REPAIRING POWER OUTPUT AUDIO CIRCUITS

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In the early transistor radios the output circuits consisted of a driver transistor coupled by an interstage transformer to the output transistors in push-pull operation ( FIG. 1). The early car radio might have only a driver transistor and one higher-powered output transistor. The early portable tape player contained a PNP driver transistor and push-pull PNP output transistors, while the early TV audio stages consisted of two transistors or an IC circuit driving two output transistors.

FIG. 1. A block diagram of the early transistor radio and cassette player PNP output circuits.

The power amplifier delivers audio to a load, such as the speaker. The power amplifier might be a tube, transistor or IC component. Today, the power amplifier delivers audio power to drive several speakers. In stereo circuits the power amplifier is found in both output stages of the left and right channels. The vacuum tube power amplifier is still found in a tube amplifier chassis. A high-powered amplifier might contain many directly-coupled transistors to provide up to 1000 watts of power or more.

THE BLOCK DIAGRAMS

Although the output circuits of the table radio are easily serviced without a schematic, the high-powered (100 watts & up) car amplifier block diagram might come in handy. A schematic diagram is a must when servicing high-powered receivers or amplifiers. You can quickly locate the defective circuit on the block diagram of the various IC’s and transistors found in the high-powered amplifier. Now locate the suspected stage from the block diagram to the components found on the chassis.

With the block diagram, notice how the preamp circuits tie into the audio circuits, where the tone controls and volume contacts are located, and what section of the power supply connects into the various circuits. Notice how the left and right stereo channels are tied together with dc to dc converter stages ( FIG. 2).

FIG. 2. The block diagram of an auto receiver 170 watt high-powered amplifier.

In the high-powered auto amplifier the preamp stages might be two IC’s (U1 and U2). Notice the tone control is tied into the preamp circuits. A muting transistor Q101 is found ahead of the output power amplifiers. Q104, Q105, Q106 and Q107 are directly-coupled driver transistors. The power output transistors consist of Q108 through Q112. There are eleven transistors used in each stereo channel to produce a 170 watt stereo amplifier.

Q110 is the overload transistor in the left channel while Q201 prevents overload in the right channel. 03 provides shutdown protection. A 14.4 volt input from the car battery provides the dc to dc converter to step up the +34V and a -34 volts to the power output transistors.

THE TRAIL DRIVER

The driver transistor provides audio power to the output circuits. A driver transformer couples the driver transistor to the driver output stage. Sometimes the driver transformer is called an interstage transformer. The driver circuit in the early transistor radio or portable cassette player consisted of a driver transistor, transformer-coupled to a push-pull PNP transistor output circuit ( FIG. 3).

Notice the PNP driver transistor has a higher negative voltage on collector and a -0.9 volts on the base terminal. Although the emitter has a -0.8 volts and the base 0.9 volts, the forward bias voltage is only -0.1 volt difference. The forward bias upon a PNP transistor is around 0.3 volts. The collector terminal of driver transistor (TR3) is a -4.4 volts supplied through the primary winding of Ti. The 200 ohm voltage dropping resistor and decoupling capacitor (470 uF) provide a decoupling filter network from a 6 volt battery source.

FIG. 3. The early PNP transistor transformer-coupled driver transistor to two push-pull output transistors.

The driver transistor might be directly-coupled or provide capacity-coupling to the output transistors. In some higher-powered directly-coupled circuits a silicon diode might be found from the driver collector terminal to the base terminal of the two output transistors. The AF transistor audio signal is fed from the volume control to the base terminal and the output of the collector terminal is tied to a coupling capacitor C216 (1 uF). The negative terminal of C216 is tied to the base of driver transistor Q206 ( FIG. 4). Q206 driver transistor collector terminal is connected directly to the base of output transistor Q208.

FIG. 4. The directly-coupled power output stages found in the cassette auto receiver or tape deck.

A no left channel symptom might be caused by an open driver transistor and an increase of voltage on the base terminals of Q208 and Q210. The leaky driver transistor in the auto radio might create only a hum in the speaker. The open driver transistor might result in a dead right channel. The driver located in a Motorola auto receiver with a dead left channel. A very weak and distorted audio was found in a Craig auto receiver with a leaky driver transistor. The weak and distorted music might result from an open driver transistor.

When a leaky driver transistor is located, always check the directly-coupled output transistor for possible damage. Check the emitter bias resistor for a burned, or a change in resistance with a shorted, driver transistor. Suspect the driver transistor with a noisy right or left stereo channel. Simply replace the driver transistor. The leaky driver transistor might cause the voltage dropping resistor to run warm or change value. A weak and distorted or distorted with no sound can result from a leaky or shorted driver transistor in the high-powered amplifier.

