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The audio circuits in special consumer electronics might consist of the telephone answering machine, speaker relay circuits, remote control, and muting circuits. Servicing the tube amplifier of an ac/dc radio, musical and high-powered amplifier might be considered special circuits. Since the vacuum tube is finding it’s way back into the consumer electronic field with Hi-Fi audio gear, that might include amplifiers, preamps, phono stages, and tuners. Troubleshooting the tube chassis can introduce different service problems.
FIG. 1. The telephone answering machine takes a telephone message when one is not home or available.
TROUBLESHOOTING AUDIO CIRCUITS IN THE ANSWERING MACHINE
The audio circuits in telephone answering machines are quite common to most cassette audio circuits. The audio circuits might consist of a recording amp, preamp, or main amplifier. The microphone (condenser or electret) circuit is fed into the recording amp, to outgoing and incoming RIP heads, audio preamp, and finally to the power output IC.
FIG. 2. A block diagram of the audio system within the telephone answering machine.
The audio from the telephone line is amplified and recorded on a miniature cassette. When the message is recorded, the cassette can be played back by pressing the play button. The electret microphone is used to change or make different recordings of outgoing messages. The recording amplifier is used in only recording messages and disabled during audio playback. The audio preamp is not used during incoming messages. Both the preamp and power amplifier are in force to repeat the incoming message.
The recorded message is taken from a recording transistor or IC amplifier circuit and capacity coupled to a volume control. The preset volume control audio is fed to the input of a transistor or IC preamp stage. The voltage preamp IC amplifies the weak recorded message and is directly-coupled to a power output IC or transistors. An electrolytic capacitor (33 uF) couples the audio to a 32 ohm pm speaker. The audio output circuits can be two push-pull transistors or one large IC. The ac power supply might consist of a fullwave or bridge rectifier circuit with a large electrolytic (2000-3000 1 filter capacitor. The function switch or push buttons apply voltage to the various answering machine circuits. Very little current is drawn when in the off position. Most power supply problems are related to improper voltages with dried-up filter capacitors and leaky silicon diodes ( FIG. 4).
A no erase or jumbled recording might result from an improperly seated cassette or one with a dirty or open tape head. The erase head circuit might be excited with a dc voltage switched to the erase head. The no-playback message might be caused from a defective transistor, IC or amplifier component.
A weak or distorted message can be caused by a dirty tape head, defective microphone and wrinkled cassette tape. Check the microphone and input circuits when the announcements and incoming messages don’t record. Suspect the input circuits when the message is not recorded. Clean up the R/P head for a weak and distorted message. A no sound system might result from a defective amplifier or power supply circuits. Clean up the function switch, shunt filter capacitors, and check leaky blocking diodes, when the amplifier oscillates in the play mode.
FIG. 4. A simple ac power supply in the answering machine.
Intermittent and erratic sounds might result from a dirty function switch or push buttons. A push button may not seat properly, appear erratically, indicating poor switching contacts. Spray cleaning fluid down inside the switch area. Rotate function switch or move push buttons up and down to help clean the switch contacts. Be careful not to apply too much cleaning spray that might drip down on the moving tape, idlers and belt area.
PHONO EQUALIZER AMP CIRCUITS
The phono equalizer amp might also be called a tone or phono head amp with a single transistor or two transistors in a directly-coupled amp circuit. The equalizer phono amp circuit is ahead of the stereo volume controls. The auxiliary and playback jacks might be switched into the volume control in the high-wattage amplifier circuits. A magnetic phono pickup connects directly into the equalizer circuits. Both stereo equalizer circuits are identical.
Within the high-powered amplifier, the stereo equalizer circuits might have a phono 1 and 2 as phono input jacks. Either of the stereo phono circuits can be switched into the input of the equalizer phono amp circuits ( FIG. 5).
A 3.3 uF electrolytic capacitor couples the input phono audio to the base terminal of Q501.
Q502 is directly-coupled to Q501 and the signal is amplified to a function switch.
This same switch assembly switches in the AM/FM/MPX, tape playback and phono circuits.
FIG. 3. Typical audio stages within the telephone answering chassis.
FIG. 5. A phono equalizer circuit with PNP audio preamp transistors.
