Servicing High-Powered Audio Circuits

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The audio components within the high-powered amplifier circuits consist of many transistors and IC components. A high-powered amplifier might operate with an output power of 35 to 1000 watts.

The early home receiver output might be less than 1 watt, while today’s receiver-amplifier might average between 35 to 100 watts.

FIG. 1. The high-powered circuits are found in a separate auto amplifier.

The early auto-cassette receiver had less than 10 watts of output power and now might average 25 to 40X4 watts peak power output. Today’s auto CD receiver might have an average of 35 to 50X4 watts peak power output with a preamp output voltage of 0.5 to 4 volts, with the average around 2 volts output.

The average high-powered auto amplifier might have a 50X2 to 400X2 RMS at 2 ohm output. The average RMS power (watts X channels) 35X2 up to 300X2 watts with 14.4 volts of battery input voltage. The high-powered amp might have a built-in low pass (LP) or a high and low pass crossover network. A signal-to-noise ratio might be from 95 to 110 dB.

The fuse rating of the auto amplifier might be 15 to 60 amps with 2X20 up to 2X40 amps of power.

The Root-Mean-Square (RMS) measures the power in watts the auto amplifier can pro duce continually. The measured output power of an amplifier depends on the automobile’s input voltage. The average voltage from a battery, with the car operating, can be around 14 volts and 12 to 13 volts when the engine is shut oft. Likewise, the amplifier will have a higher wattage output at 14.4 volts than at 12.6 volts of input battery power. For instance, a Jensen KA2500 2-channel auto amplifier has a bridged RMS power output of 300X1 watts compared to 250X2 watts of peak power.


Most commercial audio amps have both a high and low impedance input. The high level jacks are to be used with the auto-cassette speaker output. A low-level input might match the separate CD line output jacks. Usually, the high and low level inputs are found in the auto stereo frequency equalizer/booster and high-powered audio amplifier. A low-level input might connect the preamp output circuits to the low-level input of the auto amplifier.

The high-level radio and cassette player outputs might be switched into the IC1 preamp circuits of an equalizer-booster circuit or low-powered auto amplifier ( FIG. 2). The high and low level inputs in a resistance network is switched by SW-i into the input of IC1. Both left and right stereo channels are switched with dual SW-i into the preamp circuits.

FIG. 2. The high and low level audio signal is switched into the input of IC1 preamp.

The high-level input might match the input impedance with transformer coupling. A transformer input is found on both left and right audio channels. Some auto audio amplifiers have speaker level inputs from the radio receiver, CD and cassette player. The multi channel amplifiers might have both speaker level input and preamp output jacks.


Besides the high-powered auto amplifier, a complete home theater audio system of a shelf or rack system might have a 3-CD to 24-CD player, dual cassette, AM/FM/MPX receiver, VCR, and system remote control features. These stereo receivers are found in the 50 to

100 watt systems. The dual-cassette player features normal and high-speed synchro dubbing, auto reverse, continuous play, and Dolby-BNR for clean, dynamic sound. The speaker system might include two 10 inch 3-way main speakers, two 4 inch full-range speakers for rear channel, and a shielded 4 inch full-range speaker for center channel sound.

FIG. 3. The high level output audio is coupled to the dual preamp through T101 and T102.

The high-powered amplifier might appear in 50 to over 1000 watts in the auto amp, auto shelf or rack system, auto receiver, CD player and PA systems. The lower voltage amplifiers might have power output ICs while the high-wattage amplifiers might contain many transistors in directly-coupled power output circuits. A typical 150 to 200 watt amplifier might contain two ICs in the preamp circuits, one transistor in the muting system, ten power amp transistors, and a single transistor in the overload and shutdown circuits, within each stereo channel.

The 150 watt amplifier might consist of two directly-coupled AF transistors, four driver transistors and four output transistors in a push-pull directly-coupled power output circuit. Q109 and Q110 are NPN power output transistors connected to the positive (+) 35 volt source. Q111 and Q1 12 are PNP power output transistors connected to the minus (-) 35 volt source. Q109 is a 2SC3421 or 2SC600 transistor, while Q110 is a 2SC3907 or 2SC351 9 NPN transistor. Q111 is a 2SA1 358 or 2SB631, while Q112 is a 2SA1516 or 2SA1 386 PNP transistor. Replace all power output transistors with original part numbers, when available.


