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After many years of electronic experience, the electronic technician repairs thousands of electronic products each year without a schematic. The electronic TV technician knows where to look, how to test, and how to correctly replace the defective component with a given symptom. The TV technician automatically goes to the horizontal output section with a no-high voltage symptom or a dead TV chassis. A quick horizontal waveform and voltage test indicates a defective horizontal output transistor.
Another waveform test points out a leaky count down drive IC.
Likewise, the audio technician specialist knows where to look for an extremely weak and distorted sound symptom. He or she takes critical voltage and component tests in the audio output circuits, because most distorted problems occur in the output stages. Besides years of experience in electronic Troubleshooting, the audio technician can rely on many short cuts and tips on how to quickly service the audio amplifier circuits. The experienced audio technician knows where to look, make critical tests, and to put their senses to work without a schematic.
FIG. 1. Look the amplifier chassis over to locate the audio output transistors upon a large heat sink.
Besides electronic knowledge and correct test equipment, the electronic technician can rely on sight, smell and sound to help locate the defective audio component. The electronic technician can tell if a power transformer or audio output transistors have become overheated with a strange odor drifting up from the audio chassis. The sweet odor from the low voltage circuits might indicate a defective selenium rectifier or silicon diode. You can smell an overheated resistor or transistor. Overheated components in the audio amplifier circuits can always be a source of trouble.
Weak or distorted music from the speaker can be traced to the audio output circuits. No sound from the speaker might indicate problems within the low voltage power supply.
A low hum symptom might be caused by a poor ground or decoupling electrolytic capacitor. A really loud hum in the speaker indicates a poor filter action in the low voltage power supply. A vibrating or mushy sound might be caused by a loose speaker cone or a dropped cone against the magnet pole. A loud groaning noise with a dim pilot light might indicate a shorted power transformer or leaky silicon rectifiers.
Notice if the power output transistor and IC is leaky or shorted with burned marks on the body area. A defective electrolytic capacitor should be replaced when white or black substance is oozing from the bottom of the capacitor. A quick peek at the audio chassis might spot a burned resistor, choke or zener diode. The overheated connection on the pc board appears as a dark section indicating a poorly soldered joint or overheated component.
When the pilot light does not come on and the chassis remains dead, this indicates a blown fuse or problems in the low voltage source, and on it goes. Simply use three senses to help locate the defective part within the audio chassis.
FIG. 2. The audio output IC is found upon a heat sink within the cassette player.
A symptom, defined by Webster’s Dictionary, is any circumstance, event, or condition that accompanies something and indicates its existence or occurrence; sign; indication. The symptom of weak sound from the speaker might indicate to the electronic or audio technician where to locate the defective component. An extremely distorted sound in the speaker might indicate a leaky or shorted output transistor or power IC. No sound from the speaker and no pilot light might result from a defective low voltage power supply.
The inoperative speaker relay system with no audio in the speaker system can indicate a defective relay, improper voltage, and defective relay protection circuits. The erratic buzz and intermittent sound symptom might indicate a defective power IC or loose mounting screws ( FIG. 3).
A distorted speaker system in the left channel might indicate leaky or open transistor or power IC in the left channel audio circuits. One channel dead with a blown fuse symptom might point out a leaky power output transistor or IC. The dead as a door-nail symptom can be caused by a defective power switch. Remember, the symptom in the audio circuits might come from the speaker (sound), smoke drifting up from an overheated component (sight), and odor from a burned part (smell).
THE DRIVING FORCE
Go directly to the power source in the low voltage power supply circuits and check the B+ voltage. Take a quick voltage measurement across the largest filter capacitor, the first thing. All audio circuits must have adequate voltage or no audio operation from the circuit. If the voltage is normal in the power supply circuits, check the voltage at the defective stage or component for correct voltage. Suspect a regulator transistor or IC when the voltage is low at the defective stage or component.
FIG. 3. Check for loose mounting power IC screws for erratic and intermittent audio.
