the power output transistor or IC. Sometimes a large heat sink is mounted at one end of the radio and bolted to the metal chassis. The output transistor or module are bolted to the metal heat sink. Make sure all metal mounting screws holding the transistor or IC are snug and tight against the heat sink. A loose mounting screw that connects the collector terminal to the output circuit might produce intermittent or erratic reception. Now solder the transistor or power IC terminals to the PC board.

FIG. 25. The metal chassis might serve as a heat sink in TV chassis.

FIG. 26. A large heat sink provides heat dissipation in the auto radio.

REPLACING POWER OUTPUT ICS

After determining the power output IC is leaky or shorted, remove it from the heat sink and chassis. Remove the many terminal pins with solder wick or a sucking tool. Lay the solder mesh material along side a row of IC terminals and heat with a large soldering iron or gun. Apply excessive heat upon wick material and move it alongside of the IC terminals, picking up excess solder. A de-soldering bulb, iron or de-soldering pump helps to remove solder around the pin terminals.

Make sure all solder is removed from each pin terminal and pc wiring. Flick each pin terminal with an insulated tool or small screwdriver to make sure the pin is loose and free. Remove the metal screws holding the IC component to the heat sink.

Replace the IC with original part number. If not available, look up the part number in a universal replacement manual for an replacement. Universal ICs and transistors work nicely in the audio circuits. Universal solid-state components can be found in RCA, Sylvania, NTE, Howard Sams cross reference, and Japanese universal replacement manuals.

The large IC sound output circuits are often mounted upon a large heat sink with many metal fins for cooling ( FIG. 27). The heat sink is usually mounted directly on the PCB. Apply silicone grease on the heat sink and IC component for greater heat dissipation. Bolt the large IC to the heat sink with the same metal screws. You might find plastic screws on a few IC components that insulated the IC away from the heat sink. Snug up the screws or bolts, so the heat sink will make a very tight connection.

FIG. 27. Many cooling fins are found in the power output IC heat sink.

Solder up all pin connections to the PCB. Check each pin terminal for excess solder that might bond two or more terminals together. Remove excess solder around the pin terminals and solder up again. Check the resistance between each succeeding pin terminal for possible leakage. Clean off all excess rosin around the terminals for a clean appearance, with cleaning spray or brush.

TUBE STEREO OUTPUT CIRCUITS

The high-wattage tube-amplifier might have dual-triode tubes as AF and driver stages. The output circuits might consist of four power output tubes in a push-pull operation. Two of the power output tubes have the plates tied together and feed into the primary winding of the output transformer. The secondary winding might provide a 4, 8, and 16 ohm speaker impedance taps ( FIG. 28). You might find ultra wide-band toroidal output transformers. in some tube circuits. The 100 watt amplifier can operate from 475 to 550 volts of low voltage power supply.

FIG. 28. A block diagram of the 100 watt tube amplifier with four output tubes in parallel and push-pull operation.

The stereo tube circuits have identical high-powered circuits from 6 to 10 tubes with the AF and driver circuits containing dual triode tubes. Two separate single-channel tube amplifiers can serve in the stereo system. The 20 watt amplifier can contain four dual-triodes and four output tubes. A 50 watt stereo push-pull tube amplifier might have four 6550 or KT88 output tubes. The EL34 output tubes are found in 20 and 30 watt amplifiers. Some of these tube amplifiers today appear in kit form from $700 to $1400.

The distorted tube amplifier can be caused by a gassy, weak or shorted vacuum tube. A bluish-red color inside the tube might indicate a gassy tube. Check the suspected tube within the tube tester. Substitute another new tube when a tube tester is not available. A matched set of tubes should be installed in the audio output circuits. Although a complete set of tubes is rather expensive, keep one each of the preamp, driver and output tubes on hand.

A shorted output tube might destroy the screen grid resistor and damage the power output transformer. Improper negative bias applied to the grid circuits of the output tubes can destroy or damage them. Check for a negative bias supplied from a power transformer wind through a bias adjustment control. An increase in resistance of a grid or plate load resistor within the AF and preamp circuits can cause distortion and a loud hum symptom.

Suspect a burned or open cathode resistor of a shorted or leaky AF or preamp triode tube. A change of resistance in the grid circuits of a preamp or AF tube can cause slight distortion and hum in the sound. Poor soldered input grounds can result in pickup hum in the speakers. A dirty or worn volume control can cause erratic or noisy audio. Excessive hum can be found in the sound with dried-up decoupling or filter capacitors in either the positive or negative voltage sources.

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