Check each transistor with in-circuit transistor tests with the transistor tester or diode- transistor test of a DMM. A leaky transistor can cause a weak and distorted phono audio signal. A leaky or open transistor can cause different voltage measurements on the other directly-coupled transistors. The open electrolytic coupling capacitor provides a dead phono audio circuit from the pickup cartridge. Take critical voltage measurements on each transistor to determine if open or leaky. A change in an emitter resistor can produce a distorted audio signal.
REMOTE CONTROL AUDIO CIRCUITS
The hand-held transmitter can change the volume within the latest TV chassis. The remote can increase or decrease the volume in the table-top AM/FM/MPX receiver. The volume control IC driver might operate a motor that controls the volume in a digitally synthesized audio/video surround receiver or an integrated stereo component system.
The audio within the RCA CTC1 57 TV chassis is controlled by signal from the Al U (U3300) processor that is connected to the input circuits of a volume control IC. Besides controlling the sound, U3300 also controls the bass, treble, and balance of the audio output circuits. The volume control IC (U1 801) internally controls the audio signal fed to the sound output IC. Both the left and right audio stereo volume is controlled by U1801.
The left channel audio is fed into pin 4 and the right audio is at pin 6 of U1801. The microprocessor AIU (U3300) signal controls the volume internally for U1801 at pins 2 and 9. The controlled volume of U1801 is found at pins 3 and 7. Thus, the audio is coupled to the power IC (U1900) through a 1 uF electrolytic capacitor at pins 4 and 8. U1900 amplifies the controlled audio from pins 1 and 13 to each stereo speaker.
Scope the system control volume signal at pins 37 of AIU (U3300). Suspect IC (U1801) when the volume cannot be controlled with remote transmitter. Take critical voltage measurements at each IC terminal.
Replace U1900 for no or distorted audio. Solder all pins on U1900 for distorted audio, especially ground pins 6 and 14. Intermittent sound can result from poorly soldered connections of U1900. Solder the ground terminal of IC1900 for a hissing and popping noise in the speakers. Replace leaky C3313 (0.22 uF) capacitor from system control U1801 for noisy or no sound in the speaker. Take critical voltage measurements to locate a defective U101 or U1900.
FIG. 6. The volume is automatically controlled by U1801 at input 2 and 9.
SERVICING RECEIVER VOLUME CONTROL CIRCUITS
The rotation of the volume control in remote controlled receivers are both electronic and mechanical. The mechanical rotation of the motor turns the volume control up and down. A motor controlled IC signal is fed from the system control IC.
When holding or pressing the remote volume control buttons, the positive button raises the volume as long as the button is held. Likewise, the motor is rotating the volume upwards, increasing the volume level. The negative button reverses the motor direction or lowers the volume setting when the button is pressed.
FIG. 7. A close-up of a remote transmitter that controls the volume of an AM/FM/MPX receiver.
The volume control motor is controlled directly by a motor control IC. The motor volume- driver IC might be controlled by a PLL Controller IC in a stereo receiver. A volume indicator light lights up, indicating the volume is being rotated or changed. An LED is often controlled by a single transistor when the volume is rotated. The voltage out of (out 1) rotates the dc motor in one direction and out of (out 2) turns the motor in the reverse direction. A change in polarity of the dc voltage applied to the motor controls the direction (up & down) of the volume control.
Most schematics don’t show voltages for the motor control IC. Once these correct volt ages are taken, they should be marked on the schematic. Some separate voltage listings might be found on a voltage chart. Mark the correct voltages on the IC terminals before Troubleshooting the motor circuits. Measure all voltages on the driver IC terminals. Press the volume up button to apply voltage to the motor and then check for correct voltage. Likewise, measure the voltage on the down terminal of motor control IC to reverse the motor.
No motor rotation might be caused by a leaky or defective motor control IC371. Suspect a leaky motor drive IC if the voltage is low at pin terminal 2. Erratic or intermit tent rotation of the volume control knob can result from a defective motor control IC or poor board terminal connections. Suspect a defective zener diode when the volume will go way up or down, once pressed and when the button is released.
FIG. 8. A volume control driver IC circuit in a stereo receiver.