FIG. 4. Take a quick voltage test across the large filter capacitor in the low voltage circuits in a Quasar AEDC148 chassis.
Don’t overlook a leaky or shorted component pulling down the voltage source. An overloaded part can lower the voltage source. Remove the suspected overloaded circuit from the low voltage source by cutting out a section of PC wiring. This can easily be done with a pocket knife or sharp cutting tool. Now check the voltage at the voltage source.
Repair the overloaded circuit when the voltage returns at the voltage source. Don’t forget to place a bare piece of hookup wire across the cut-break of PC wiring to restore the circuit.
The most frequent problems in the low voltage power supply are leaky or shorted silicon diodes, dried-up filter capacitors, and open or leaky regulator transistors. A shorted silicon diode can destroy the primary winding of the step-down power transformer or blow the main fuse. Excessive hum in the speaker might result from a defective filter capacitor in the low voltage power supply. The open regulator transistor results in a no-output voltage source and a leaky regulator transistor might lower the voltage source with poor regulation. For the first service operation, remember to check the voltage within the low voltage power supply when a dead or weak symptom exists.
QUICK AND CRITICAL VOLTAGE TESTS
Take a quick voltage measurement across the large filter capacitor to determine if the supply circuits are normal without a schematic. The voltage should be a few volts lower than the working voltage rating marked on the filter capacitor. If the working voltage at the largest filter capacitor is 25 volts, the actual measured voltage should be between 15 to 20 volts across the capacitor ( FIG. 5). Likewise, check the decoupling electrolytic capacitor working voltage of 15 volts, and the measured voltage might be 9 to 12 dc volts.
Although these voltages might not be true in every case, you might find one with a lower voltage. By quickly checking the voltage across the various electrolytic capacitors, you can determine whether the voltage sources are normal or improper without a schematic.
FIG. 5. The working voltage upon a filter capacitor should be 5 or 7 volts lower than marked upon the electrolytic capacitor.
Quick voltage tests on a transistor will indicate if the transistor is an NPN or PNP type. The collector terminal of an NPN transistor is always the highest positive voltage (10v) with a low positive voltage (2v) on the base and very low (1 .7v), or the least positive on the emitter terminal.
A PNP transistor has the highest negative or the lowest positive voltage (0.69v) on the collector terminal with a more positive or low negative voltage (7.3v) on the emitter and a very low negative or more positive voltage (7.06v) on the base terminal.
The collector voltage is the highest positive voltage on the NPN transistor while the emitter is the most positive voltage in a PNP transistor circuit. Just compare the suspected transistor terminal voltages to the same transistor in the normal audio stereo channel without a handy schematic.
Locate the transistor terminals with C, E and B stamped on the pc board wiring when no schematic is available. Take transistor in circuit tests with a transistor tester or diode- junction tests of a DMM, when the terminals are not marked on chassis and no schematic is available. Locate the base terminal with common measurements on collector and emitter terminals. The highest voltage measurement, either positive (NPN) or less negative (PNP), will indicate a collector terminal. Check from chassis to emitter terminal with a low resistance measurement, since most audio emitter resistors are connected to common ground; the terminal remaining is the base terminal.
The voltages found on audio transistors can be checked with another similar model or chassis amplifier schematic. Compare these voltages with the defective chassis. Another method is to compare voltages on the identical transistor found in the normal stereo channel. For instance, if the left channel is dead and really low voltages are found upon the AF transistor, compare these voltages to the normal right stereo AF transistor.
FIG. 6. Compare the voltages found upon another similar circuit of another audio chassis.
A quick in circuit voltage test on a suspected transistor can indicate if the transistor is open or leaky. A higher than normal voltage upon the collector terminal might indicated the transistor is open. A very low voltage on the collector or on all three terminals, that are about the same voltage, indicates a leaky transistor.