The overload protection output circuit is designed to shutdown the DC to DC power supply when the output circuits become unbalanced. The directly-coupled output circuit to the left and right channels is balanced at zero volts. No dc voltage is found on the voice coil of a connected speaker (usually a woofer type speaker).

FIG. 4. A directly-coupled power output circuit of a transistorized 150 watt amplifier.

When a driver or directly-coupled power output transistor becomes open or leaky, the output voltage of Q1 09 and Q1 11 change, resulting in a dc voltage at the speaker output terminals.

If a dc voltage is left too long on the voice coil of a speaker, the winding of the voice coil will become warm and drag. The voice coil might result in a frozen or burned coil on the speaker magnet. Now the speaker is damaged and must be replaced. The overload output circuit is suppose to protect the woofer speaker or the speaker connected to the output terminals.

The dc voltage on the emitter terminals of Q109 and Q111 is zero. This same voltage is connected to the base and emitter circuit of the overload transistor Q113. D1 Q1 isolates the DC to DC power circuits from the overload transistor. When the power audio output circuits fail, a dc voltage is found at the base and emitter of Q113, upsetting the balanced output circuits. The overload transistor conducts, applying a signal voltage to the protection circuits in the dc power supply.

The protection circuit shuts down the power supply, disconnecting the positive and negative 35 volts to the output transistors, and removing the dc voltage from the speaker terminals. Each stereo channel has the same type of overload protection circuit.

The defective audio output circuits might come up and then shut down at once. Some times the audio output transistor or IC might become leaky and blow the main power fuse. The dc voltage on the speaker fuse might cause the fuse to open. When a blown fuse is found or the audio chassis fires up and shuts down, check the dc voltage at the speaker output jacks or terminals.

FIG. 5. Q113 provides overload protection to power output transistors Q109 and Q1 11.

A test speaker should never be connected to the amplifier that has a dc output voltage at the speaker terminals. Connect high-powered output speaker load resistors instead. Then take a low voltage measurement at the left and right output speaker terminals.


The high wattage audio amplifier symptom was a loud hum and distortion, then quit operating. The chassis was dead. The line fuse was blown with shorted and open power output transistors. The woofer speaker voice coil was found open. The left channel loading resistor was running warm, while the right speaker load resistor was quite cool. A - 24.1 volts was measured at the left channel speaker.

A quick in-circuit transistor test of both output transistors indicated Q109 was open and Q112 appeared to be leaky. Both transistors were removed from the circuit and both proved to be defective. Before replacing the power output transistors, the emitter bias resistors were tested on the low ohm scale of a DMM. Although, emitter bias resistor (R137) had correct resistance, the 5 watt resistor’s ceramic body was cracked and showed burn marks. R137 (0.15 ohms) was replaced with another 5 watt replacement. Both bias resistors were replaced.

The amplifier was powered up, the fuse blew and the chassis shut down. Q112 was damaged with a dead short between emitter and collector terminals. All transistors were checked in the left channel and appeared normal. Although voltage measurements were off at the output transistors, it was difficult to check voltage on the other transistors when the chassis would shut down. The voltages were way off with Q109 and Q112 out of the circuit.

Starting at the output speaker terminals, a resistance test was taken from the left speaker terminal to common ground. This same reading was compared with the normal right speaker terminal.

It’s best to use an analog ohmmeter for these resistance measurements, as the DMM numbers will charge up and down, and take a long time to recover. Although, the analog meter is not as accurate as the DMM, a good comparison resistance test can be made. Naturally, the defective channel output resistance was off compared to the normal right channel.