The dead right channel and a normal left channel can be caused by burned bias resistors, leaky driver transistor, and leaky output IC in a J.C. Penney 683-3845 amp. The speaker relay might click on and then hum with poor soldered driver transistor terminals; solder all three driver transistor terminals. Check for a leaky driver transistor when only the power switch operates in a Kenwood receiver. Intermittent audio can result from an intermittent driver transistor under load; the driver transistor might test normal in-circuit, replace it anyway.

SINGLE-ENDED OUTPUT CIRCUITS

Only one power output transistor was found in the early car receiver or table radios. The single audio output transistor might be coupled with an interstage transformer from the driver transistor. The single output transistor in the auto radio might be driven by two directly coupled AF amp transistors. A tapped output transformer had only one winding with the tap tied to the 10 ohm PM speaker ( FIG. 5). The rear metal chassis serves as a heat sink for the output transistor.

FIG. 5. Only one power output transistor is located in the output circuits of the car radio.

The AF amplifiers and output transistor work in a directly-coupled circuit. All three audio transistors are tied together. When Q9 becomes leaky or shorted the power output (Q10) might also be destroyed. A weak and distorted audio symptom might be caused by an open AF transistor. Replace the emitter bias resistor (0.68 ohms) when Q10 becomes leaky or shorted. If the car radio is left on too long, the T2 winding might be damaged. Replace T2 when the outside cover shows a dark brown and burned area. Check Q10 for a dead and distorted hum in the speaker. Replace output Q10 if there is a loud popping noise in the speaker.

YESTERDAY’S TRANSISTOR OUTPUT CIRCUITS

Besides the single-ended audio circuit, the early low-powered transistor audio circuits contained a driver and two output transistors in push-pull operation. This type of circuit was found in the table radio, auto receiver, tape deck, and phono amplifier. Driver transistor Q103 was coupled to the output transistors Q104 and Q105 with an interstage trans former. Notice that the driver transistor is an NPN and the outputs PNP types ( FIG. 6). Q104 collector terminal is at ground potential.

FIG. 6. The driver transistor is coupled via an interstage transformer in the early low wattage amp circuits.

An open driver transistor might provide a weak or dead audio symptom. You might find 0104 leaky and Q105 open with distorted reception. Replace both output transistors when one is found leaky or shorted. Check the bias resistors and thermistors for a change in resistance with a shorted output transistor. Suspect the 470 uF electrolytic speaker coupling capacitor when intermittent reception occurs.

TODAYS TRANSISTOR OUTPUT CIRCUITS

The transformerless audio output circuits have returned in the low-priced 13 and 19 inch TV chassis ( FIG. 2) In the RCA CTC145 chassis, an NPN audio amp is coupled through two diodes to the base terminal of Q1202. The output base terminal of Q1203 is directly coupled to the collector terminal of AF amp transistor (01201). The collector terminal of Q1203 is grounded, while a higher voltage, 18.2 volts, is fed to the other audio output Q1202. The 32 ohm speaker is coupled to the emitter terminals by C1 207 (100 uF) capacitor. The audio voltage source is fed from a scan-derived flyback secondary circuit.

FIG. 7. The transformerless audio output transistor circuits are found in today’s 13 inch TV chassis.

A weak and distorted audio symptom might be caused by a leaky AF transistor Q1201. When either Q1202 or Q1203 become leaky or open, replace both output transistors. Low distortion with weak sound might be caused by CR1201 or CR1202. Check bias resistors R1209 and R1210 (2.2 ohm) when a leaky output transistor exists ( FIG. 8). Intermit tent and no sound can result from a defective C1207 (100 uF). Replace both Q1202 and Q1203 and bias resistors R1209 and R1210 with a no audio or dead speaker. Check CR4120 and C4135 (470 uF) for no or improper 18.5 volt source in the flyback power supply circuit.

FIG. 8. The AF amp and output transistors in recent RCA CTC145 TV chassis.

Suspect open R1 211(5.6 ohm) with no voltage on the collector terminal of output transistor Q1202. Replace a leaky Q1202. Shunt electrolytic capacitor C1 206 (680 uF) when motor boating is heard in the speaker. Check both Q1202 and Q1203 when a frying noise is heard in the speaker. Replace both transistors since they cost only a few cents.

POWER IC OUTPUT CIRCUITS

The early IC circuits were found in the preamp and AF circuits of the cassette player, auto radio, and home receiver. Then along came the power IC mounted on a large heat sink. Next appeared the IC with both stereo output channels in one IC component. Today, you see large IC’s with the entire audio stages in one component of a high powered receiver ( FIG. .9)

FIG. 9. You might find a complete audio circuit within one large IC of a Technics radio-receiver AM FM-MPX chassis.

