|Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting|
The stereo amplifier can be found in a tuner-amp component, radio receiver, or a separate amplifier with input jacks for a tuner, phonograph, and cassette player. Some of the new amplifiers have a separate input for the compact-disc player. The average amplifier power output can be under 20 W, the intermediate group can be under 50 W, and the high-power amplifiers ( FIG. 1) can be over 100 W. A few super-power amplifiers can go above 250 W of power.
The frequency response can vary from 20 to 20,000 Hz. The output speaker impedance can be 4, 8, or 16 ohms. Several speakers can be switched into the output with a speaker switch assembly. Most new amps have earphone operation.
Because all amplifiers break down sooner or later, the trouble symptoms are about the same. The only difference might be in the higher amperage and wattage of the output components. The high-wattage output system calls for more high-power transistors and operates at higher voltage and greater amperage than the low-voltage power supply. Of course, these defective components must be replaced with the same components or with components having better specifications. Critical high-power components must be replaced with the original part numbers.
Denon DRA-435R 55-watt power-output circuits
The CD player, video, tape playback, tape recording, and radio input circuits are electronically switched into the amplifier circuits with 1C201 (FIG. 2). Each input passes through the left- and right-volume controls that can be controlled with the remote motor. The stereo signal input from tuner is connected to pins 4 and 25 of 1C201.
The left stereo channel output at pin 12 is fed to preset control VR201 and to pin 1 of volume control. The right stereo channel outputs at pin 17 through VR201 (100 k-ohm) to pin 3 of the volume control. The center control tap goes to each stereo channel.
FIG. 1 The electronic service technician is making critical voltage measurements in a high-power amplifier.
The right channel input to preamp circuits are capacity coupled through C374, C302 and R304 to base of preamp TR304. Here TR304 and TR306 are PNP transistors with a collector voltage of —40.1 V ( FIG. 3).
The left channel signal input to preamp TR303 and TR305 is capacity coupled through C373, C301, and R303 to the base of TR303. The audio preamp signal is directly coupled to the base of AF transistor TR308.
The audio-output transistors in the left channel—TR319, TR317, T323, and TR321—are directly driven power output transistors with the 55 W audio signal taken between R355 and R357 resistors. TR325 is used as a mute switching transistor. The right audio-output transistors—TR320, TR318, TR324, and TR322—provide 55 W between R358 and R356 resistors. TR326 is a right-channel mute switching transistor.
Often, when one transistor is found defective, you might find several more directly coupled transistors that are damaged. Always replace the directly driven and output transistors if only one is defective. These transistors might be balanced, matching transistors. Double check the bias resistors and diodes while the transistors are out of the circuit. Although universal transistors operate in sound circuits, replace these with original part numbers, if available.
Always use a speaker load-bank resistor instead of speakers in high-power output, directly coupled circuits. Using a resistor will prevent future speaker damage. Critical voltage, resistance measurements, and transistor tests solve most power output circuits. Correct measurement of resistance from output pin to common ground can locate defective transistor and IC components.
FIG. 2 The electronic switching 1C201 in the front end of the Denon 55W amplifier.
FIG. 3 Both left and right channels of a 55 W per channel Denon DRA-435R amplifier.
Radio Shack STAV-3 100 power-output circuits
The Optimus STAV-3100 is a powerhouse receiver with 100W per channel, feeding into 4—8 ohm loads with a frequency range of 20—20,000 Hz ± 1 dB. The amplifier contains 34 transistors, 6 IC components and 3 triacs within the audio, mute, voltage regulator, and relay circuits.
The left and right input audio circuits consist of capacitor coupling with mute transistors. Directly driven transistor circuits are found throughout the audio circuits. The driver amp output stages are connected to the impedance detector circuit ICs.
FIG. 4 Block diagram of Radio Shack STAV-3100, 100 W per channel output circuits. Low voltage
Impedance circuits---Impedance-detector circuit 1C501 through 1C503 protects the power amplifier and the power transformer when a low-load impedance is connected across the output terminals.
When the impedance is about 4 ohm, and the output power becomes 1—2W, the output of the current-detector amp 1C503 becomes larger than the output of the voltage- detector amp 1C502, and then the dc voltage appears at pin 1 and/or pin7 of 1C501 ( FIG. 4). This dc voltage will turn on the thyristors Q502 and Q503. Turn-on of Q502 and 503 results in the dc voltages supplied to the power amplifier changing from the higher one (the rectified outputs of D13) to the lower one (the rectified outputs of D7). This condition is held until the power is turned off. If the speakers A switch and the speakers B switch are pushed at the same time, the impedance detector also operates.
Speaker protection circuit---The relay 501 protects the speakers when an abnormally high current flows in Q516 L/R and Q517 L/R due to excessive input drive or if too low of a load impedance is connected across the output. If current increase is excessive, the voltage across R547 L/R turns on Q514 L/R. Then Q512 and Q515 turn off Q509, causing relay 501 to turn off.
The power supply:
You might find higher dc voltage with greater voltage regulation in the separate amplifier. The average output voltage might vary from 24 to 85 Vdc. Several different low-voltage circuits can be located in the power amp ( FIG. 5). The higher voltages feed the power-output transistors and IC components, and the lower voltage feeds the preamp and AF circuits. You might find both positive and negative voltage sources in the low-voltage power supplies.
FIG. 5 Circuit diagram of a low-voltage power supply with full-wave bridge rectifiers. Note the center-tapped secondary winding goes to the negative source.