The resistance measurements were made on each audio transistor terminal and com pared to the same transistor in the right channel. When the emitter terminal of Q107 was checked, no resistance was found, even on the RX1 OK scale ( FIG. 7). Upon checking the Q107 emitter circuit, R127 (47 ohms) appeared open. No resistance was measured across the 47 ohm resistor. One end of R127 was removed from the POB and checked again. Underneath the small resistor, a small crack was found; which could not be seen by looking down at the resistor.

FIG. 7. Power output transistors Q107, Q109, Q112, and resistors R128 and R137 were replaced in the output circuits.

Since the emitter resistor was open, Q107 might have arced-over or become leaky, dam aging the emitter resistor. Q107 was replaced with a 2SC2229 replacement. Once again, Q112 was replaced and a resistance check was made. The left and right output speakers resistance were within 10 ohms of each other to common ground. Resistance measurements of Q109, Q111, and Q107 were quite close in both channels.

The amplifier was plugged in and the chassis remained alive. A quick voltage measurement on the output transistors was fairly normal. By replacing Q107, Q109, Q112, R137, and R128, the music was restored in the high-wattage amplifier.

When the chassis shuts down with damaged outputs and voltage measurements cannot be made accurately, try resistance measurements to common ground to locate a defective audio circuit. Before taking resistance measurements, discharge the large filter capacitors in the low voltage power supply with the amp turned off.


The high-powered amplifier symptoms are the same found in regular home receiver amps, except more extensive component damage. A leaky AF or driver transistor can cause distortion in the output circuits. Most distortion problems are found in the audio output circuits. The leaky bias diode in the base circuits of the output can cause distortion and sometimes a whistling noise. A distorted right channel might be caused by burned bias resistors and leaky output transistors. A leaky directly-coupled driver transistor can destroy the output transistors and result in an extremely distorted channel ( FIG. 8). Check for an intermittent voltage regulated transistor when the audio becomes distorted after operating for several hours.

FIG. 8. Go directly to the audio output transistors with a main fuse blown in the high-powered audio amplifier.

Go directly to the output transistors when the main fuse is blown. You might end up replacing a leaky driver transistor and four power output transistors in the high powered amplifier. The dead amp with a blown fuse might result from a leaky 2200 uF filter or coupling capacitor.

The left channel might be dead with a blown fuse caused by open emitter resistors in AF or driver circuits. Check for ring and board cracks on the PCB when the main fuse blows. Suspect leaky driver, output transistors, and leaky zener diodes for a blown fuse symptom.

Suspect a defective component on the volume control board when the amplifier cuts out at low volume. Check for poorly soldered component leads or a cracked board when the audio cuts out. Replace defective volume control with intermittent volume in one channel. When one channel cuts out as volume is increased, check for a defective speaker relay. Intermittent audio with some noise might result from a small electrolytic capacitor (0.47 mF to 1 mF) connected to the volume control. Clean up volume and tone controls with cleaning fluid sprayed down inside the lug terminals.


The high-powered IC output circuits might consist of one large IC on a large heat sink. The entire audio circuit might be found in one large IC component. The audio circuits might be included from the volume control to the speaker terminals. The power output IC can cause many different sound problems. A high-powered IC might be found in home receivers, auto radios, and separate amplifiers.

Suspect a leaky output IC when the main fuse is blown with dead receiver and amplifier circuits. The dead left channel might be caused by a leaky output IC. Inspect the body of the large IC for blown out areas with a dead chassis. Check the body for overheated marks. Check for a change in resistance or burned bias resistor on the IC terminals, when the IC appears to be overheated. Suspect the output IC when both audio channels are dead. Replace the power output IC when the amplifier warms up and cuts out.

Check the signal in and out of the power output IC when both channels are distorted. Test out each electrolytic capacitor (47 to 2200 uF) that is connected to the IC terminals for leaky or open conditions. The left channel might be distorted and the right channel weak with a warm or red hot output IC. Suspect the power output IC for only a hum in the speaker and no sound. Both channels were distorted in a Fisher SUV7 amplifier with a leaky power output IC (STK8O5IC).

Check all hot resistors tied to the power output IC terminals with hum in the speakers. Replace the power output IC when running hot with a loud howl or no sound in both channels. Check and resolder all output IC terminals with a buzzing and motorboating symptom.