Suspect a defective volume control or speaker relay when the sound is intermittent or volume is increased. Check for open volume control when one channel has no volume. Suspect a defective system control IC, motor control IC, and transistors on the synthesizer PCB for no control of volume. Suspect a leaky capacitor off of the volume control terminal or system control IC for no volume control action.
When the volume is changed and a squeal is heard in the speaker, suspect a defective electrolytic coupling capacitor off of the volume control IC in the TV chassis. Readjust the audio bias control or when the volume is increased, check for open control at high volume level.
REGULATED MOTOR CIRCUITS
The motor circuits in the cassette player might operate directly from a fixed or regulated voltage source. The cassette player might have two motors for high-speed dubbing and normal record-play modes. In lower-priced models, one motor might drive two different tape decks (play only and record/play decks). A long belt arrangement with one motor rotates both capstan drives in each tape deck. When two separate motors are found in two separate tape compartments, one motor operates the record/play head unit in play, fast-forward and reverse modes. The play only deck is operated by another motor with a single play head and one erase head.
The early cassette motor circuits were quite simple with a motor leaf switch (SW1-1), electrolytic bypass capacitor and dc motor. The motor operated from several batteries in series or from a step-down power transformer in the ac power supply. No voltage regulator circuits were found in the play only cassette player.
A cassette motor might operate from a fullwave or half wave ac power supply. The cassette motor can operate from a dc regulated source or within an auto-stop motor circuit. When the cassette tape reaches the end of rotation, the motor stops, and is shut down by a transistor auto-stop circuit. The typical motor regulated circuit consists of one or two separate voltage regulators, controlling the voltage source applied to the motor winding. Q101 regulates the 11.lv dc voltage applied to a cassette motor (M). The zener diode (ZD101) helps to set the regulated voltage source.
FIG. 9. One dc motor controls a belt driven capstan in each cassette department.
FIG. 10. The cassette motor transistor regulator circuit.
FIG. 11. The cassette motor transistor regulator circuit.
When the motor won’t rotate, measure the dc voltage across the motor terminals. An open regulator transistor provides no or very low voltage to the motor terminals. When the regulator transistor becomes leaky zener diode ZD1 Q1 can overheat, resulting in improper voltage at the motor terminals. The defective cassette motor might have an open winding, worn brushes or a dead spot. Replace the suspected motor, when tapping the end-bell and the motor changes speeds, or the motor begins to rotate, when the motor pulley is turned by hand.
The cassette player with high/normal speed and motor control dubbing might contain several different transistors to control the speed of the cassette motor. The dubbing switch applies dc voltage from the dubbing/record switch to a dubbing control transistor (Q641). The dubbing motor control transistor controls the motor off/on transistor that applies a negative or positive voltage to the cassette motor terminals. Regulated motor control is supplied by Q642 and Q644. A normal or high speed is controlled by transistors Q643 and Q647. Often the dubbing speed is twice the normal tape speed. The dubbing feature enables the operator to duplicate a recorded tape cassette in tape deck 2 into a blank cassette placed in tape deck 1.
FIG. 12. A block diagram of a dubbing control circuit with normal or higher dubbing speed circuits.
When the motor won’t change speeds, check voltage on Q643, Q647 and the cassette motor. Suspect a dirty motor switch for erratic or intermittent rotation of cassette motor. Test motor control transistors Q642, Q644 and Q641 for no dubbing action. Most motor control circuit defects are caused by leaky transistors and dirty switching contacts.
AUTO RADIO-CASSETTE REVERSE CIRCUITS
In the early auto cassette-radio chassis, a motor reverse circuit would reverse the motor to play the other side of the tape. When the music came to the end of the tape, the motor would stop, then start up in the reverse direction. The auto reverse circuit might contain two or three transistors, fixed diodes and a commutator. The commutator can have wire-like tongs, built on top of the take up reel. In other models, a magnetic switch underneath a rotating magnet opens and closes, triggering the automatic-sensing circuit. The magnet is mounted on the bottom of the turntable reel.
When the turntable stops or does not rotate, the switch remains in a closed position and the relay energizes, causing the motor to change directions rapidly ( FIG. 13). If the auto-cassette keeps changing direction without playing, suspect dirty tongs and commutator rings. Clean up with alcohol and a cleaning stick. Make sure the commutator is rotating and the tongs are seated properly.