The full-wave, low-voltage supply might have very large filter capacitors ( FIG. 6). The capacitance might vary from 1500 to 4700 uF in the average amplifier with four separate 10,000 uF filter capacitors in the high-power amplifiers. The 50 W amp might have 12 A bridge rectifiers. In the high-power amps, a 30 A bridge circuit might be found. Amplifiers over 250W might have electrolytic capacitors up to 144,000 uF. Replacement of these large filter capacitors must be taken into consideration if there are filter hum problems.
System dead—nothing operates Inspect the fuse for blown sign marks or test with the low-ohm scale of the DMM or VOM. Note if the ac solenoid comes on in some models. Next take a voltage measurement at the large filter capacitor or power-output transistor. No voltage might indicate a leaky diode or power transformer. Check each diode with the diode function of the DMM, if diodes are found separately (FIG. 7). Most bridge circuits are found wrapped up in one component. It’s best to remove the single bridge-rectifier component from the circuit for accurate leakage tests. Check the On/Off switch for burned contacts.
Quickly check the primary winding of the transformer if there are no pilot lights or voltage at the filter capacitors. Leaky audio-output transistors or ICs can destroy the primary winding if it does not have fuse protection. Suspect an overloaded circuit, leaky filter capacitors, or a leaky voltage regulator transistor if the LED lights dim and the amp groans when it’s plugged into the power line.
FIG. 6 Very large filter capacitors are found in the typical power amp (1500 to 4700 uF), but in higher-wattage amps, the filter capacitor can vary from 8000 to 144,000 uF.
FIG. 7 The positive probe of the DMM is connected to the anode (—) terminal, and the negative probe is connected to the cathode in a normal test. The opposite is true with a VOM tester.
Keeps blowing fuses---Visually inspect the low-voltage power and audio-output circuits for signs of overheated or burned components when the fuse won’t hold ( FIG. 8). Remove B+ fuses to the various circuits to eliminate an overloaded circuit found in some large amplifiers. Cut the circuit at the first large filter capacitor and take ac voltage measurements, if the fuse blows without ‘the external circuits being connected, suspect leaky silicon diodes or power transformer.
Remove the secondary leads from the silicon diode rectifiers and take ac voltage measurements, if the transformer makes a noise with low ac voltage, suspect shorted turns within the transformer. Feel the case of the transformer and notice if it’s very warm. Some power transformers will run warm after many hours of use, but should never be too hot to touch. Most overloaded circuits occur in the power-output transistor or IC components. Replace the power transformer with one having the original part number.
Dead on both channels---Suspect the low-voltage power supply or a dual IC circuit when both channels are dead. Quickly check the voltage at the large filter capacitor. Measure the high dc voltage at the output transistors or IC circuits. Check the low-voltage regulator circuits for leaky transistors or zener diodes if there are no voltages at the output circuits ( FIG. 9). A large, leaky IC found in the output circuits might cause both channels to be dead. Don’t overlook blown speaker fuses.
Dead left channel---Identify the dead channel with the external audio amp or from the chassis parts layout chart. Check the left-channel speaker fuse if one is used. Measure the voltage at the audio-output transistors or IC. Compare these voltage measurements with the good channel. Improper or very low voltages can be caused by leaky output transistors or low-voltage power supply circuits. Normal voltages at the audio output stages might indicate a defective driver, AF, or preamp stages. Note if the left channel VU meter is working, indicating what section of the channel is dead.
FIG. 9 A simple low-voltage power-supply regulator circuit might consist of a transistor and zener diode. Suspect a leaky transistor with higher output voltage. A leaky zener diode might lower the regulator output voltage.
FIG. 8 Some large amplifiers have each voltage and speaker source fused. Suspect leaky output transistors or ICs when fuses won’t hold.
The dead or weak audio stage can be hum checked with both channel speakers connected and the volume raised up one-quarter turn. Switch the amp to phono. Place your finger on the center terminal of the phono jack. You should hear a loud hum. Now check the other channel. Don’t turn up the volume too high, or you might damage the test speaker. Signal trace the preamp and AF circuits with the external amp or the audio 1000 Hz signal generator with the speaker or scope as indicator.
Go directly to the audio-output circuits if there are signs of distorted hum in the speaker of the defective channel. Measure the voltage at the collector (body) of each power-output transistor and IC. Compare these voltage measurements with the normal channel ( FIG. 10). Check each transistor for leaky or open conditions if there are improper voltage measurements. Remove each transistor from the chassis and test each individually if there are signs of leakage in the circuit. Make both open and leakage tests.
Mark down the condition of each transistor. Put back each transistor that tests good. You might find four large output transistors in push-pull operation. Now check the condition of the driver transistors. Check all transistors in directly driven output circuits. Remember, Darlington transistors cannot be checked accurately in the circuit with ohmmeter or DMM tests. Sometimes it’s best to replace all directly coupled audio-output transistors when two or more are found leaky or open.
Check all voltages on the suspected audio IC output component with no sound. Low or improper voltages found at the IC terminal might indicate a defective IC. If one channel is dead and the other normal with one IC feeding both channels, replace the defective IC. Check all electrolytic bypass capacitors and resistors connected to the IC terminal. Don’t overlook an open condition in a large coupling capacitor in the speaker circuit ( FIG. 11).
Weak right channel--- A very weak channel is relatively easy to service compared to a channel that just does not balance. The weak channel can be located with the audio-signal generator and scope. The external audio amp is ideal for locating a weak or distorted channel. To help locate the defective component, you must have a schematic diagram (FIG. 12).