A noisy and motorboating sound in the left channel can be caused by a defective output IC.

Suspect the power output IC when both channels are noisy, crackle, and pops in the speaker. Replace power output IC when the right channel pop and music is distorted.

Retighten power IC mounting screws to the heat sink for hum and noisy reception.


The woofer speaker in a Pioneer amplifier was damaged with -24.7 volts found on the left channel speaker terminals. To prevent damage to the test speaker, a 100 watt 10 ohm resistor was clipped to the left speaker output terminals. Of course, after a few minutes of operation the resistor began to heat up. The left output IC (STK-0050A) also ran warm ( FIG. 9).

FIG. 9. The audio output IC STK-0050A was replaced with an RCA SK7661 universal replacement.

The voltages found in pins 8, 2, 1 ,and 0 were highly negative. The normal voltage on terminals 0 and 1 should be a +1 .4v and -1 .4v respectively. The same comparable voltages found on the right channel output IC indicated a leaky left output IC.

Since Q11, Q1 3 and Q15 were tied directly to the output IC, they were checked in-circuit for open and leakage with the DMM. All transistors tested normal, except Q15 was in stalled backwards. Someone had worked on the amplifier, as both Q15 and the Darlington transistor 07 were replaced. Replacing Q15 did not solve the problem. Sometimes additional problems might be found when working without a schematic. Critical voltage tests were made on each transistor and recorded upon the schematic.

Servicing the amplifier chassis was a little more difficult since no parts were marked on the bottom PC wiring. The correct transistor terminals were located with the diode-test of the DMM. The base terminal of a transistor is always common to both the emitter and collector terminals. For instance, with the diode transistor test, the resistance between base and collector terminals will be a few ohms lower (.722 ohms) than from base to emitter (.731 ohms) on a normal transistor. Remember that all transistors might have a different voltage measurement, except the comparable voltage will be quite close in resistance. Both measurements are quite close with the base to emitter test, a little higher in resistance.

Since the voltage was high on pins 0 and 1 of output IC (Q1), maybe output IC was leaky. Someone had replaced Q1 with an RCA SK7661 universal replacement. Perhaps, Q1 had overheated and was destroyed once again. Naturally, the output IC was not on hand or locally available. So the next best thing was to exchange the two output ICs (01 and 02). After the exchange of parts, the sound was normal in the right channel indicating both ICs were normal.

All components were checked on Q15 with accurate voltage measurements. The collector voltage was oft (-26.9v) and should have been a positive 1.4 volts at pin 0 of output IC (Q1) ( Figure8-1Ø. Diode Dl and all corresponding resistors were normal when one terminal lead was removed from the circuit. The wiring was traced and double checked with the RX1 scale of the DMM. Q15 circuits appeared normal.

FIG. 10. A negative -26.9 volts and -24.7 volts was found on power output IC (Q1).

Next the Q11 and Q13 voltages and components were checked. The collector voltage on Q11 and Q13 was quite high (-24.7v) and should be -1.4 volts. All components tested normal within the Q11 and Q13 circuits.

Transistors Q5, Q7 and Q9 were tested with voltage and resistance measurements. Since the Darlington transistors (Q7 and Q9) looked like flat IC components with five terminal leads, no leakage was found between Q7 and Q9 terminals. The voltage on Q9 was negative except at the collector terminal. All resistors on the terminals of Q7 and Q9 were fairly close in value.

When the wiring was checked and traced in the emitter circuits, R245 and R243 to the - 28.5 volt source, indicated an open circuit at Q13. The -28.5 volt source was traced out again since there were no markings on the PCB. Both resistors were quite close in value (2.2k ohms). Since there are about five different voltage sources within the amplifier, the - 28.5 volts was located at emitter terminal of Q13. Again a resistance measurement was made from connection of R245 and R245 to the -28.5 volt source with no resistance measurement.