The power output IC might have a single IC part in each stereo channel of the portable radio-cassette player, tape deck, console radio-phonograph and auto receiver. A preamp IC and recording amp IC might precede the single output IC in a deluxe cassette car stereo recorder. The single output IC might be driven by transistor AF and driver stages in audio circuits of the table and auto radios. The main output IC has capacity coupled input and output to the PM speaker and headphone circuits ( FIG. 10).

FIG. 10. A single power output IC is found in each stereo channel of the early portable radio-cassette tape deck and auto receivers.

DUAL CHIP-DUAL SOUND

The dual-IC output might be included in the home AM/FM/MPX, tape and phonograph player with a couple of speakers. A boom-box, table-top receiver and cassette player, compact stereo amp and cassette player/recorder deck might contain a dual-IC in the power output circuits. The dual-IC component might have only the output IC’s with driver IC or transistor AF driven circuits. The compact amp, radio and cassette recorder might have a smaller dual-IC for both stereo channels. This dual-IC might contain all of the audio circuits, from volume control to the speaker terminals ( FIG. 11).

FIG. 11. The black lines represent the audio path from volume control to each stereo speaker through dual-IC201.

The left and right input signal is controlled by a volume control and coupled to the power IC circuit by a 0.1 F coupling capacitor to terminals 1 and 15. The audio signal is amplified by IC201 and capacity coupled to the left and right channel speakers through C227 and C229 (1000 uF). Notice the large dc supply voltage on pin 7 of IC201. Terminal pin 8 is the ground terminal.

Replace the dual-output IC when both left and right channels are dead. You might find the left channel is dead with a normal right channel caused by a leaky power IC. Replace dual-IC output (LA41 26) when both channels are dead in the Panasonic RX51 00 amplifier. You might find a leaky driver transistor, burned bias resistors and a leaky output IC with a dead stereo channel. Replace the dual-IC with distortion in both channels. Check for burned or open low-ohm resistors with a leaky output IC.

Suspect the dual-output IC with a noisy-crackling sound in both output speakers. Replace the power output IC with a frying noise in both or one audio channel. Check for open bypass electrolytic capacitors off the output IC terminals for a low hum noise. A constant motorboating sound in the speakers can result from poor ground connection of the power IC or the IC. The loud-crackling noise can result form a defective output IC. Suspect a defective output IC with a loud-popping noise in the speakers. Replace the open protection IC for no sound in a Hitachi receiver. Replace the dual-IC when a hum is heard in the left channel and no sound in right channel.

HEAT SHIELD PROBLEMS

The large heat sink can cause many audio problems within the amplifier section. When the large power IC is grounded directly to the heat sink and the metal sink has a poor ground, a dead, noisy, and intermittent sound can be heard in the speakers ( FIG. 12). Intermittent sound might result from a defective output IC terminal and heat sink not being properly grounded.

FIG. 12. The large heat sink helps dissipate the heat from the large power IC in a stereo receiver.

Check the heat sink for overheating of the shorted power IC instead of touching the IC component. Be sure and check for open jumper wire connections that tie from power IC to PC wiring. Tighten the power-IC mounting screws or those on the PCB where the IC is mounted, for intermittent low hum noises. Check for poor IC screw connections when moving or touching the cabinet, when the speakers make a cracking noise. Sometimes it’s best to run a separate ground wire from power IC screw and shield to chassis ground.

SPEAKER RELAY PROBLEMS

The defective relay might have dirty or bad points that can cause no audio or intermittent sound. One channel might cut out with weak volume with a bad relay. The volume was intermittent with a bad relay in a Fisher CA272 amplifier. The relay might not click on or off with a dried-up 4700 uF filter capacitor and open 4.7 ohm resistor in the power supply.

The relay might click on and off with a hum in the sound caused by poor soldering connections of driver transistor. Replace the relay for intermittent sound in the speakers. Replace relay or volume control when the sound is intermittent in a Fisher amplifier. Check the relay for one channel intermittent and the other channel low in volume.

An unusual problem existed in a Pioneer receiver when the audio was intermittent and both the power amps were replaced with a defective relay.

The speaker relay cuts out when the power IC’s were normal, the audio was intermittent, and the speaker relay clicks off and on with an open D51 9 bridge rectifier in a J.C. Penney receiver amplifier. Always replace the defective relay instead of trying to repair it. The relay might not energize with a shorted output power IC.