The weak channel might be caused by a leaky transistor or IC circuit. Open or dried electrolytic coupling capacitors might cause weak audio. A slight loss of audio in one channel might be difficult to locate. Sometimes the balance control is thrown off just enough to warrant locating and replacing the defective component. A change in resistors or coupling and bypass capacitors in the preamp stages might throw off the balance control ( FIG. 13). Signal tracing with a 1000 Hz tone injected at the auxiliary or phono jack of both channels with the scope as indicator might locate the weak stage. Compare the scope waveform at the same point in the circuit as the normal stage.
Won’t balance--- Look for a defective coupling capacitor in the preamp and AF circuits when one channel won’t balance up with the other. Small 1, 3.3, 4.7, and 10 uF electrolytic capacitors cause a lot of balance problems. Check the signal on both sides of the suspected capacitor. Slowly signal trace each stage until you find the small loss of audio signal. Check the amplifier circuits for balance or level controls and follow the manufacturer’s literature for correct balance alignment.
FIG. 10 Suspect a defective dual-channel power IC if one channel is distorted and the other dead. Check voltages and signal with the good channel and compare the measurements. To left speaker; To right speaker
Weak channel case history The right channel was a little bit weaker than the left and would not balance up in a JC Penney Model 3222. The signal was traced to the preamp circuits of Q403 and Q404. The right channel was weak compared to the left channel transistors. Both Q403 and Q404 were tested in the circuit and were normal. The signal was scoped with the test tape. Actually, capacitor C414 (10 uF) was dried or had lost some of its capacity ( FIG. 14). Replacing coupling capacitor C414 balanced the channels.
FIG. 11 A dead stereo channel might be caused by an open electrolytic capacitor between transistor or IC and speaker. Signal trace the audio right up to the PM speaker.
Magnetic preamp Because the audio signal is much lower in the magnetic pickup compared to a crystal cartridge, one or two stages of audio must be added in the preamp circuits. The separate magnetic preamp circuits start at the magnetic input jacks and are switched into the circuit of the preamp audio stages ( FIG. 15). The magnetic amplifiers might be low-noise transistors or IC components. Often low collector voltages of under 10V are found in these circuits.
FIG. 12 A schematic diagram is a must for locating defective parts or taking correct resistance and voltage measurements.
FIG. 13 Signal trace the audio through the preamp, AF, and audio-output circuits to locate the defective stage when one channel won’t balance up. Suspect small electrolytic capacitors if there is weak sound. From function; Components that might be defective are circled; Preamp stages
Dead right channel—distorted left channel Here both channels have some thing wrong with them. Check the circuits that would cause problems in both channels. Improper low-voltage conditions can often cause low audio and distortion in both channels. It’s possible to have a leaky or open transistor in the right channel and a leaky transistor in the left channel, producing distortion. Likewise, two separate IC output components might be found defective at the same time. Both channels might be defective in a dual-IC in the output circuits ( FIG. 16). Always check signals in and out with the scope or external amp. A leaky electrolytic bypass capacitor in the dual-IC circuits might cause one channel to be dead and other channel distorted.
Distorted left channel ---Excessive distortion found in either channel might be caused by leaky driver or output transistors and IC components. Weak volume might appear along with distorted audio ( FIG. 17). Distortion found in both channels might be caused by a leaky dual-IC output or an open filter capacitor in the negative voltage source of a transistorized output circuit. Spray the suspected transistor or IC with coolant when the distortion occurs after the chassis has been on for a few minutes to determine which one is leaky.
FIG. 14 In a JC Penney Model 3222, a defective C14 (10 uF) capacitor caused a weak right channel.
FIG. 15 The AF and preamp stages are located right next to the power output transistors mounted on a heat sink.
FIG. 16 With a dual-power IC, you might find only one or both channels defective. Signal trace the audio in and out of suspected IC; then take critical voltage measurements.
FIG. 17 Check the circled components in the audio output circuits for distortion. You might end up replacing four or five transistors and the corresponding bias resistors. To left speaker; Check components circled for distortion:—29.5 V, + 29.5 V
Always check the bias resistors when a leaky transistor is found. Remove one end of each resistor for accurate resistance measurements. Look for leaky bias diodes in the base circuits of the audio-output transistors for a small amount of distortion. Again remove one end of the diode and check for leakage. Lower negative voltage in the high-power amps might cause some signs of distortion.
The distorted stage can be located with the audio-signal generator and scope. The external audio amp can isolate a distorted circuit. Go from base to collector of each transistor or from input to output of each IC component. Replace both audio output and driver transistors when you find one transistor shorted. While the transistors are out of the circuit, check each bias resistor. Compare the voltage and resistance measurements with the good channel after replacement.
Noisy right channel---The noisy channel can be isolated by turning down the volume control. If the noise stops, the defective component is in the AF and preamp stages. Check the driver and output stages if the noise continues. The noisy transistor or part can be located with the scope or external audio amp. Heat and coolant treatments might help in locating the noisy component.
Check the driver and output transistors for a loud popping and cracking noise. Most noise symptoms are caused by transistors and IC circuits. A high-resistance junction in a transistor might cause noise with a lower level of audio in the same channel. The high-resistance junction can be located with the diode-transistor test of the DMM. The normal junction-resistance measurements of a good transistor are quite close. For example, a normal AF transistor might have a resistance measurement from base to collector of 0.670 ohm and from base to emitter of 0.675 ohm. The measurement from base to emitter might increase to 0.980 ohm indicating a high-resistance junction between these two elements.