Another method to double check the resistance or wiring between the emitter terminals and power supply source (-28.5v) is to take a resistance measurement between the emitter terminals of 09 and Q13. The resistance measurement should be less than 2.4K ohms. Double check this resistance measurement by checking the same terminals in the normal right channel. No resistance was found even on the RX200K scale. Perhaps there was a break in the pc wiring of the emitter circuits. Remember, the voltage on both emitter terminals of Q9 were -27.1 volts and only off 1.4 volts from the schematic.

A quick resistance measurement between the emitter terminals of Q9 and chassis ground were made to determine if the wiring was open. The 20K ohm range was made for these resistance measurements. Another resistance measurement from the emitter of Q13 to common ground should be quite close. These two different resistance measurements are made through the -28.5 volt power source to common ground. No measurement was noted.

When tracing out the return of R243, R245 and R253 to the -28.5 volt source, one end of a wire tie bar was poorly soldered. Both sides of the tie bar were re-soldered, the voltage returned to normal, and the music was restored in the amplifier. And to think, the poorly soldered terminals of a small tie bar, caused all these servicing problems in the left channel circuits.


The four channel or quad amplifier circuits were found in early auto, home, phono and cassette players. Four separate tape head directly-coupled preamp circuits were switched into a 4 or 2-channel cassette circuit. After the four channel input circuits, a function switch placed the phonograph, AM/FM/MPX and tape head circuits into the AF amp section.

The four identical auto output circuits contained a right and left audio output to four different speaker circuits. In the high-powered auto amp circuits, the 30X4 up to 80X4 watt multi channel amplifiers are located. These amplifiers can be bridged from 70X2 up to 240X2 watts.

The tape head circuit is coupled through a 1 uF electrolytic capacitor to the base terminal of Q10. The base terminal of Q11 was directly-coupled to the collector terminal of Q10. The output preamp circuits are coupled with a 33 uF capacitor to a selector output switch. All four tape head circuits are identical and each channel can be checked against the others for correct voltage, resistance, and component tests ( FIG. 11).

FIG. 11. The directly-coupled preamp transistor circuits within the 4 channel amplifier.

The multi channel amplifier might contain preamp circuits. Check the preamp circuits with signal of a test cassette, from tape head to the output of each preamp circuit with the scope or external amplifier. Compare each stage with one of the normal circuits.

The output transistor circuits are switched from the AF amp transistor with the function switch. An AF amp transistor is directly-coupled to the function switch and into the balance, bass and treble control circuits. A volume control provides audio signal to the driver and push-pull audio output transistor circuits. Two separate left and right channel output circuits are found at the output of the four audio channels (two-left and two-right).

FIG. 12. There are four identical transistor audio output circuits found in the 4-channel amplifier.


The 2-channel auto amplifier can be bridged for a greater wattage output. For instance, a Kenwood PS-300T 2-channel amplifier that has a peak power output of 200X2 can be bridged to a single output of 400 watts. The outputs of the 2-channel amplifier (L and A) are tied together to achieve high wattage output into a 2 ohm load. In mono-mode, you combine the stereo outputs to power a subwoofer pm speaker. A bridged amplifier cuts in half the output impedance to the speaker. A 2-channel 4 ohm output amplifier, when bridged, cuts the output impedance to 2 ohms and doubles the wattage output.

Some auto amplifiers have multi channels like the Kenwood model PS500F (75X4) with a peak output power of 600X2 or when bridged (300X2). The bridged output provides a left and right channel of 300 total output watts. The RMS at 2 ohms equals 1 50X4. Most of these type amplifiers have built-in high-pass and low-pass filter crossover networks.

A Tri-way speaker output hookup powers a pair of stereo speakers and subwoofers. This is a cheap way to drive two main speakers and one subwoofer from a 2-channel amplifier. A Tri-way crossover network might have a low-pass filter to send frequencies below 100 Hz to the subwoofer, while the high-pass filter sends frequencies above 100 Hz to the main speaker. Of course, the amplifier that the Tri-way crossover connects to must have Tri-way capabilities.