RESISTANCE RUNS DEEP
A quick resistance junction-diode test between any two transistor terminals can indicate if the transistor is open or leaky ( FIG. 7). Take a resistance test on transistors with the diode-test of a DMM. A normal and comparable resistance test between base and collector, base and emitter should be quite close, with the red probe at the base terminal. A low resistance in both directions indicates a leaky transistor. The leaky transistor often has a low resistance measurement between the collector and emitter terminals.
FIG. 7. The transistor resistance measurements with the diode test of DMM.
Resistance tests within the audio circuits might have an improper measurement, if a diode or coil is found in the base or collector circuit. If the resistance measurement is quite low from base to emitter, remove the base terminal from the PCB wiring with a solder wick and iron. Now take another measurement between the base and emitter terminals. Replace the leaky transistor if a low resistance measurement is found with the base terminal removed from the circuit.
FIG. 8. A quick resistance test with the diode-test of DMM can locate a defective transistor.
A quick resistance measurement across a capacitor, diode, and resistor can indicate if the component is leaky or open. Remove one end of the diode or resistor from the circuit for a correct measurement. Sometimes another component is shunted across the part to be measured and results in an improper reading. When locating a leaky or shorted transistor in the audio circuit, always check the emitter bias resistor for correct resistance. Some times the resistor can become overheated, burn and have a change in resistance. A normal silicon diode should have a resistance measurement in only one direction.
A low resistance measurement between a fixed capacitor terminal indicates a leaky capacitor. The normal capacitor should have an infinite resistance measurement. A leaky capacitor will show a low measurement in both directions. The electrolytic capacitor should charge up and down with a normal component on the ohmmeter. Remove one end of the capacitor from the circuit for an accurate resistance measurement. Now check the suspected capacitor with the capacitor tester, if handy.
EASY TRANSISTOR TESTS
You can quickly locate a leaky or open transistor with an in-circuit voltage, resistance and diode-junction test with the DMM. Locate the suspected transistor with signal tests. Take a quick voltage test on the collector terminal and common ground. Now take a voltage bias test between the base and emitter terminals. A really low collector voltage might indicate a leaky transistor. A higher than normal voltage on the collector terminal indicates the transistor is open ( FIG. 9). No voltage measurement on the emitter terminal results in an open transistor or emitter resistor. To determine if the emitter resistor is open, take a resistance measurement between the emitter terminal and common ground.
FIG. 9. Zero voltage measured on the emitter terminal indicates the transistor or emitter resistor is open.
Take a quick resistance test with the power off, between the collector and emitter terminals. Locate the collector terminal and take a resistance measurement to common ground. A low resistance measurement indicates a leaky or shorted transistor. Check the resistance between collector and base, base and emitter, and collector to emitter. If the resistance is low between any two or all three elements, replace the leaky transistor. Remove the suspected transistor and test it out of the circuit. Sometimes the solid-state component will test normal after heat is applied and defective before removing it from the circuit; then, just replace it.
DAY IN AND DAY OUT
The quickest method to locate a defective IC component is with audio signal in and out tests. Locate the input terminal by tracing the audio signal from the volume control, through a coupling capacitor or resistor to the IC input terminal. The output terminal is generally capacity coupled from the IC output terminal to the speaker terminals. Check for a 220, 470, or 1000 uF electrolytic capacitor on the chassis, close to the power IC, to locate the output terminal.
Another method of locating the various IC preamp or power output terminals is in the universal semiconductor manual. Take the part number stamped on top of the IC and look up the universal replacement. Now, check the input, output and supply voltage terminals on the universal IC drawing ( FIG. 10). Besides finding the correct terminals, the required operating voltage might be found upon the supply voltage terminal (Vcc).
FIG. 10. Check the part number of the IC against the universal replacement outline for correct terminal connections.
Check the supply voltage applied to the IC component. This voltage is always the highest and is called the supply voltage. Suspect a leaky or shorted IC if the supply pin voltage is quite low. Feel the body of the IC and notice if it’s running quite warm. Remove the supply pin terminal from the pc wiring with solder wick and iron. Now take another voltage measurement on the supply pin pad and common ground.