Suspect a noisy transistor or IC component when the noise does not appear after the amp has been on from three to four hours. Apply coolant to each transistor until the noise disappears, indicating the noisy transistor. A noisy bias or level control might produce noise in the channel. Sometimes a dirty wiper blade or a change in resistance between wiper and resistance might cause noise. Check for an increase in resistance of the high-value resistors in the base and collector circuits of the AF and preamp circuits. The increase might cause a hissing noise in the audio. Replace with carbon-film resistors if available. Suspect a noisy bypass or coupling capacitor in the AF and preamp circuits. Look for a loose component or screws grounding the PC board to the chassis if you hear movement or noise from inside the cabinet when it’s moved or jarred.
A loud plop or frying noise at the On/Off switch turn-on point might be caused by arcing between On/Off switch contacts. The sound might stop or continue for several minutes. In either case, replace the switch. If you must order a switch, try taking the old switch apart and cleaning it (if it’s a large switch). Clean off burned or pitted contacts with a knife, sandpaper, or fingernail file. Wipe off contacts with alcohol and cloth. This temporary repair might keep the amp operating until the new switch arrives.
Locating noise in a Sansui Model 2000---A frying noise was isolated to the audio-output stage by lowering the volume control in the left channel. The noise would begin only after about four hours of operation. Coolant applied to each AF, driver, and output transistor did not locate the noise. The noise was signal traced to the audio-output transistors with the external audio amp.
Because the output transistors were not readily available, and to prove they were noisy, the two left output transistors were interchanged with the right-channel transistors (2SC793). Simply remove the two mounting screws of each transistor and lift the power transistors. After three hours of operation, the frying noise was heard in the right channel, indicating the two output transistors of the left channel should be replaced with the originals. Sometimes a transistor tester might not locate the noisy transistor. Replacement is the only answer.
Intermittent left channel---Most intermittent conditions are caused by transistors and IC components. Apply heat or coolant to the transistor or IC to make it act up. Small electrolytic-coupling and large speaker capacitors can cause an intermittent audio. Sometimes a defective capacitor or resistor can be found by moving it with a plastic rod or pencil. Dirty contacts in the plugs from the preamp to the main chassis might produce intermittent sound.
The intermittent channel can be signal traced with the scope or external amp to locate the intermittent stage. Play music from either the tuner or phonograph. Divide the circuits in two at the volume control to save time. Of course, monitoring the intermittent channel takes a lot of time, especially if the amp does not act up for several hours.
Excessive hum---A loud hum is much easier to locate than a slight hum. Hum with distortion can occur in the power supply or audio-output circuits ( FIG. 18). A defective or open electrolytic filter capacitor might cause loud hum in both channels. Low hum in both channels might be caused by a dried electrolytic decoupling capacitor ( FIG. 19). Always clip large filter capacitors into the circuit for hum tests with the power off. Suspect the audio-output circuit with excessive hum in one channel.
A loud hum with weak sound might occur in the audio-output transistors or ICs. Check for leaky and open transistors. Signal trace the input and output terminal of the power IC component. If normal signal is found at the input and distortion at the output terminal, suspect a leaky IC. Take accurate voltage measurements on each IC terminal. Replace the leaky IC if the voltage measurements are quite close.
Check for burned or overheated bias resistors with poor terminal connections when the hum begins after several minutes of operation. Double check variable bias controls. Inspect the PC board for burned areas around transistors and electrolytic capacitors. Note if black or white material is leaking out of the filter capacitors. Replace filter or decoupling capacitors that are warm to the touch.
Loose shields or poor board grounds might cause intermittent sound or motor boating. Push up and down on the chassis to see if the hum comes and goes. Poor connections and ground wires of cable plugs and harness might produce pickup hum. Suspect a dried decoupling filter capacitor if there is motor-boating on both channels. A defective audio-output transistor might produce a motor-boating symptom. Low hum levels and motor-boating stages can be located with the external audio amp.
Signal tracing ---The audio circuits can be signal traced with the scope and external audio amp as indicators. Use the cassette tape player with a 1 kHz tone cassette connected to the audio amp. A dead, weak, or distorted stage might be located quickly.
FIG. 19 Check for dried or open decoupling filter capacitors in the regulator or in the preamp AF stages to locate the source of a slight hum in the audio.
FIG. 18 Check the various components that might produce hum in the audio circuits. Turn off the power and discharge electrolytic capacitors before shunting large filter capacitors. Be sure power is off before clipping filter capacitors into the circuit.
Take the external audio amp with test leads and signal trace from stage to stage to locate the defective component. Of course, any external amp can be used, but here is a circuit that can be placed right in a speaker cabinet.
You can purchase a small amplifier already mounted on a PC board or build your own ( FIG. 20). The chassis can be mounted in a small speaker enclosure that costs a few dollars at electronics outlets. Select one with an 8 1 speaker at least four or six inches in size for normal sound. Many of the parts can be found in your junk box. This little external amp can be used in signal tracing radio, phono, and cassette player circuits besides the regular audio amplifier.
Smoking amp---Where there is smoke, there are burned bias resistors, over heated power transformers, and transistors in the audio amp. Burned or smoking bias resistors can be caused by leaky transistors in the audio-output circuits. You might find burned voltage-dropping resistors caused by leaky or shorted filter and bypass capacitors. A smoking power transformer might occur if there are shorted or leaky output transistors or diode rectifiers. Sometimes lightning damage or line-power outage might destroy the power transformer.