In a 3-channel speaker hookup, a 4-channel amplifier is built with enclosed crossover networks. You bridge the 4-channel amp to run a subwoofer while the front channels drive a pair of regular stereo speakers. The 3-channel output provides greater control over the sound output ( FIG. 13). The amplifier with preamp outputs allows a non-amplified signal to pass through the amplifier and into another amp in the system without separate crossovers.

FIG. 13 A 2-channel amp output with a tri-way crossover and 4 channel amp hookup connected to the speakers.


Most high-powered amplifiers provide speaker hookup information with a new unit. A simple auto amp and speaker hookup might contain the AM/FM/MPX receiver and cassette player, equalizer, 100 watt amplifier, and two mid-range with large rear subwoofer speakers. The average auto installation might consist of an AM/FM CD receiver, 6 disc changer, center control unit, equalizer, 120 watt amp for two mid-range tweeters, and a 250X2 watts amplifier for two woofer speakers.

FIG. 14A. A typical 100 watt amplifier hookup with a front midrange and sub woofer speakers.


The high-powered amplifier provides audio to multiple-driver, enclosed, and raw speakers. The multiple-driver speakers might include a tweeter, mid-range, mid-bass, woofer or sub- woofer. A typical multi-driver might consist of a compact 2-way speaker system with a soft-dome tweeter and a 6.5 inch woofer. The multiple-driver system is more than one speaker on one speaker channel. The driver system might contain two or three speakers.

The multiple-driver speaker channel might have 2, 4, and 8 ohms voice coil impedance and from 15 to 300 watt power range. The woofer speaker cone material can be copolymer, tuff-kote, polyprop, graphite, kraft pulp, pulp, carbon fiber, carbon filled polyprop, fiber, polymica, polypropylene, paper, laminate fiber, poly, mica, black resin, carbonic, acrylic resin, neodymium, injected graphite, poloygraphite, carbon mica, copolymer, treated pa per, coated paper, mylar, polylaminate, silicone treated, injection w/butyl, injection cone, kelvar, kelvar-normex, poloymica cone, mica polymer, cellulose, polyglass, polylam, polymineral, graphite/copolymer, pmp, pmt, and polytex.

The tweeter cone material might be made from copolymer, silk, poly dome, polymide, polymer, titanium, poly-ethyl-imide, mylar, paper, textile, aluminum, poly, cloth, rubber- dome, fabric, strontium, whizzer cone, polycarbonite, soft dome, neodymium, treated silk, kelvar, tioxid, glass fiber, phenol, polymineral, polycell, ceramic, cellulose, dynamic cone, whizzer, piezo, pure titanium, titanium laminate, hybrid composite, soft linear, polyneo 11, supronyl dome, silk-cotton, crystal fiber, crystal paper, polyester, and trilaminate.

The raw speaker impedance might be 2, 4, and 8 ohms with a power range up to 1600 watts and a frequency response of 20 Hz to 20 kHz. The driver material is made from copolymer, polyprop, kapton, cellouse, graphite, paper, polycarbonate, plastic pulp, epoxy pulp, poly, polycarb, polymer, carbon fiber, organic fiber, fiber, titanium, treated NCP, silk, treated paper, fiber laminate, mica graphic, glass fiber, PVA paper, treated linen, resin lam, polycoated paper, coated silk, poly/butyl, polyproplene, pcp, aluminum, mica, kraft pulp, polymineral, propylene-mica, and polyprop coated.

FIG. 15. The large 12 or 15 inch PM speaker can handle high-powered auto amplifiers.

A typical Hyper-Throw model HT156D subwoofer 15 inch speaker has a 400 to 800 watt power range, a 3 to 12 ohm impedance, and a 15 to 150 Hz frequency response. Another resonance model RW91 5 subwoofer is 15 inches in diameter, with a power range of 1000 watts, 4 ohm impedance voice coil and a frequency range of 20 to 300 Hz. The raw woofer or subwoofer speaker might vary between 8 to 15 inches in diameter.

The high-powered amp speaker damage results from too much power applied to blow out the voice coil, have an open winding, and a dragging cone. Of course, the open voice coil has no or infinite resistance measurement. A voice coil can be damaged by placing a dc voltage across the winding with speakers connected to a directly-coupled leaky or open power output transistors or IC within the amplifier.