NO SOUND-DEAD AMP

Check for an open or blown fuse in the receiver or large amplifier. If the fuse keeps blowing, check for leaky silicon diodes, filter capacitor, and output transistor or IC components. Next, measure the voltage across the filter capacitor to determine if the low voltage supply is normal. Check the supply voltage source at the audio output transistors or power IC’s to determine if the voltage source is dead or improper voltages ( FIG. 13). A defective power switch can cause a dead amp or receiver.

A dead left audio channel might be caused by a leaky driver and power output transistor.

The left channel might be dead with a burned bias diode and leaky output transistor. Both channels were dead in a Pioneer amplifier with a leaky dual-output IC. In a J.C. Penney 3226 amplifier the fuse kept opening with leaky right channel output transistors, Q209, Q210, Q211 and Q21 2. The audio channels can be dead with poor soldered connections on the volume control or a shorted volume control.

A defective channel switch can produce a dead amplifier. The Soundesign amplifier circuits were dead with an open winding in the power transformer. Both channels might be dead with a leaky 2200 uF electrolytic capacitor in the power source. A shorted decoupling capacitor can cause a dead left channel with a good right channel. The dead right channel might result from a hole blown out of the output IC. The blown speaker fuse produced a dead channel with high voltage on the speaker terminal, and caused by a shorted output transistor or IC. A dead left channel might result from a power amp poor board-ground terminal.

Most dead receivers or amplifier circuits are caused by leaky power output transistor or IC components. Replace the driver transistor when the output transistors are found open or shorted. Check for leaky bias diodes and burned bias resistors in the transistor output circuits. Sometimes all components should be replaced in the output circuits when leaky output transistors and components show signs of overheating ( FIG. 14) Don’t over look a dead channel with open (470 uF or 1000 uF) coupling capacitor to the speakers.

FIG. 13. Check the supply voltage at silicon diodes, large filter capacitor, or upon the large power IC to determine if voltage source is normal.

WEAK SOUND

Check all transistors, IC’s, and electrolytic coupling capacitors for weak sound in the amplifier. Inject a 1 kHz signal at the volume control to determine if the front-end preamp or AF stages are weak or the AF, driver and output transistors or power IC’s are defective. Measure the voltage at the power output transistors and IC components to determine if proper voltage is found at the output circuits. Signal trace the weak front-end circuits with an external amp or scope. Compare the good channel with the weak one in stereo audio circuits.

Suspect a leaky or open driver transistor with weak sound. A weak volume with distortion can occur with a leaky driver and output transistor and low voltage on the emitter terminals. Replace the leaky bias diode with a distorted left channel and low volume. Check the electrolytic coupling capacitors (1 uF to 4.7 uF) when one channel is weaker than the other. Replace a 10 p coupling capacitor for a weak right channel.

The output transistors, driver transistor, and bias resistor were replaced with a weak left channel. Suspect an open bypass capacitor on the emitter terminal of the driver transistor for a weak audio channel.

In a Silver Marshall amplifier the driver transistor was replaced with a universal replacement (SK3122) with a weak left channel. Replace both high powered output transistors when the right channel is weak and distorted. Suspect a defective volume control with no volume in one channel. Check for low ohm resistors or an open voltage regulator source that feeds the AF or driver circuits ( FIG. 15).

DISTORTED AUDIO

Go directly to the audio output circuits for extreme distortion in the speakers. Check the output transistors for open or leaky conditions. The left output transistor might be open and the right channel leaky causing distortion with very weak audio. Remove one end of each bias resistor or diode and check for correct resistance or leakage. A leaky driver transistor might destroy both output transistors and bias resistors.

FIG. 14. Check the following parts when replacing defective audio output transistors.

FIG. 15. Check the following parts for a weak audio channel.

Check for a leaky coupling capacitor that can produce distortion in the audio circuits. The right or left channel can be distorted with a leaky AF or driver transistor. Replace both output transistors and bias resistors when either channel is distorted and has a loud hum.

Suspect a leaky dual-power IC with distortion in both stereo channels. After 5 minutes the right channel became distorted with a defective power output IC. Spray coolant on the body of a suspected IC for distortion and noisy reception to make it act up.

A slight hum was heard in a J.C. Penney amplifier circuit when the output IC voltage source dropped to 36.1 volts from 40.1 volts, caused by a defective 6800 uF filter capacitor. A loud popping noise with distorted sound can result from a defective output IC. Check the output IC for overheating when distorted audio is heard in the speaker.

ERRATIC OR INTERMITTENT SOUND

Erratic or intermittent sound might be caused by a defective driver or AF transistor. A defective speaker relay can cause intermittent sound in the speakers. A bad power switch might cause a noisy sound when turned on and off. The right side of a Sony large boom-box player had erratic sound with a loud rushing noise and was caused by a 47 pF 50 volt coupling capacitor to the volume control.