Expensive repairs might be needed if the power transformer must be replaced along with other components. Power transformers are rather expensive. Determine what caused the transformer damage. If the bridge diode rectifier becomes leaky or shorted, knocking out the transformer, the additional component cost is not worth it. If several power-output transistors damaged the power transformer, the repair might be costly. In fact, you might find four or six audio transistors open or leaky in the output circuits in addition to burned bias resistors and a transformer.
By taking critical resistance measurements on the power output transistors or ICs might help locate a defective component in directly driven output circuits. Connect a speaker bank load resistance across the defective speaker terminals, especially in directly coupled circuits. When a power transistor or IC becomes shorted or leaky, dc voltage might be placed directly across the speaker terminals, destroying the woofer speaker. The output of the balanced, directly coupled amp circuits is always at zero potential.
To protect any connected speaker, install a power load bank instead of the speaker. Then measure the voltage across the speaker jack terminals. Any dc voltage measurement indicates a defective directly coupled amp circuit. Although the power load resistors will run warm, you might have time to find the trouble and not ruin another speaker in the process.
Resistance measurements in the output stages might turn up the defective component, especially when the unit keeps knocking out fuses or destroying the output transistors or IC parts when the chassis is fired up. First, take resistance measurements from each transistor or IC terminal to common ground. Compare each reading with the good channel. For instance, if the left channel keeps destroying the IC output and the right channel is normal, use the right channel as a reference point in the circuit.
A VTVM or VOM might work best in resistance tests, but the DMM must count down or change numbers and stop for the correct resistance. Of course, the DMM is much more accurate in low resistance measurements. Start with terminal 1 of the defective IC and compare each terminal with the good channel. Write down each resistance measurement.
FIG. 20 This is an audio amp you can build to signal trace audio circuits. The amp can be placed in a small speaker enclosure.
You will find this might take longer to locate a defective stage, but sometimes after all voltage tests have failed, it works. You will note that within low-ohm bias resistor tests, the measurement might be under 1 !1 and should be the same in both channels. Rotate the resistance control to a higher range if no measurement is found. In higher resistance measurements, each resistance reading should be within several ohms of the good channel ( FIG. 21).
FIG. 21 Take accurate resistance measurements between each IC or transistor terminal and common ground. Compare these resistance measurements to the normal channel to locate a defective component and after replacement.
When you find a big difference in resistance measurement, the defect is in that circuit. Often, you can replace the power output transistor or IC and the trouble still exists. If the different resistance on a certain pin goes to an input AF, driver, or Darlington transistor, proceed to that circuit with resistance measurements.
Sometimes, the suspected transistor or IC might appear normal when voltage measurements are made on each terminal. In resistance circuit measurements, the transistor might not be shocked into normal condition. The resistance measurement method can isolate defective bias resistors and transistors in the AF-driven circuits, using resistance measurements on the power output transistor or IC.
After installing a new power output or IC, take resistance measurements from each terminal or pinto chassis or common ground. To avoid damaging the replacement, take these measurements before applying power to the amp chassis. Make sure the rest of the audio circuits are normal with the good channel. If the resistance measurements are normal, fire up the chassis and take critical voltage measurements. For normal operation, these voltage measurements should be within a volt of the good channel. Check each power transistor or IC for running excessively warm.
Power output transistor replacement--- Try to replace each power output transistor with the original, If not available, replace with universal replacements. Cross reference the defective transistor number with those found in manufacturer’s transistor replacement guides. RCA, GE, Sylvania, Mallory, and Workman have universal replacement guides.
If the power-output transistor burns out after replacement, suspect a defective driver transistor or bias resistor. Recheck those bias resistors. It’s always best to replace four or five of the transistors in the driver and output stages. Sometimes an intermittent transistor won’t act up under test but will break down under load. Note if the transistors are running red hot after replacement. Excessive idling current might destroy the output transistor. Adjust the variable-bias resistor for correct current and voltage settings as specified by the manufacturer. Check to see if the operating voltage is too high compared to the normal channel. Always keep speakers connected to each channel while servicing the amplifier. Replace the speaker fuse, if blown, before applying power to the chassis.
Always replace the mica insulator found between transistor and heat sink. Some of these transistors in push-pull operation might have the transistor collector (metal body) bolted directly to the heat sink. Apply silicone grease on transistor body and heat sink and on both sides of the mica insulator ( FIG. 22). Be careful not to crack or break the insulator. If the corners are torn or broken off the insulator, throw it away and get another. Clear silicone grease is easiest to work with because it does not stain hands or clothes.
Idling current---In some large receivers, idling current is adjusted for correct adjustment of bias voltage within the power-output stages. This adjustment is to be made after the unit has warmed up for a few minutes. Actually, the voltage measurement is taken across the two emitter resistors of the power output transistors, through two 10 k-ohm isolation resistors. Idling current is adjusted in both the left and right power-output channels ( FIG. 23).
In Denon DRA-435R/335R receivers, the digital voltmeter is connected to test points TP1 and TP3 and to TP2 and 4. Adjust both VR305 and VR306 so that the digital voltmeter reads 5.0 mV ± 0.5 mVdc ( FIG. 24).
Darlington transistors---The Darlington transistor can be used in a driver, balance, or power-output stage. These transistors might have two directly coupled transistors internally operating in parallel. In high-power circuits, the Darlington transistors might have diode and bias resistors inside the metal case. You might find either NPN or PNP transistors.