Suspect a leaky IC when the supply voltage rises higher than the normal supply voltage. Double check by taking a resistance measurement on an unsoldered supply pin and common ground. Replace the leaky IC if the resistance is below 100 ohms.
Signal in and out tests with the audio signal generator can locate defective transistors, ICs, and coupling capacitors. If one stereo channel is weak or dead, signal trace the audio circuits with the function generator. Check the signal in and out on the suspected IC. Suspect a defective transistor when audio signal is applied to the base terminal and no or weak audio on the collector terminal.
Check the suspected coupling capacitor by injecting audio on one side and then the other side. When one side of the capacitor won’t respond in the speaker, check the coupling capacitor for open conditions. Sometimes the intermittent capacitor might pop on with a loud signal tone in the speaker; replace it.
FROM HERE TO THERE
A quick continuity or resistance test across any electronic component might determine if the part is defective. Rotate the DMM to the 2K ohm scale and place the test prods on each side of a coil, diode and capacitor for a quick continuity measurement. The meter reading can indicate if the component is open or leaky. The continuity measurement of dial lights, LED5, fuses and transformer windings can indicate if the part is open.
When locating a defective component in the low voltage power supply, a quick continuity measurement of the primary and secondary windings of a power transformer indicates if the winding is open or has continuity. A continuity measurement across the silicon diode can quickly locate a shorted or leaky diode. The shorted filter capacitor can be located with continuity measurements across the positive and negative terminals.
FIG. 11. An open coil, detective resistor and poor connection can be located with a quick low ohm continuity measurement.
A leaky regulator transistor can be located with a low continuity measurement. Although, the continuity test can quickly locate a defective part, remove one end of the suspected component and take accurate resistance, transistor and diode tests.
NO LIVE ACTION
Of all the troubles that might exist in the audio amplifier, receiver, CD and cassette player, the quick and easy problem to locate is the dead symptom. Nothing usually operates in a dead chassis. Most dead chassis problems originate in the low voltage power supply and power output circuits of the audio amplifier. One stereo channel of a receiver might be dead and the other normal.
The dead audio amplifier symptom might be caused by no sound in the speaker of either channel and no pilot lights light up. The problem might indicate a blown power fuse or defective power transformer. Check for leaky silicon diodes after replacing the line fuse. The dead amplifier might result from an open regulator transistor and power resistors. A leaky power output transistor and IC can cause a dead symptom. The dead chassis might be caused by a defective power switch. A defective speaker relay might cause a dead chassis symptom with no sound in the speakers.
The no left channel symptom in the deluxe AM/FM/MPX amp receiver might be dead from an open AF, driver or output transistors. You might not be able to hear even a slight hum in the speaker. Both audio channels might be dead with a defective dual-power output IC. A dead receiver might result from a defective voltage regulator transistor in the power supply.
Both speakers might be dead with a defective speaker relay or relay circuit. The right channel might be dead with a blown speaker fuse. The receiver chassis might be dead with a blown power fuse that was caused by leaky and shorted output transistors. A dead symptom with arcing noise might result in a defective power push-on switch.
A no sound symptom with pilot lights on, might be caused by a leaky zener diode, open regulator transistor, or filter capacitor in the power supply circuits. Don’t overlook a leaky or shorted output IC for no sound from the speakers. The audio speakers were dead in a Fisher CA270 amplifier with defective Q114, Q115, Q104, D412, R165 and C153 components.
FIG. 12. Suspect an open regulator transistor for a dead receiver system.
The dead symptom in the auto-stereo receiver might be caused by a blown fuse and leaky output transistors. Locate the suspected output transistor or IC on a heat sink. A shorted output IC might result in no audio in the speakers. Suspect burned “A” lead wires and pc wiring with a dead auto receiver. The defective on/off switch can cause a dead receiver symptom. Check for burned bias resistors after replacing leaky output transistors. The dead auto speaker might result from an open voice coil or broken speaker wires.