FIG. 16. Check the following parts for intermittent reception in the power output circuits.

Sometimes just moving components around with an insulated tool can cause a capacitor or resistor to act up. Pushing up and down on the PCB can uncover a poor soldered connection or a tie-wire circuit connection. By snugging up the mounting screws of the main chassis PCB solved the intermittent noise in a Westinghouse output amplifier. A bad soldered connection on the volume control can cause an intermittent and noisy left audio output channel.

The right channel was intermittent within a Soundesign amp caused by a poor speaker ground connection. The volume increased all the way up and then cut out and was caused by a 10 volt zener diode.

When one channel is weak and the other cuts in and out, suspect a dual-power output IC. Intermittent noise in the left channel and then the audio jumps in and out can be caused by a large power output IC. Check the large power IC when the sound pops in and out with distorted audio. Resolder all output IC terminals with intermittent sound symptom in both channels.

LEVEL METER PROBLEMS

The VU (volume units) meter indicator monitors the volume level in the audio channel. Very few electronic problems are found within the VU sound meters of a large amplifier or cassette deck. The stereo meters might be connected after the AF or driver transistors within the transistor output stage or after the power IC in the output circuits ( FIG. 17). Both VU stereo meters have the same type of audio circuits. When one or both audio meters don’t register, signal trace the audio signal right up to the VU meters with an external amp or scope tests.

FIG. 17. The level recording on VU meters are found in the output circuit of audio amplifier.

The LED amp signal strength or VU indicators might be operated from one large or two separate IC’s. In the large AM/FM stereo receivers a separate array of LED’s are operated from one large IC. The input audio signal from the left channel appears at pin 11 and pin 10 is the B+ supply source (Vcc). Each stereo channel has its own IC amp and set of LED indicators. The various LED’s are tied to terminals 1 through pins 7 ( FIG. 18).

FIG. 18. LEDs are used to indicate the signal strength or recording level.

Some LED sound indicators might have an array of LED’s for each channel while on small recorders one LED array is switched into the play and record circuits. In record mode, the LED’s might be switched at the output of preamp IC and in the playback mode at the output of power amp IC. When found in large stereo receivers, the LED’s are used as VU indicators.

The audio signal can be signal-traced right up to the LED IC or VU meter with scope or external audio amp. Very seldom do LED’s cause any problems. Check the suspected LED with the diode tests of the DMM. Most VU problems are related to a defective IC or improper applied voltage source. When the VU meter becomes erratic or intermittent, suspect a bad wire connection or meter. Sometimes the meter hand might stick in one spot if the cardboard indicator warps. Remove the front cover and re-glue the meter scale. If the meter hands go backwards to the incoming sound, reverse the meter leads.

SERVICING OUTPUT CIRCUITS IN THE EQUALIZER/BOOSTER

The stereo equalizer is a circuit that compensates attenuation to achieve equalization. The stereo equalizer/booster provides stereo frequency equalization with a boost of power. Besides a stereo frequency equalizer stage with separate frequencies (60 Hz, 150 Hz, 400 Hz, 1 kHz, 24 kHz, 6 kHz, and 15 kHz), the output circuits contain separate power output IC’s to boost the audio to several speakers. The stereo frequency equalizer-booster circuit might be found in the auto CD, cassette or receiver output circuits.

FIG. 19. A block diagram of an equalizer/booster output circuit.

The stereo frequency equalizer IC2 is coupled through C201 to input terminal 3 of IC4. The 20 watt power IC output is fed to a fader control and to the auto speakers. The output audio signal is also coupled to the level meter driver amp IC3 of the left stereo channel. IC3 provides drive signal to the five different level indicator LED’s. The audio level LED’s are tied to terminals 1 through 5 of IC3 ( FIG. 20).

FIG. 20. The left channel output lC4 and level meter IC3 circuits.

Signal trace the audio signal from the input of IC2 through IC4 and the output speakers with an audio external amp or scope. When the audio signal stops suspect that stage or circuit. Check for a blown fuse with no pilot light or dead amp. Measure the dc voltage input at pin 6 of IC4 and pin 9 of IC3. If no sound, suspect defective lC2 or IC4, with the pilot lamp on. Check all electrolytic coupling capacitors for open conditions.

Check IC4 applied voltage at pin 6. Suspect IC4 for distortion or insufficient sound. Check all electrolytic capacitors attached to IC4 terminal pins for open or leakage conditions. A defective fader control and speaker can cause audio distortion.

Shunt C126 (470 uF) for a motorboating sound in the audio. Check C127 (0.1 uF) for oscillation within IC4 circuits. Suspect IC3 when all of the LED’s don’t light up. Check each LED with the diode test of the DMM when one of them won’t light up. Measure the 14.3 volts on the anode terminal of each LED.