FIG. 13. The auto-reverse cassette motor circuit in the auto receiver.
Test all transistors with in-circuit tests of the transistor tester, when the motor won’t reverse directions or rotate. Check the polarity of the voltage across the motor terminals when motor switching occurs. If there is no voltage at the motor, suspect a defective Q6 transistor. Remove the positive wire from the motor terminals. Inject a 12 volt source across the motor terminals and notice if the motor rotates. Reverse the motor polarity injected voltage and notice if the motor reverses direction. Replace the defective motor, if it won’t rotate, with original part number.
TROUBLESHOOTING CD PLAYER MUTE CIRCUITS
The mute circuits within the CD player are usually located in the audio line output jacks. The mute circuits operate when the power switching is applied and when any noise might be created by switching or changing of different CD operations. The mute circuits consist of transistors that ground out the audio of the line output jacks. A separate mute transistor is found in each line output circuit. The mute transistors are controlled by a system control IC or micro computer.
FIG. 14. IC805 controls the muting operation of Q807 and Q805 in a CD player.
Scope the muted signal at the base of each mute transistor when the CD player begins to operate. If the voltage at the base of each mute transistor does not change, suspect IC805, Q807 and Q805. Suspect the system control IC when noise is heard in the speakers during operation. Check each transistor with in-circuit tests.
No noise or sound should be heard in startup and during switching operations with a normal mute system. Determine if the audio is dead at the line output amp IC or at the mute transistors. Remove the excess solder from the collector terminal on the PCB to remove the mute transistor from the circuit. Repair the defective mute circuit if the sound is now heard.
SERVICING THE CASSETTE PLAYER MUTE CIRCUITS
Most cassette players don’t have mute circuits at the power output stages; except large and expensive models might contain certain mute circuits within the record amp and play circuits. In playback mode, the muting simultaneously controls the input and output of the decoder and encoder IC603. The circuit also eliminates the momentary noise of the play back/stop switching for both tape decks.
The record muting controls the output equalization amplifier so that the signal is applied to the head of tape deck 1, only at the time of the recording. The muting takes place between the recording equalizer amp and record tape heads. A mute transistor might be found in each left and right channel tape head circuit. Q607 and Q608 mute the playback amplifier circuits.
The amplifier muting is in effect when signals other than tape signals are recorded. The amplifier muting controls the audio signal before the recording buffer amplifier so as not to record noises caused by switching of the function selector buttons and the power switch. The record input left and right signals are muted with transistors before entering the de coder and encoder IC. Q609 and Q610 mute the record/play tape head signals.
FIG. 15. In the deluxe cassette player, the mute circuits are controlled at the input and output of a two deck head circuit.
Determine what muting circuits are not functioning. Test transistors Q607 and Q608 when the signal is dead or no muting action occurs in playback mode. Likewise, test transistors 0609 and 0610 when muting does not occur in the record/play modes. Scope the signal from each mute controller IC or transistors. Take critical voltage measurements at each transistor. If the right channel muting system is not operating, compare the voltages and signal in the left muting system that is normal.
Check for leaky 2.2 to 4.7 uF electrolytic capacitors within the muting circuits with no audio muting symptom. Suspect leaky mute transistors when one channel is dead. Check the system control IC or transistor with no muting during switching modes. Suspect a leaky mute transistor when one channel won’t record and the other channel is normal in the recording mode of the deluxe cassette players.
RECEIVER MUTE CIRCUITS
Besides relay protection circuits, the large integrated stereo component system and high powered amplifiers might contain several mute circuits. The sound can be muted after the volume control or within the preamp circuits. Some high powered amp circuits might mute the power output before the speaker relay switching. Each right and left channel might have a separate mute circuit.
The system control IC or microcomputer controls each muted circuit ( FIG. 16). The amplifier input circuits are muted so as not to hear the operation of power or function switch and recorded noises. Q501 and Q503 mute the incoming signal from entering the high power amp IC501. The headphone muting transistors (Q509 & Q511) are controlled by the speaker relay control transistors.