KEEPS BLOWING FUSES
When the main power line fuse keeps blowing, a short or leaky component is loading down the low voltage power supply. A blown speaker fuse might be caused by a dc voltage placed on the speaker terminals or too much volume applied to the speaker. Although you can easily see a blown fuse, test the continuity of the fuse with the ohmmeter. The black internal glass area of the fuse indicates a directly-shorted component. When only the center area of the fuse opens up, a power line outage condition might occur or too much power may be applied to the speakers. Test run the amplifier or receiver for several hours and notice if the fuse opens.
Suspect a leaky or shorted component within the low voltage power supply when the fuse keeps blowing. Check for shorted or leaky silicon diodes. The dead chassis with the blown fuse might be caused by leaky power output transistors in the audio amplifier. You may have to replace both output transistors. When the speaker fuse blows each time the chassis is fired up, check for dc voltage on the speaker terminals caused by a shorted transistor or IC. The fuse did blow in a Pioneer SX-1 0001W amplifier with leaky 2SC897 output transistors.
The power line fuse might keep blowing in the large receiver with a dead chassis symptom. Check the output transistors and power output IC for leaky conditions. One channel can be out with a blown speaker fuse in a Fisher CA65 receiver. A quick voltage test on the speaker terminals was around +65 volts. An open Q111 (2SC1 5011) and R140 (78 ohms) were replaced to restore the dead audio channel.
Extreme distortion symptoms are generally located in the power output stage of the audio amplifier or receiver. The left channel was extremely distorted at high volume with an open R713 in a Sony STRAV1 020 receiver. Readjustment of the bias control helped solve the distorted left channel.
After the receiver warms up, both channels might contain strong distorted music with a leaky voltage regulator and audio driver IC. Two or more leaky output transistors and a dual-output IC can cause extreme distortion in both channels. Look for the power IC on a large heat sink. Check leaky output electrolytic capacitors to the speaker terminals when distortion exists.
After several hours of warm-up, a Magnavox receiver (AS305SM3701) contained extreme distortion with a leaky regulator Q7300 (IC7520). Remember, that extreme distortion in the audio channels of a receiver can result from leaky transistors and IC components. Don’t overlook an improper voltage source. Check for a change or burned bias resistors for distortion. These bias resistors are tied to the emitter and base terminals of the power output transistors.
The extreme distortion found in the audio circuits of a TV chassis can be caused by a shorted output IC. Extreme distortion might be caused by leaky SIF/Amp IC in the TV chassis. Improper alignment of the discriminator coil can cause distortion in the speaker. A leaky Multi-Sound IF Decoder and balance sound IC can cause extreme distortion in the recent TV chassis. Replace L201, C208 (91 pF) and C209 (7.2 pF) for extreme distortion in the RCA CTC1 10 chassis.
FIG. 13. Replace C208, C209, and L201 for extreme distortion in the sound circuits of an RCA CTC110 chassis.
Exact replacement components are easy to replace for they were designed for a certain chassis and circuit. So try to obtain the original parts when possible. Replace high powered dual-IC components with exact replacements. Replace those high-powered output transistors found in the high-wattage auto amplifiers with originals. Obtain the original part number from the manufacturer or replacement depot, and from electronic mail order firms.
Low signal and general purpose transistors and ICs can be replaced with universal parts. Simply cross-reference the semiconductor to the universal semiconductor manual. Most resistors and capacitors can be replaced with regular replacements. Silicon and zener diodes can be replaced with parts found at the local electronic distributor.
The power and special resistors should be obtained from the manufacturer. The dual- volume controls should be ordered from the parts depot. Replace the stereo volume control and switch found in the auto receiver with the exact part number. Special type coils, chokes and transformers should be ordered from the manufacturer. Replace the special correct size and correct voice coil impedance speaker with original part number. Special function and power switches should be replaced with original components.