SERVICING BOOM-BOX PLAYER OUTPUT CIRCUITS

The early boom-box radio and cassette players might have transistor output circuits while others can have a separate output IC or dual-IC. The dual-output IC is found in the radio, cassette-CD player amp circuits. The dual-IC output is usually switched by earphone jacks into a tweeter and woofer speaker in each audio channel. The headphone jack is connected to each speaker output terminal through an audio dropping resistor (220 ohms).

The left volume control controls the input audio to the left input (pin 5) of IC201 ( FIG. 21). A right volume control (20 k-ohms) provides audio signal to the right input terminal 8 of IC201. The left amplifier audio signal is found at pin 2 and right signal at pin 10. Notice the large coupling capacitors of 1000 p that couple the audio to the speaker and earphone circuits. Pin 1 of IC201 is the voltage supply pin and pin 12 is at ground potential.

FIG. 21. The boom-box output circuits might consist of one dual-output IC.

The dead left channel might be caused by an open C205 or C21 1. A defective IC201 or open capacitors tied to the IC output pins can cause a dead circuit. Check the volume control for an open control or broken connection. Both channels might be dead from an improper voltage supply source.

If the headphones are operating and no sound to the left speaker, suspect a dirty ear phone switching terminal or speaker. When only the tweeter speaker is operating and no woofer sounds, sub another speaker across the woofer terminals. Sometimes too much volume applied to the speakers can blow open a voice coil. Suspect a dropped cone in the woofer speaker with a mushy sound.

Check for improper voltage at pin 1 and defective IC201 when both channels are distorted. A distorted right channel can be caused by a leaky C206, C21 2, or IC201. Remember the power output IC might be defective when either channel or both are distorted. Leaky output C21 2 (1000 uF) can cause distortion and a dc voltage upon the speaker terminals. C211 and C212 have been noted to cause an intermittent channel. Check the earphone jack for poor connections when one channel earphone is dead, erratic or intermittent.

TROUBLESHOOTING THE CASSETTE PLAYER OUTPUT CIRCUITS

The early cassette player might have a driver transistor transformer coupled to the output transistors. The radio-cassette player might have transistor or IC output circuits. The AM/FM/MPX receiver with a cassette player can have a separate IC or a dual-IC in the output stereo circuits. A monaural cassette player might have a transistor preamp and a power IC output.

The monaural output IC input signal is coupled from the volume control by C10 to input terminal 9 ( FIG. 22). The output audio is switched into the earphone and speaker by headphone jack (J1) from coupling capacitor C14 (470 uF). Both the preamp transistor and power output IC1 provide play and record audio signal.

FIG. 22. The power output IC circuits found in a typical cassette player.

Signal trace the audio signal from the tape head to the output IC with an external amplifier or scope. When the audio stops, check the coupling capacitors, transistors, and power IC’s for open conditions. Check all voltages upon the suspected IC or transistors. Take critical resistance measurements upon each IC pin terminal to locate a leaky IC. Suspect a leaky IC when low or improper supply voltage is found upon the output IC.

A weak sound with a loud hum can be caused by an open tape head. The weak channel can be caused by weak batteries or improper voltage source. Replace the power IC for a slightly weak right channel in a J.C. Penney cassette player. The weak left channel can result from a poor grounded tone control. Check for a leaky low value coupling capacitor for low audio in the play mode. A broken wire in the leaf switch might result in a weak or dead audio symptom.

Check and clean up the tape head for extreme distortion. An open preamp transistor under load might cause a slight distorted channel. Test the output transistor in-circuit for leakage and burned bias resistors with a distorted speaker. A bad radio-cassette switch can result in distorted and garbled audio. Suspect a power dual-IC with distortion in both channels.

Suspect coupling capacitors, transistor and IC for intermittent audio. Clean up the bias variable resistors for intermittent audio in one channel. Check the function switch for intermittent record or playback modes. First clean up the function switch. Replace if broken or has worn contacts. Check the volume control when you cannot turn down the volume and the sound becomes intermittent. Check all bypass electrolytic capacitors off of the power output IC for an intermittent left channel. Replace the output IC for intermittent right or left channel. Check for a bad muting switch for intermittent audio in the cassette player.

Check for open tape head or lead connection for a rushing noise in the cassette player. A low rushing or frying noise can result form a defective output IC. The noisy right channel can be located by pushing up and down on output IC with insulated tool. Check for a noisy right channel with an open electrolytic capacitor across the emitter resistor.