The muting circuits can be signal traced with an external audio amp or scope. Monitor the audio signal at pins 5 and 9 when the system control IC is functioning. The base voltage on Q501 and Q503 are a -3.4 volts in normal operation. Both transistors act as a switch when the system control IC is in the muting mode. The audio on both left and right channels is grounded between the two 4.7 uF electrolytic capacitors with transistors Q501 and Q503 ( FIG. 17).
System control IC411 controls the mute audio input and headphone output circuits.
FIG. 17. System control IC401 controls the input circuits to the high-powered output amp ( IC501).
When the muting takes place, no sound should be heard at pins 5 or 9 of power amplifier IC501. Most audio signal tracing methods and critical voltage tests can locate the defective component in the muting circuits.
SERVICING THE CENTER POWER AMP CIRCUITS
The surround receiver or amplifier might have a center amplifier circuit identical to the left and right channels. A center power amplifier might consist of transistors and IC components. The most powerful center amp might contain 10 or more transistors in one channel. The center amp circuits might contain directly-coupled transistors with speaker relay and overload circuits in the output circuits.
First, locate the center power amp circuits on the chassis. If a schematic is not available, trace the center amp output IC directly tied to the center speaker terminals. Connect a load resistor across the left and right channels with a large speaker connected to the center amp terminals to monitor the audio. Determine if the defective center amp terminals contain a dc voltage. If so, connect a 100 watt load resistor across the center amp speaker terminals, for Troubleshooting the audio circuits. Now repair the defective center amp circuits.
Remember, the center amp circuits are usually identical to the left and right stereo channels. If a schematic is not available, take critical voltage measurements on the normal stages and compare them to the center amp transistors.
For extreme distortion within the center amp circuits, go directly to the power output transistors or ICs. Check the voltage on each power output transistor. Sometimes you will find a leaky or shorted output and an open transistor connected to it. The directly-coupled driver transistor might also be open or shorted.
If a dc voltage is found at the center power amp output transistors and the dc detector overload circuits shut down the amp circuits, disconnect the center amp output circuits from the overload circuits. Remove one end of a coupling resistor (10K ohms in this circuit), defeating the overload circuits tied to the center amp speaker terminals.
FIG. 18. To determine if overload protection circuits are defective, remove the 10 Kohm resistor.
Test all transistors in circuit for open or leaky conditions. When one or more transistors are leaky or shorted, replace the output transistors, directly-coupled driver transistor and directly-coupled preamp transistors. If in doubt, replace all of the output transistors in that channel. Check for correct resistance of all bias resistors. Replace them if they are over heated or cracked. Remove one end of the bias diode and test using the diode-test of the DMM.
After all defective transistors have been replaced, take a resistance measurement from each collector terminal to common ground and compare the reading to a normal channel. Check the resistance to common ground at the emitter terminals of each output transistor and compare to the normal left and right channels.
FIG. 19. Compare the resistance to ground off all output transistors to determine if defective component has been replaced.
Now fire up the chassis and take critical voltages on each output transistor. Sometimes the voltage is listed on the schematic and sometimes not. Compare the voltage measurements with the normal identical output circuits.
SPEAKER RELAY PROTECTION CIRCUITS
The speaker protection circuits might consist of two or more transistors that control the speaker relay, disconnecting the speaker from the amplifier output terminals. When a fault or dc voltage is found at the speaker terminals, it’s detected by the protection circuits. A protection signal is sent to the protection control transistors to switch off the speaker relay and protect the amplifier and speakers. The power output IC might have a built-in output protection circuit in some amplifiers.
The speaker terminals are usually connected directly to the power output transistors or Cs. If a power transistor, IC or component breaks down in the power output circuits, a dc voltage might be found at the speaker terminals, damaging the speaker voice coil. With speaker protection circuits, the speaker relay contacts open up or disconnect the speakers from the amplifier output terminals.
FIG. 20. IC501 might control the speaker protection circuits.
A defective protection system might keep the relay solenoid energized and the speakers connected to the speaker terminals. Some protection circuits shut down the whole chassis, when a leaky component occurs in the output circuits.