When an electrolytic coupling capacitor is found between the output circuit and speaker terminals, the speaker seldom was damaged. The auto speaker might become warped, with the voice coil dragging or frozen against the magnet pole, and might contain water damage. A mushy speaker sound can be caused by a dropped speaker cone.


In the large power amp receivers or amplifiers, the left and right speaker relay switches in the left and right channel speakers. The transistor-relay circuits are controlled by a micro processor. A separate transistor controls the left and right relays, providing audio output to each speaker. The defective relay circuits might be dead or inoperative, intermittent, or after the amp warms up the speaker relay cuts out. If the power output IC is normal and the speaker relay kicks out, check for a poorly wire soldered wire connection and jumper wire bridges.

FIG. 16. The speaker relay transistors are controlled by microprocessor IC701.

When one channel cuts out at low volume, suspect a defective relay. Check the relay for poor contacts when the volume cuts in and out on one channel speaker. Suspect a defective relay when one channel cuts out at low volume and as the volume is turned up, the sound cuts in.

Suspect the speaker relay when the right or left channel goes out with a loud static noise. Check the control microprocessor or output IC when the relay won’t energize. Check the muting switch transistor if one channel is intermittent and cuts out. Suspect an open solenoid or speaker relay when dc voltage is found across the solenoid winding. A dead, no power, loud arcing noise symptom might be caused by a defective ac power switch and not the speaker relay.


The tabletop AM/FM/MPX receiver might have several transistors in each audio channel or one large IC. The audio transistor circuits might consist of an AF, driver and push-pull output transistor circuits. In the latest stereo receiver amplifiers, only one large IC is found.

Just take the receiver symptoms and apply them to the schematic and components on the PC board. Check the line fuse if the receiver is dead. Measure the voltage on the largest electrolytic capacitor in the power supply with a dead chassis. If correct voltage is found in the power supply source, go directly to the power output transistors or IC. Scan the chassis for any burned components. Suspect leaky output transistors or IC with very low voltage at the output components.

Suspect a leaky or shorted transistor and IC when the fuse keeps blowing. A leaky output IC and directly-coupled transistors can blow the main line fuse. A dead receiver can result from a defective power switch. In a Pioneer SX737 receiver, the symptom was no sound, no relay click with a defective transistor regulator and diode lowering the voltage source.

Check for a weak left channel with open resistors and zener diodes in the audio voltage sources. Suspect an open or leaky driver transistor when the right channel becomes weak and then cuts out. Signal trace both sides of a small electrolytic coupling capacitor for weak sound. A change in emitter bias resistors and electrolytic bypass can cause a weak audio stage.

When one channel is out or dead, check the power output IC or transistors. Suspect the power output, driver transistors, power source, and emitter resistors when one channel is dead. Check for a leaky electrolytic coupling capacitor (1 to 2.2 uF) with a dead right channel. Replace the open driver transistor for no left channel.

Resolder plated feed-through holes with a weak or distorted left channel. Replace the power output IC when the sound in the right channel is distorted, pops and cracks in the speaker. Check for poorly soldered joints on the output IC and defective relay with intermittent audio. When the left channel shuts down at once, check the output and driver transistors, open or burned low ohm emitter resistors, and burned pc wiring.


Today the in-dash auto cassette-receivers have a greater wattage output than yesterday’s receivers. The peak power output might be around 25X2 up to 40X4 watts, while the RMS wattage range is 1 5X2 up to 22X4 watts. The auto receiver might have four speaker outlets and 1 or 2 sets of preamp outputs. The preamp jacks at the rear of the radio can be connected with an RCA patch cable to another outside amplifier. Some auto receivers have two sets of preamp outputs, to which an external 4-channel amp or a second 2- channel amp can be added for a higher power output.

In the early auto cassette receivers, you might find a directly-coupled transistor or IC preamp stage. The power output amplifiers might contain transistors or separate IC components. Each channel output IC was separate and fed one or more pm speakers. The AM and FM stereo radio signal was tied to the base terminal of each left and right preamp transistor. When the cassette was inserted, the dc battery source was switched out of the radio circuits.