Inspect poor soldered connection or tie wires on the PCB. A bad function switch can produce a hissing noise in the speakers. A noisy and intermittent left channel can be caused by a defective output IC. Replace dual-output IC when one or both channels are noisy. Replace the power switch for an arcing noise when first turned on.

Within a large Sony portable radio and cassette player, the right channel had a rushing or frying noise like that found in a transistor or IC. The large output IC was replaced (TA7215P) and the results were the same. The cassette output circuits were signal traced with the external audio amp. The noise was found upon pins 15, 16, 17, and 18 and not on the volume control ( FIG. 23,). The 47 uF 50 volt electrolytic capacitor was found to be open. Replacing the electrolytic coupling capacitor solved the rushing noise.

FIG. 23. The open 47 uF electrolytic coupling capacitor caused a rushing noise in a Sony portable radio and cassette player.

A ground wire that had come off of the erase head caused a jumbled recording in a Sanyo cassette player. Check for a defective function switch in record or playback for intermittent recording. The intermittent recording can be caused by a defective microphone coupling capacitor. A noisy recording might result from a defective microphone. The weak recording on the left channel was caused by a packed tape head. The same sound circuits in the small cassette player might combine the record and play modes. Check both play and record modes to locate outside recording symptoms.

REPAIRING THE CD PLAYER OUTPUT CIRCUITS

The analog sound circuits within the CD player starts at the D/A converter (digital-analog). The left and right stereo circuits are developed at the output of the DIA converter. Often, one or two stages of audio in each channel amplifies the audio signal to the line output jacks and an earphone jack. Most CD players have a muting transistor in each output channel.

The early or low priced portable CD players might have a transistor amp in each stereo channel. One dual-IC might be located in today’s CD line and output circuits. The boom- box stereo radio, cassette and CD player might have the radio, cassette and CD signals from the D/A converter switched into a dual IC preamp and dual-output IC to each speaker. A separate headphone IC amp might be connected to the line output jacks.

Signal trace the audio signal from pins 10 and 9 through the preamp IC2 to the line output jacks for weak, intermittent or distorted audio. Often, distortion is caused by leaky coupling capacitors, transistors, IC’s and defective mute transistors. The weak channel might be caused by an open electrolytic coupling capacitor, transistor and IC amplifier. Improper voltage source to the IC amplifiers can result in a weak and distorted audio symptom. The normal stereo channel can be used to check out the defective or loss of signal in the other stereo channel.

Check the low pass filter (LPF101) for no audio in the left channel. Replace the D/A converter IC with no audio output. Check the Mechanism Micro IC when no muting during program mode. Replace the audio preamp IC for no audio at the line output or headphone IC. A dead-no power output can be caused by defective filter capacitors in the power supply. Check the 14 volt regulator zener diode when the protection relay won’t turn on the speakers. Suspect a defective voltage regulator with no audio output and LCD display.

Check the left preamp IC with distorted left channel. Suspect a defective RAM IC with distorted sound. Replace the audio output IC with distortion after the CD player has warmed up. Check all IC preamp terminals for poor connections with audio distortion. Replace the D/A converter IC for distorted audio.

Suspect the preamp IC with noise in either stereo channel. Replace the headphone amp IC with noise in both earphones. A background noise can be caused by a defective D/A converter IC. Check those small electrolytic coupling capacitors for a noisy channel. Replace the RAM IC when there is a ticking noise in the audio. Remove and replace the preamp IC for a popping noise in either channel.

SERVICING TV AUDIO CIRCUITS

The defective sound IF stages might produce hum and distorted sound. Check the discriminator coil adjustment for garbled and distorted audio. Simply touch up the coil adjustment until the sound returns to normal. Poor soldered connections upon the coil can cause intermittent and buzzing audio. Resolder pins 15 and 16 oft of IC101. Distorted audio can result from a coil or terminal connection of T101.

FIG. 24. The headphone circuits are connected to the line output circuits in a portable CD player.

The complete audio circuits might be included in one large IC201, except the sound IF circuits. The audio signal is taken from pin 23 of the SIF (IC101). The volume is controlled by an MPU. C209 couples the audio from IC101 to pin 2 of audio output IC20l ( FIG. 25). The amplified audio signal at pin 8 is coupled through a 220 uF electrolytic to output transformer T203. Some speaker circuits might be switched out of the circuit when the headphones are plugged in.

Check IC201 for weak and distorted audio. Suspect IC201 for intermittent and noisy sound. No audio might be caused by leaky IC201 or open 220 uF capacitor. Check all electrolytic capacitors on IC201 pin terminals for weak or garbled audio. Inspect the heat sink screws for weak and intermittent sound. Inspect for poor IC201 grounds for repeated failure of the output IC. Check IC201 for a hum and buzzing noise. Suspect IC201 for hum when the set is turned off.