FIG. 23 In some amplifiers, idling current adjustments should be made after the amp has warmed up for a few minutes.
FIG. 22 Here both power output transistors are being replaced in a Pioneer SA9 100 amplifier with insulator between each transistor and heat sink.
FIG. 14 Make both idling current adjustments with VR305 for TP1 and TP3, with VR306 for TP2 and TP4.
Actually, the Darlington transistor looks just like any other transistor with three leads or terminals, but they are very difficult to test with the DMM and with some transistor testers. The different ohm tests can be compared to a known good one in the other stereo channel. Signal trace the audio up to the suspected Darlington transistor and replace it. The suspected Darlington output transistor can be replaced with one in the good channel. Use the original part number when replacing Darlington transistors.
IC power output replacement---Be careful when unsoldering IC terminals to prevent damage to other components and PC wiring. Use solder-wick material to pick up the excess solder. Some smaller IC power-output components might have extra shields over the wiring area. Remove and replace all shields to prevent hum pickup. The power IC is rather easy to replace. Apply silicone grease to the back of the IC that touches the heat sink. Bolt the IC in place ( FIG. 25). Solder each terminal with a low-wattage soldering iron.
Poor bass---The treble and bass controls are often found after the preamp stages in the audio circuits. With stereo channels, the bass control has two controls in one. The bass is adjusted in both channels when the control is rotated ( FIG. 26). Check for a broken wiper blade or connecting wire to the center bass terminal if the bass does not change when adjusted. Lower the treble control and now turn the bass control. Check for an open electrolytic capacitor, bypass capacitor, or small resistor in the bass circuits if the treble control affects the music.
Cannot lower volume---Suspect a broken control or wire connection if the volume cannot be lowered in one channel with the other channel normal. Check for an open ground wire on the control terminals. Sometimes the dual volume and tone control grounds are connected together. Inspect the wire or soldered terminals for a badly soldered connection.
Keeps destroying the speaker---Excessive volume might blow the voice coil right out of the woofer speaker. Too much volume can destroy the speaker. Insufficient wattage speakers can be damaged by a very high power output amplifier.
Speakers can be damaged when operating directly from the power output transistors or IC circuits. A shorted IC or transistor might place excessive voltage at the speaker terminals, burning out the voice coil. Some speakers are protected with a speaker fuse ( FIG. 27), preventing damage to the speaker. If the fuse blows and is replaced with a larger fuse or wrapped with tin foil, the speaker will have no protection. Always replace the speaker fuse with one having the same amperage as the original.
In many of the speaker output circuits, a large electrolytic coupling capacitor is connected between the speaker and output transistors. Here the transistor voltage is isolated from the speaker voice coil. With larger, high-power amplifiers, the speaker output is connected through a fuse or resistor to the output transistors or IC component ( FIG. 28). Zero voltage should be found at these terminals. If the IC or transistors become leaky, higher voltages might be applied to the voice coil, knocking out the expensive speakers unless the speaker fuse opens. Always check the voltage at the speaker output terminals when a voice coil is damaged. Repair the leaky output stage, or you might quickly burn out another expensive speaker.
FIG. 25 The large power ICs either bolt to a large heat sink or metal chassis. You might find only one large IC for both stereo channels.
FIG. 26 Check the circled components when the bass control has no effect on the music. Sometimes a broken board connection or control can prevent bass control.
To right speaker; left speaker
FIG. 28 A resistor, fuse, or relay contacts might be in series with the power IC or transistor output to the speakers for protection. A dirty relay contact can cause a noisy or dead speaker channel.
FIG. 27 Check the speaker fuse if one channel is dead. Often too much power applied to the speaker knocks out the fuse. Replace the fuse with one with the exact rating.
Blown components---Sometimes it’s very difficult to explain why various components blow up. Electrolytic capacitors might blow due to a buildup of gas inside the plastic containers, which then become leaky and overheat. Small pieces of tin foil and insulation from the blown capacitor might be found inside the amplifier. This was the case in a JC Penney MCS series Model 3233 amp. A 470 uF capacitor was found with only the terminal connections left and pieces scattered all over the chassis ( FIG. 29). Transistor Q601 was found leaky and replaced. No doubt, higher dc voltage was applied to C602, causing it to blow its top.
FIG. 29 C602 was found blown apart in a JC Penney Model MCS-3233. Replacing both C601 and C602 corrected the defective regulator circuit.
Lighted power indicators---Like the LED tuning lights, some audio amplifiers have LED power-level indicators ( FIG. 30). The level meter circuits might connect to the audio-output speaker circuits. Often each channel has its own IC driver with corresponding LEDs. The LEDs and IC components are mounted on a separate PC board ( FIG. 31).
The peak or power level meter can be serviced like any other audio-meter circuit. Signal trace the sound from speaker circuit to the input terminal of the level meter IC ( FIG. 32). If the audio signal is going into the IC but is not applied to the LEDs, suspect a defective IC. Measure the dc voltage source (pin 9). Check the voltage at each LED pin number of the IC. Suspect a defective LED if only one is out.
FIG. 30 A lot of solid-state integrated amplifiers have power level LEDs on the front side of the panel. Actually, the rectangular LEDs poke through the metal front panel.
FIG. 31 The LEDs and IC components might be located on a separate PC board. Signal trace the audio un to the input of LED IC driver.
FIG. 32 Note a portion of the audio is tapped off the speaker output circuits of IC901. R968 is adjusted so D901 is just lit at normal sound. Suspect a defective LED when only one bulb is out.