Remove the protection circuit from the power output IC or transistors by removing one end of the resistor or capacitor from the circuit. Check the speaker circuit for a low dc voltage. If no voltage is found at the speaker terminals and the relay is continuity energized, check the defective protection circuits. Take critical voltage and resistance measurements on each transistor. Test each transistor for open or leaky conditions in the protection circuits.
SERVICING THE AC-DC TUBE RADIO AMP CIRCUITS
The “All American 5” tube radio utilized 1 2BA6, 1 2BE6, 1 2AV6, 50L6 and 35Z5 tubes. The 50C5 and 35W4 tubes were found in later models instead of the 50L6 and 35Z5. The 50L6 and 12AV6 are in the audio circuits.
The heater elements of all tubes were wired in series and connected directly across the ac power line. When the heater of one tube opened up, the entire bunch of tubes remained dark; a dead radio.
The audio circuits begin when a radio IF signal is detected by a diode detector element inside the 1 2AV6 audio amplifier tube. A 500K ohm volume control applies the audio to the input grid terminal of a triode section in the 12AV6 tube. Here the audio is amplified and capacity-coupled by C116 (0.02 uF) to the grid of the power amplifier tube (50L6GT). The audio is again amplified and coupled to the pm speaker through a matching output transformer to a 3.2 ohm speaker ( FIG. 21).
Check the 1 2AV6 and 5OL6GT tubes for weak or distorted audio. A leaky or gassy amplifier tube, leaky coupling capacitor, and burned bias resistor can cause distortion in the speaker. A weak amplifier tube or open coupling capacitor can cause weak audio reception. Check the amplifier tubes for weak or leaky conditions in a tube tester or substitute a new tube.
FIG. 21. The heater and audio amp circuits found in the early “All American 5” radio.
When the output tube (5OL6GT) becomes shorted, suspect a burned cathode resistor. If the shorted output tube is left to operate too long, the primary winding of the output transformer might appear overheated and burned, changing the resistance of the winding. Of course, a weak sound is heard after replacing the leaky output tube. Shunt each electrolytic capacitor for a loud hum in the speaker.
Any tube within the series heater circuit can become intermittent, causing the radio to come on and off and shutdown. The thermal heater will cool down, then make contact and the whole string will heat up again. After becoming very hot, the intermittent heater element opens up again, until the whole set cools down.
The intermittent thermal heater circuit can be located by checking the ac voltage across each heater terminal. Set the voltage meter to 200 volts ac and place both meter leads across the heater terminals of each tube. You have located the intermittent tube heater or filament when the entire 120 ac volts is found across the heater terminals. Replace the intermittent tube at once.
THE TUBE AMPLIFIER CIRCUITS
There are many different tubes found as audio amplifiers within the audio circuits of a radio receiver, musical instrument amp, or audio amplifier. The triode tube has three elements called a plate, grid and cathode, besides the heater or filament elements. A tetrode tube has four elements, a cathode, control grid, screen grid and plate electrodes. A pentode tube has five electrodes, a cathode, control grid, screen grid, suppressor grid, and plate elements. The suppressor grid is added for electrons that are bombarded against the plate element and leak off to common ground.
The cathode element is connected to ground through a voltage bias resistor and has the lowest voltage on any tube element. A suppressor element is at ground potential or tied internally to the cathode element inside the tube envelope. The control grid operates at a negative voltage to control the amount of electrons applied to the plate terminal. The screen grid has a high potential voltage to help pull the electrons to the plate circuit. The outside or plate element attracts the electrons emitted from the cathode element and has the highest voltage on a tube terminal.
Tube problems result from weak, gassy or shorted elements. A weak tube occurs when very few electrons are emitted from the cathode and reach the plate element. This is usually caused after the tube has had many years of operation. The tube becomes gassy (a soft tube) and might cause distortion. A shorted tube might result from the cathode emitting material flaking off and lodging between the grid elements. The microphonic tube might have loose elements that vibrate or produce ringing noises when the tube is tapped or with sounds of music. Check the tube using the tube tester for shorted or weak conditions. Substitute a new tube when a tube tester is not handy.
TUBE BIAS CIRCUITS
In the early or musical amplifiers, the audio power output circuits contained negative bias voltage developed from the low voltage power supply. A separate tap off of the high volt age winding of the power transformer is rectified and fed to a bias control or resistor net work. This negative voltage is applied to the grid resistors of the high-powered amplifier tubes.