Suspect C209 (4.7 uF) and C214 (220 uF) for intermittent and weak sound. The improper voltage source might cause weak or no sound. The dead audio speaker might be caused by open C209 and C214.

SERVICING THE HIGH-WATTAGE AUDIO AMP CIRCUITS

A 200 watt or higher audio amplifier circuit might have ten or more power amp transistors with several preamp circuits. The typical 170 watt auto amplifier has nine power amps, one muting, and two transistor for overload protection ( FIG. 267. Of course, the high- powered stereo amplifier has twice this amount of transistors. Both left and right stereo channels are identical. You can use the normal channel to compare audio signal and voltages with the defective channel.

After removing the top cover, inspect the chassis for burned or charred parts and wiring. Signal trace the audio from stage to stage with the external audio amp or with a scope and function generator. Connect high-powered loaded resistors across the left and right speaker output terminals. Don’t fire-up the high-powered amplifier without speakers or a load attached to the amplifier. Keep the gain or volume control as low as possible. Inject a 1 kHz audio signal at the input terminals. Check and compare the signal in each stage until the signal is lost in the defective channel.

FIG. 25. The sound output circuits are found in one C201 component.

Some technicians prefer to check all transistors first, before taking signal or voltage tests. Sometimes transistor tests might not be accurate when directly-coupled transistors or fixed diodes are found in the base and collector circuits. Take critical voltage measurements upon each transistor and compare them to the normal channel. A schematic is a must item where ten or more transistors are found in the audio output circuits. When a dc voltage is found upon the speaker terminals, the last two or more power transistors might be leaky or open.

Check the bias resistors for correct resistance when the power output transistor is leaky, shorted or open. Test both directly-coupled transistors when one is found leaky or open. A leaky driver transistor can damage one of the directly-coupled power transistors to open and the next transistor to become leaky. Replace all three transistors.

A dead left channel can be caused by a leaky coupling capacitor, driver and leaky output transistors. The weak and distorted output circuits can result form a missing 40 volts of either positive or negative voltage.

The woofer speakers can be damaged when the output transistors become open or leaky, applying a dc voltage upon the voice coils; this might occur when no coupling electrolytic capacitors are found between speaker and amplifier. The right channel might be distorted with a leaky driver transistor. The weak and distorted left channel might be caused by the power output transistors.

Suspect output power transistors for a popping and cracking sound in the speakers. replace both power output transistors when the fuse keeps blowing. Test each transistor for a short or leakage. Check the filter capacitors and voltage regulator circuits when a slight hum is heard in one channel. Suspect leaky or shorted power transistors when coupling or bypass electrolytics have a blown top. Check the output transistors when the speakers begin to hum after operating for 5 or 10 minutes. Check for burned bias resistors in both output transistor circuits. All audio problems related to any power amplifiers can occur in the high-powered amp circuits.

FIG. 26. The final directly-coupled output circuit in the 200 watt high-powered audio amplifier.

SERVICING TUBE OUTPUT CIRCUITS

The dual-triode tubes such as a 12AX7, 12AT7, 12AU7, 6BK11, 6U10, 6SC7, and 7025 were used in a driver stage. A push-pull power output tubes as the 6L6G, 6V6GT, 7027A, 7591, 6AQ5, 5881 and EL34 were found driving power to the speakers. Notice the high voltages found upon the screen grid and plate elements of each output tube. A choke coil and resistance filter network with small electrolytic capacitors are found in the B+ circuits.

The defective tube should be replaced when the amplifier becomes weak, noisy or distorted. Check the tubes in a tube tester, if available. Simply replace the tube with another known good tube is the best method. Sub another tube when one becomes noisy or microphonic. The output power tubes should be replaced with a matched pair. Pickup hum in the input circuit can result from resistor increases or open coupling capacitors ( FIG. 27).

FIG. 27. A typical tube output amplifier circuit employing a 12AX7 and two 6L6GT output tubes.

A gassy or leaky tube can cause extreme distortion. The leaky or shorted tube can dam age transformers and cause resistors to overheat and burn or change value. A quick resistance measurement from grid to common ground upon the output tube can indicate if the grid resistors have changed value. Likewise a check of the screen grid to common B+ can quickly find a change in the screen grid circuits.

Check the resistance from the output plate elements to the center tap of the output trans former can locate a defective winding. Extreme hum and distortion in the output circuits can be caused by output tubes, damaged choke winding, and filter capacitors. An improper negative bias voltage from the power supply can cause distortion in the audio output circuits.

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Updated: Wednesday, 2014-12-24 23:50 PST