Loudness control--- You might find a loudness control circuit in the audio-amp input circuits in some models. Usually the loudness switch is a DPDT (double-pole, double- throw) variety placed between the balance and volume controls (FIG. 33). When switched on, part of the signal is paralleled across the top of the volume control. In the off position, two small resistors are placed in the circuit. The small resistors and capacitors might vary in resistance and capacitance, but they serve the same purpose. Suspect a dirty loudness switch or broken wire on the volume control if the loudness switch has no effect on the audio.
Audio output resistance checks---After replacing audio-output transistors or IC components, take a resistance measurement from each terminal to ground. Compare the same resistance measurements between the ones just replaced and the good channel. This might take a few minutes, but it can be done rather rapidly with the DMJVI. Take one terminal measurement and compare this with the same terminal in the normal channel. These resistance checks should be within a few ohms of one another.
You might locate a poorly soldered connection, a shorted connection, or a defective component in the circuit. Many times, the burned or open bias resistors were actually defective in the first place. If you forgot to check out the new transistor replacement before installation, the leaky transistor will show up in the measurements. However, you have located a defective component connection before firing up the amplifier and damaging another part. Resistance and voltage comparison tests in the audio-output circuits with the normal channel are quite valuable in amplifier servicing.
Audio adjustments---Idler current, power meter, and peak-level meter adjustments should be made according to the manufacturer’s specifications. Because most idling current adjustments are made across the emitter terminals of the output transistor, the voltage and current requirements are different in each manufacturer’s output circuits.
FIG. 33 The loudness switch is usually located between the balance and volume controls.
Idling current is taken in milliamperes when no audio signal is present in the circuit. The same might hold true for power and meter adjustments. Idling current and bias adjustments should be made after replacing high-power output transistors.
Motor volume control---When controlled by a remote-control unit, the volume controls must be rotated by a dc motor. IC251 controls the small volume-control motor. The VD signal at pin 6 controls IC251. The motor is connected to pins 2 and 10. The motor shaft directly drives both right- and left-channel volume controls ( FIG. 34).
FIG. 34 In remote-control amps, the volume control might be rotated with a small dc motor driven by a motor IC.
Twelve case histories
Here are twelve problems that were found in different sound amplifiers. They are listed here in the hope that they will illustrate how the defective component was located. The audio problem discussed might be the same or similar to the one you now have on your service bench.
Keeps blowing fuses---The fuse would blow as soon as it was replaced in a Realistic STA-740 deluxe stereo amp. Because the low-voltage diodes and power circuits seemed normal, the power-output transistors were tested. Output transistors D896, B776, and driver transistors D600K were found leaky in the right channel. All bias resistors were checked while the transistors were out of the chassis. One 4.7 ohm bias resistor was found open and replaced. Universal transistors were used for replacement. Diode D896 was replaced with an ECG36, B776 with an ECG37, and D600K with an ECG373.
Dead—no right channel---The left channel was normal in a Sanyo Model 2050 with a dead right channel. The audio signal at the output transistors were the same volume on both channels with the external audio amp. On checking the circuit, L801, S7-2 and speaker terminal connections were suspected. The signal was normal up to S7-2 ( FIG. 35). A dirty relay contact S7-1 was cleaned, restoring the right speaker.
FIG. 35 A dead channel in a Sanyo Model 2060 was caused by dirty speaker relay controls of S7-1. Clean contacts with cleaning fluid and sandpaper. Check R720 and L701 if the woofer speaker cone is damaged.
Dead left channel --- The fuse would open in the left channel of a Pioneer Model SX-1000 TW. Right away a burned 0.7 C bias resistor was spotted. Both 25G897 output transistors were replaced with a universal SK3535 replacement. All other transistors and bias resistors were checked in the same output circuit. Because voltage measured at the power output transistor was 75 V, replacement with either high-voltage types or originals was necessary.
Intermittent left channel --- Besides the intermittent left channel, a low noise could be heard in a AKAI Model CR-80 T. The intermittent signal was monitored at the input terminal of the left output IC. Then the speaker cut out, the signal was normal at the input terminal. The input sound was compared to the normal right channel. Replacing the STK-011 IC output with a universal SK3152 replacement solved the intermittent left channel.
Noisy left channel --- After operating for three hours, a low frying noise was heard in the left channel of a Sansui Model 2000. The noisy channel was isolated by turning down the volume control. The signal was the same with the volume control turned down, indicating the noisy component was in the output circuits. Starting at TR8O1, the external amp noise was traced right up to the left output transistors. Both 2SC793 output transistors were replaced, eliminating the noisy left channel.
Keeps blowing speaker fuse---Sometimes the speaker fuse will blow ii too much volume is applied to the speakers. The 3A fuse was replaced and again blew out in a JC Penney Model 3235. All right transistors within the left channel tested normal. A quick check of the circuits found R537 burned with the top of C516 blown off. On checking the circuit, D501 was found to be leaky ( FIG. 36). After replacing all three components, left channel audio was restored.
FIG. 36 The fuse kept blowing in a JC Penney Model 3235. Replacing R537, D501, and C516 solved the speaker fuse problem.
Low-level hum---A slight hum could be heard in a JC Penney Model 3835. The customer had complained of smoke coming out of the amplifier. A visual inspection of the chassis did not turn up anything. The voltages measured at the IC output were +36.1 V and —40.1 V. The two voltages should be the same but with opposite polarity. Finally, arcing seemed to occur at the bottom of the large filter capacitor (6800 F, 45 V). With the bottom cover removed, the capacitor was found to be arcing between the terminals and PC board. The capacitor was removed and part of the burned PC board was cut out. Replacing the large filter capacitor and repairing the PC wiring solved the low-hum problem.