FIG. 14. Replace the defective volume control and switch in the auto receiver with the original part number.
Filter capacitors can be placed in series or parallel when the original is not available. In the high-voltage tube amplifier chassis, 450 volt electrolytics can be placed in series to operate at a high working voltage and shunted to add capacity. Most bypass and coupling capacitors can be obtained from the local electronic distributor. Test all new components before replacing them in the audio circuits.
YEARS BEFORE AND AFTER
By keeping a record of the unusual or tough dog repairs, the electronic technician can use a case history on another chassis to quickly solve the same electronic symptom. The different audio case histories can be recorded using a card file system, the computer, or right on the schematic. By circling the defective component and writing the symptom on the edge of schematic, the service problem is there when you need it.
Case histories can save a lot of valuable service time. It seems the same electronic trouble in one model can occur again or in another chassis. Sometimes the same trouble hap pens again and again. The most common service problem might be remembered, while the unusual one might be forgotten in this busy world of servicing. Just take a minute and mark it down for a hurry up and rush-critical repair down the road.
There are many case histories sold today in literature form and on computer disk. The case histories might be repairs on TVs, Projection TVs, camcorders, CD players, and audio equipment. Check the classified adds in the magazine, Electronic Servicing & Technology for the different listings.
HOT CIRCUITS-HOT PARTS
The dead or excessively distorted chassis symptom usually consists of a hot audio output transistor or IC component. A shorted or leaky output transistor not only provides a dead or loud-hum symptom, but might be running red hot. Some normal audio output transistors and power ICs operate warm on the large heat sink. The red hot transistor or IC has a dark gray body appearance and is too hot to touch. Be very careful when touching the body of a suspected power output component; you might end up with burned fingers.
Remove the overheated part from the audio chassis. Test the output IC or transistor for leakage. The red hot output transistors can have a direct short between collector and emitter terminals. A leaky power IC might have a very low resistance be tween the voltage supply pin (Vcc) and ground terminal. Don’t install the red hot component even if it tests normal; discard it.
FIG. 15. Remove the hot output transistor from the heat sink and test out of the circuit.
Before replacing the red hot output transistors, test the driver or AF and all directly-coupled transistors in the output circuits for leaky or open conditions. Sometimes it’s best to replace all output and driver transistors within the audio output circuits, when one or more is found to be leaky or shorted. Double check all bias resistors and diodes in the output circuits. Likewise, check for leaky or open electrolytic capacitors within the output circuits. Replace all burned resistors or those with a change in resistance when connected to the red hot IC.
LOCATING THE WEAK COMPONENT IN THE CASSETTE PLAYER
Clean the tape heads for a weak or distorted sound within the cassette player. Make sure all packed tape oxide is removed from the tape head. Sometimes a flat plastic dowel helps to remove the packed oxide from the shiny surface. Make certain that the magnetic area of the tape head is free and clear of oxide dust.
FIG. 16. Make sure the tape head is clean in the cassette player for weak audio symptom.
A weak audio channel is generally caused by a defective transistor, IC, coupling capacitor and improper voltage source. Measure the voltage at the large filter capacitor. Check the voltage at the power output transistor or IC. Signal trace the audio circuits if everything appears normal at this point.
Insert a test cassette and check the signal at the volume control and ground with the scope. Check the weak signal against the normal channel at the volume control. If the signal is weak in one channel at the volume control, proceed toward the preamp transistors. When the signal at the volume control is normal or at the same audio level, signal trace the audio output circuits.
Another method used to locate a weak stage is to signal trace with an external audio amplifier. Plug the cassette player into the isolation transformer. Check the signal at the stereo tape heads. Proceed from the base to the next coupling capacitor and base of the preamp, to base of AF amp transistor or IC. When the signal becomes very weak, you have located the defective circuit. Locate the defective component with voltage, transistor and resistance test methods.