FIG. 17. The early auto receiver audio output IC were separate components.

A dead cassette receiver might have a blown fuse caused by a leaky or shorted output IC. Check for a leaky filter or in-line capacitor that can damage the line fuse. Suspect a shorted output IC when the fuse won’t hold. Check the audio signal with the external audio amp or scope at the volume control for weak or distorted sound. The audio can be checked at each preamp transistor and output IC. Compare the defective left channel to the normal right channel or vice-versa.

The IC preamp circuits can easily be serviced by inserting a tape or cassette and checking the audio at the input terminal (1) of IC205. If no sound at pin 8 of the left channel IC205, suspect a defective IC or improper voltage source. Measure the voltage on pin 4 (9.1 v). A leaky IC205 might lower the supply voltage at pin 4. Remove pin 4 from the PC wiring and notice if the voltage source increase, indicating a leaky output IC. Check capacitors C205, C213 and C211 before removing the suspected preamp IC ( FIG. 18).

FIG. 18. Check electrolytic capacitors Q205, C211 and Q213 to see if they are open or leaky before replacing 10205.

The medium-wattage output cassette-receiver-player might have an AF, driver and a separate power output IC in the 4-channel audio circuits. There are four different audio output circuits and IC components. All four output circuits are identical with the same components and voltage measurements. When a schematic is not available, compare the audio signal and voltages of the normal channels to the distorted or weak channel.

Determine if the radio receiver or cassette circuits are functioning. If the cassette circuits are not heard, suspect the tape head preamp circuits or the tape head itself. When the radio circuits are dead, proceed to the radio input circuits. If one channel is dead, distorted or weak, check that audio channel.

When the front left speaker is dead, start at the speaker output and check the audio at pin 9 (output) and pin 4 (input) of IC201 terminals. Check the audio at the volume control to determine if the driver circuits are defective. When the signal is normal at any given point in the circuit, you have located the defective audio circuit.


The AM/FM 4-channel stereo amplifier has identical audio amplifier circuits. Besides the AM/FM receiver circuits, the 4-channel receiver might have cassette and phono inputs.

The phono and cassette inputs might contain directly-coupled transistor or IC preamp circuits. After the audio function switch, the audio circuits contain identical AF, driver and IC output circuits. A treble and bass controls are found in the driver input circuits. Each channel has identical volume controls at the input of each power output transistor (Fi, 8-20 Isolate the defective sound at each volume control and proceed either way until the bad stage is located.

FIG. 19. Signal trace the audio into pin 4 and out pin 9 of IC201 for weak, dead or distorted music.

FIG. 20. To locate the defective output IC, signal trace the signal in and out and compare critical voltage measurements.

Check small electrolytic coupling capacitors (1 to 4.7 uF), diodes, and output ICs or transistors for a noisy weak channel. A loud pop and arcing noise might result from a bad power switch. The noisy right channel might be caused by the audio IC function switch. A constant buzzing in the audio might result from loose IC mounting screws. Check the output ICs for a pop, crackle or hum noises. Suspect the output IC for a crackling noisy sound in the speaker. Clean up the volume control for noisy and intermittent sound. replace volume control if excessively worn. Resolder the driver and output IC terminals for a low hum in one channel.

Suspect leaky (1 uF) coupling capacitors on the driver (Q15) input terminal for no audio in the left front speaker channel. Replace Q10 for a no audio symptom in the left rear channel. For no audio and a blown fuse, check for a leaky or shorted power output IC4. Suspect poor speaker relay terminals on the right rear speaker for intermittent audio. Measure terminal 8 of IC1 for a low -6.7 volts or improper voltage source that results in distortion at the left rear speaker. Check Q17, when the audio quits after several minutes of warm up. Suspect power output lC3 or small electrolytic capacitors (47 to 220 pF) connected to the output IC terminals for extreme distortion.

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Updated: Friday, 2014-12-26 20:55 PST