Low hum in sound---Low hum was heard in all functions of a Sylvania Model SCT6734P receiver-phono combination. Low hum was heard at all times. When a radio station was tuned in and volume turned up on phono, the audio would motorboat loudly.
At first, filter capacitors were suspected in the low-voltage power supply. Shunting each capacitor with a larger one did not help. After taking critical voltage tests in the low voltage circuits, 1C504 regulator was found to be leaky ( FIG. 37). Replacing the IC with a universal ECG763 replacement solved the low-hum problem.
FIG. 37 Low hum in the sound was caused by a leaky 1C504 in a Sylvania SCT6734P stereo console.
Weak volume on left channel---The signal was traced from the volume control to the preamp circuits in a JCPenney Model 3230 with weak volume in the left channel. Q202 and Q204 were checked in the circuit, and Q204 was found leaky between the base and emitter. Voltage measurements on Q202 were quite high compared to the schematic ( FIG. 38). Replacing Q204 with a GE-20 universal replacement restored the weak channel and returned the voltages to normal.
No volume, distortion---Very little volume was found in the left channel of a Sansui Model 5000X. There was no voltage at the left-channel output transistor. Eighty volts was found at the right channel transistors. The missing voltage was traced back to an open 4 A fuse (F002). Because the left channel was weak and had distortion, the power-output stages were checked first. Replacing the left channel B + fuse solved the problem.
Dead-power--transformer warm---After several power line fuses were blown, the power transformer was running quite warm with a dead Pioneer SA9 100 amplifier. A dead short was noticed across filter capacitor C502 (8000 uF). The ground lead was removed from C502, and the short remained across the capacitor terminals ( FIG. 39). Replacing C502 with an exact replacement solved the warmed-over power transformer.
FIG. 38 A leaky preamp transistor (Q204) in a JC Penney Model 3230 caused a weak left channel. Higher than normal voltage was found on Q202 since it was directly coupled to Q204.
FIG. 39 The dead and overheated power transformer in a Pioneer SA-9100 amp was caused by a shorted filter capacitor.
Left channel dead—distorted right channel---Both channels were found defective in a Sanyo DXT-5004 amplifier with two separate IC output circuits. The symptoms pointed to a defective low-voltage power source. Normal voltages were found in the low-voltage power supply. After signal tracing both channels, each separate power output IC was replaced. Here the power IC in the left channel was open, and the power IC in the right channel was leaky. Both ICs were replaced with the original part number (ICLA4O51P). Occasionally, you might find defective components in both stereo channels.
Building a high-power speaker load:
You can build a high-power speaker bank load for less than $20.00. Buy the heavy-duty resistors from electronic mail order firms. These high-wattage resistors can be chosen from 25, 50, 100, 150, and 175W sizes. Either wire them in series for higher resistance or parallel for greater wattage.
Here a 0 to 175 W speaker load was made up of two 10 ohm 175 W variable resistors. One 10 ohm resistor might be adjusted for the 4-ohm load, and the other set for 8 ohm impedance. The two resistors can be put in parallel and set for either a 4 or 8 ohm load with 350 W of power. You can take one 175 W resistor and half the resistance of 5 ohm for the 4 ohm impedance and parallel the resistance, making it a 4-ohm load at 350W ( Fig. 40).
FIG. 40 Build your own high-wattage speaker load bank out of 175W resistors for under $20.00. Use the load down the high powered amp while working on an amplifier.
Parts list for the high-power speaker load
10 ohm 175 W power resistors from: Three banana plugs; One metal cabinet
Twelve do’s and don’ts:
1. Don’t place too large an ac line fuse in the amp, or you might damage the primary winding of the power transformer.
2. Don’t insert too large a speaker fuse or wrap the defective one with tinfoil. A leaky transistor or IC in a directly coupled output circuit might place high dc voltage on the voice coil of the speaker and burn it out.
3. Do check all transistors in the power-output circuits for leakage both in and out of the circuit. Replace both output transistors when one is found defective.
4. Do be sure to test the new transistor replacement before mounting. It might take even more time to diagnose the same problem the second time.
5. Do double check each transistor replacement for correct terminal connections before soldering. Check both top and bottom sides of the PC board.
6. Do use a low-wattage soldering iron when soldering the IC and transistor terminals. Inspect each terminal for a good soldered connection.
7. Do replace the heat sink before trying out the amplifier after transistor or IC replacement. If you don’t replace heat sinks, you could damage the replaced component within minutes.
8. Do be careful when shunting filter capacitors in the power-supply circuits. Clip them in with the volume off. Always discharge the capacitor. Make sure you have the right capacitance and voltage ratings.
9. Don’t forget to have a speaker or a load resistor connected to the speaker output terminal while working on the amplifier. You might damage the output transistors or IC with the volume turned up.
10. Do use the normal stereo channel as a reference point when signal tracing and taking voltage and resistance measurements.
11. Do take comparable resistance measurements between transistor terminals of output transistor and ground. Compare these measurements with the normal channels. You might have burned or open bias resistors if the resistance measurements are way off.
12. Don’t assume right away that the defective component will be difficult to locate. First look for the obvious. Usually burned resistors, leaky or blown capacitors, and other similar problems can be spotted right away.
Prev. | Next