In a Philco 11-75198-1 chassis, the audio was weak in the left stereo speakers. Since the audio was also weak in the AM/FM/MPX receiver and phonograph, the defective component must be in the audio output circuits. A local radio station was used as the signal source. The volume on both left and right channels were normal at the volume controls.
When signal tracing with the external audio amp, a weak signal was noticed on the base terminal of the Voltage Amp (Q502). Again the signal was checked at the volume control in the left channel. C504 (1 uF) electrolytic was found to be defective just off of the left volume control terminal ( FIG. 17).
FIG. 17. The weak audio in a Philco chassis was caused by defective C504 (1 uF electrolytic).
Intermittent conditions within any electronic product are very difficult to locate. Most components producing intermittent audio are transistors, ICs, coupling capacitors, and poor terminal board connections. Try to divide the audio circuits in half by monitoring the audio at the volume control. Proceed towards the preamp or AF circuits, when the defective channel cuts up and down.
Sometimes a suspected transistor or IC can be sprayed with coolant to make it act up. It’s best to let the chassis operate until it acts up and then signal trace the intermittent channel.
Monitor the right intermittent stereo channel at the driver transistor base terminal, when the signal is normal at the volume control. Attach the external audio amp probe to the base terminal of the audio output transistor or input terminal of power output IC. Notice if the sound pops up or cuts down. If the sound quits, the defective part is located ahead of the power output stage. Scope the driver and AF circuits. Sometimes when the intermit tent transistor, IC or coupling capacitor are touched with the scope or external amp probe, the audio snaps back in.
The sound might appear intermittent when simply touching or moving a part with the probe tip. Monitor the input and output of a suspected component until you are sure the part is intermittent. Apply heat from a hair dryer or apply coolant from the spray can to make the component act up.
The sound would cut up and down in a Marantz 2230 audio amp’s right channel. Some times the sound would cut in and become distorted. Signal tracing the audio circuits with the external audio amp located an intermittent driver transistor. The driver transistor (2SC959) was replaced with an ECG-128 universal replacement.
REPAIRING DEAD RECEIVER CIRCUITS
The dead chassis has no sign of life or even a hum in the speaker. Most dead receiver symptoms are caused by leaky power output transistors or ICs. Check for a blown line or speaker fuse. Both audio channels might be dead with an open power line fuse; only one channel is dead with an open speaker fuse.
Next, check the low voltage power supply at the main filter capacitor terminals. If the pilot lights are on in the latest receiver chassis, usually the low voltage power supply is okay. If a low hum is heard in the speaker, voltage is applied to the audio output circuits.
Touch the center terminal of the volume control with a test probe or metal screwdriver and the audio circuits are usually normal, if a loud hum is heard. Proceed toward the AF and preamp circuits if the receiver and input signals are dead. Signal trace the input stages until you have located the dead stage.
Look the chassis over to possibly locate a burned, cracked or damaged component. In a J.C. Penney 3233 model, several pieces of blown electrolytic capacitor paper and foil covered the low voltage circuits. Several resistors were burned around a filter regular transistor Q601. Since several of these models had been worked on, the low voltage power supply circuits were well known. The C602 capacitor had blown into pieces, R601 and R604 were burned. Replacing D201, R601, C602, R604 and Q601 solved the dead chassis.
FIG. 18. C602 was blown open and 0601 was replaced with a dead symptom in a J. C. Penney 3233.
SERVICING NOISY AUDIO TV.CIRCUITS
Extreme cracking and frying noises in TV audio circuits might result from a defective transistor, IC or electrolytic capacitor. A popping or cracking noise might be caused by a defective power output IC. The frying noise, hum and distortion in the speaker can be cause by electrolytics, ICs and transistors. You might find several different components with noise in the audio circuits. Remember you cannot locate the defective noisy component with regular transistor or IC tests.