FIG. 22. The tube bias supply is taken from a separate transformer winding in the high-wattage tube amplifier.
The negative bias voltage is fed to the cathode of a fixed silicon diode with the anode dc voltage supplied to the bias control. The bias control can apply the correct negative bias voltage to the grid circuits of the 6L6G tubes. The dc bias voltage is filtered with a 75 uF electrolytic capacitor. Improper adjustment or dirty contacts on the bias control (10K) can cause distortion within the high-powered audio output circuits. Sometimes cleaning up the control with cleaning fluid and readjustment can remove the distorted or noisy music from the speakers.
The weak amplifier circuits can be caused by weak or gassy tubes, open coupling capacitors, leaky bypass capacitors and an improper voltage source. Check each tube or sub another new one. Signal trace the audio circuits by inserting a 1 kHz signal from a function generator to the amplifier input jack. Scope or signal trace the audio with an external audio amp until the weak stage is located. Take critical voltage and resistance measurements to locate the defective component. Shunt coupling and electrolytic capacitors with known ones to locate a defective coupling or bypass capacitor. Remember, the voltages within the tube amplifier are quite high compared to solid-state circuits and should be treated with extreme care, using proper test equipment.
HIGH-VOLTAGE TRANSFORMER PROBLEMS
The power transformer located in the tube’s high-powered amplifier are physically very large, have a high-voltage secondary winding and a 6.3 or 12.6 volts ac winding to light up the heaters for 6 or 8 vacuum tubes. The low voltage power supply might have either tubes or silicon diodes as rectifiers with a 500 to 600 volt ac applied voltage. A rectified 550 to 625 volts dc is applied to the audio output transformer secondary tap to the output tubes in push-pull operation. The negative bias voltage might be taken from one side of the secondary transformer winding.
When one or more silicon diodes or tube rectifiers become leaky or shorted, either the line fuse blows or if left on too long, the power transformer becomes hot and is damaged. Sometimes leaky or shorted filter capacitors break down and provide a heavy load, destroying the power transformer. The transformer can be damaged by replacing the line fuse with a larger amp fuse than required when the silicon rectifiers break down. The defective transformer runs red hot and has a burning tar odor. Sometimes the shorted transformer begins to smoke and has an overload-humming sound ( FIG. 24).
FIG. 24. Remove all leads from suspected amp transformer for accurate tests when the transformer runs hot and smokes.
Usually the amplifier pilot lights dim with a heavy overload when the ac switch is thrown without audio. Remove all wires from the smoking transformer to the rectifiers and heater circuits. Mark down the color code of each removed wire if a schematic is not handy. With wires detached on the secondary windings, fire up the transformer and notice if it immediately becomes warm. The hot transformer might groan and hum if shorted windings are found inside the transformer.
If the transformer does not heat up, let it run for one half hour with the primary winding connected to the ac power line. Check the ac voltage on each secondary winding. A normal power transformer runs quite cool without a load on the secondary. Repair the leaky low voltage components before attaching the secondary transformer leads.
A Dolby system is an electronic method or circuit for improving audio quality of the cassette player. For low-level sounds, the gain of the amplifier is increased during record - modes and the low-level sounds are reduced in playback modes. The Dolby A system might have four frequency ranges while the Dolby B has only one band for reducing noise in the amplifying circuits.
In some cassette players, Dolby noise-reduction circuits are included to reduce the level background noise normally found during recording modes. Dolby B noise reduction occurs only in the middle and upper portions of the audio spectrum. With the Dolby circuits, noise (NR) has been reduced to a minimum in portions of the recorded program. Dolby reduction does not change the frequency response of the audio signal. At high levels, the noise is suppressed. At low input-audio levels, the signal-to-noise (S/N) decreases and the noise is heard. The amount of boost alteration depends on the level and frequency of the signal.
In the early cassette player several transistors were found in the Dolby audio circuits. In the recent cassette players, the Dolby noise reduction circuit ICs are generally located between the preamp and output IC circuits.
FIG. 25. The Dolby noise reduction circuits are found after preamp circuits in the recent cassette players.