In the RCA CTC1 40 TV chassis, intermittent popping was cause by a defective power IC U1900. A high-pitched sound in both channels resulted in a defective C1807 (4.7 uF), off of pin 7 of U1800 ( FIG. 19). A no sound symptom with static in the audio was solved by readjustment of the discriminator coil (L2306). A distorted and noisy sound in the main chassis resulted from a leaky C1600 and C1695, off of pin 12 of U1601. These actual noisy problems were taken from case histories listed on the CTC1 40 RCA schematic.
FIG. 19. Replace C1807 for a high-pitched squeal in both audio channels on RCA CTC140 TV chassis.
When trying to locate defective components within the audio circuits, check the layout of parts on the chassis. Most stereo left and right channels are located to the left and right side of the chassis; sometimes they are lined up in a row. Most audio components are located close to and around the power output transistors and IC parts located on a large heat sink. If in doubt, simply trace the audio circuits back from the speaker terminals to the output transistors or ICs; the left and right channel components are found in the same manner.
Locate the low voltage power supply with large filter capacitors and silicon diodes mounted close together. Locate audio transistors with a body part number and look them up in the universal replacement manual. Compare the audio section with another product or schematic when the exact diagram is not available. Remember, the highest voltage is found on the collector terminal of a transistor and IC power supply terminal.
SERVICING REMOTE CONTROL CIRCUITS
Besides the TV, a camcorder, VCR, and AM/FM/MPX receiver might have a remote to control the various functions. The hand-held remote contains an infrared transmitter that triggers a infrared receiver inside the product to be controlled. The AM/FM receiver remote transmitter might also control a TV, VCR, tuner, tape and CD player in one remote package ( FIG. 20). The TV remote might control functions in the TV, VGA, audio amp and DSS cable disc operations.
FIG. 20. The infrared remote control transmitter might control the TV, VCR, tuner, tape, CD player, and receiver functions.
The separate receiver remote transmitter might control the on/off switch, volume up and down, and mute the audio at any time. If the receiver has a built-in CD player, the remote controls the disc/deck, right or left skip, fast forward skip and what disc to select. All of these functions are controlled by two self-contained AAA batteries.
When the remote won’t trigger any functions of the electronic products, check the small batteries. Remove the batteries and insert new ones. Clean the battery terminals with cleaning fluid. Rub the ends of the batteries on a towel or cloth to help clean the contacts.
The intermittent remote control operation might result from loose batteries or dirty contacts. Clean battery contacts and spread out the battery terminals to make a tight connection. Since remote controls are rather inexpensive, it’s best to replace the remote control instead of replacing components inside the transmitter. Try another remote before determining the infrared receiver is not functioning.
The infrared remote receiver is located inside the electronic product and consists of a photo-transistor pickup that connects to a preamp circuit. The new infrared receivers have an IC component as amplifier within the TV chassis. The infrared signal is then fed to a microcomputer or system control IC. The control IC controls the many functions within the TV.
When the infrared signal strikes the infrared element CR3401, the signal is amplified by IA amp (U3401) in an RCA CTC157 chassis. The IR signal is fed to pin 36 of the microcomputer AIU (U3300) IC. Here, the volume is controlled out of pin 37 to the volume control IC (U1801). Try to check another similar schematic of the control system when the schematic is not available, since there are many pin terminals on the system control IC.
FIG. 21. The IR signal is amplified by an IR preamp IC and fed to the AIU IC of an RCA CTC157 chassis to control the volume in the audio circuits.
Although the infrared receiver produces very few service problems, the IR preamp IC has been known to break down. Check the supply voltage at pin 8 (5.2v) and all other pin terminal voltages. Suspect a leaky IR preamp IC or zener diode (CR3401), when the voltage is low on pin 8.
Don’t overlook the various electrolytics connected to the IC pin terminals. Take a quick resistance test from each pin terminal to common ground, to locate a leaky component. When other remote functions seem to operate and there is no volume control action, suspect the AIU IC (U3300) or volume control IC (U1801) in the TV chassis.