Troubleshooting Inexpensive Electronics Circuits

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The early table model radio and portable cassette player contained a mono or one source of audio. Today, the clock radio and portable cassette player might have AM/FM/MPX reception. The monochrome TV provides a black and white picture. A mono-amplifier consists of only one channel of amplification with a speaker or headphones as indicator. The simple mono-amplifier might have only one audio IC or two and three audio transistors.


In the early transistor table model radio, the audio was detected with a germanium diode detector (1 N34A) from the 2nd IF transformer. This weak audio signal was controlled by a 5K ohm audio tapered volume control. A 5 uF electrolytic capacitor coupled the audio from the volume control to the base terminal of an NPN driver transistor (TR5). Interstage transformer Ti coupled the audio from TR5 to the base terminals of push-pull output transistors TR6 and TR7. The output transformer T2 stepped down the amplified audio to match the impedance of an 8 ohm pm speaker. The low voltage power supply consisted of two silicon rectifiers in a fullwave ac circuit from a step-down power transformer.

FIG. 1. The early transistor radio audio circuits with interstage and output transformer circuits.

The early transistorized audio circuits within the table model cassette player might consist of two preamps, driver, and two push-pull output transistors. A 3.3 uF electrolytic capacitor coupled the tape head signal to a driver transistor. The driver transistor was transformer coupled to two output transistors. Instead of an output transformer, a 220 uF capacitor couples the amplified audio to the 8 ohm speaker.

FIG. 2. The audio output is coupled to the pm speaker through 220 uF electrolytic capacitor.

The portable tape player operated from batteries or the ac power line. A fullwave or bridge rectifier circuit provided a 6 volt dc source with a 2000 uF filter capacitor.


The most common IC audio amplifier found in many different electronic projects were designed around the 8-terminal IC chip (LM386N-1). This small low voltage audio IC only provided 250 mW with a 6 volt supply and the LM386N-3 provided 500 mW with a 9 volt source.

The early mono cassette player-recorder might require greater audio output with an IC component that was equivalent to 15 transistors within one IC. The simple cassette recorder had one transistor stage ahead of the power output IC. A separate ALC (automatic level control) transistor controlled the level of recording.

When in playback mode, the tape head was switched into the base circuit of Q1 through a 1 uF electrolytic capacitor. The weak audio signal was amplified and capacity-coupled to the 50K ohm volume control. VR1 adjust the correct audio applied to the input terminal of IC1 ( FIG. 3). A 1 p electrolytic couples the controlled audio to pin 9 and the output terminal 1 is coupled to an 8 ohm pm speaker. The cassette amplifier might be powered by five 1.5 volt “C” batteries or from a fullwave rectifier circuit of a step-down power line transformer.


FIG. 3. A very simple cassette player amplifier circuit in play mode.


A typical inexpensive audio stereo output circuit might consist of only one dual-output IC. The left channel input is taken from a 100K volume control, coupled to pin terminal 4 of IC1 with coupling capacitor of 4.7 uF. Likewise, the left channel amplified audio output is coupled to pin 2, through a 470 uF electrolytic capacitor to the pm speaker ( FIG. 4). The right channel is identical to the left stereo channel. This amplifier circuit might be powered by “0” cells or a step-down transformer rectifier circuit.

FIG. 4. Large electrolytic capacitors couple the audio from dual power IC to speaker terminals.


Before audio output ICs were included in the TV audio chassis, the early solid-state audio circuit consisted of a sound IF/Detector/audio preamp IC and a single-ended output transistor. The weak detected IF audio was amplified by a preamp audio stage and the volume was controlled inside the same IF-IC. IC201 was directly-coupled to the audio output transistor with a 680 ohm resistor ( FIG. 5).

FIG. 5. IC201 is directly coupled through a 680 ohm resistor to base of single-ended output transistor.

The audio output transistor (Q201) operates at a very high dc voltage through a step-down coupling transformer to the 3.2 ohm pm speaker. A distorted sound symptom can be caused by a leaky IC201 or Q201.

When the output transistor shorted or became leaky, the emitter resistor was found burned or overheated and was replaced.

Sometimes only a touch-up of the discriminator coil can cure the mushy-distorted speaker. Only a slight turn of the 4.5 mHz discriminator coil (L202) was needed. Slowly adjust the coil in one direction and then reverse to clear the distortion and obtain a clear sound.


The early output circuits consisted of one large IC with a heat sink glued to the top to provide audio to a pm speaker. IC201 consisted of the sound-IF input, preamp and audio output circuits. The volume was controlled by applying a dc voltage to pin 6 of IC201. The audio output at pin 8 is coupled to the 8 ohm speaker.

Excessive distortion can be caused by improper adjustment of coil (T202) and leaky IC201. When a touch-up of T202 does not entirely erase the mushy sound, replace the small bypass capacitors inside the shielded coil. The noisy volume control can be cured by spraying cleaning fluid down inside the volume control lugs. Rotate the control knob back and forth to clean up the carbon-control area.

Intermittent or no sound can result from the defective 470 uF speaker coupling capacitor. Sometimes, just touching the capacitor can make the sound pop in and out. A defective IC201 or open 470 uF electrolytic can cause a weak sound symptom. Take critical voltage measurements on IC201 terminals to locate a leaky or defective output IC.

FIG. 6. The early TV audio IC output circuits.


The low-priced compact or portable cassette audio circuits might consist of only one large IC component to power an 8 ohm speaker. The audio output might have only 700 mW to 750 mW of audio power. The record/play tape head is coupled through SW1 -A through a 1 uF electrolytic capacitor. SW1 -B switches the other head lead to common ground. The preamp audio from pin 3 is coupled to the volume control through a 1 uF capacitor and switched into play circuit at pin 6 of IC-1.

FIG. 7. A simple cassette IC audio output circuit in play mode.

A 100 uF capacitor couples the audio signal to the earphone jack J5 and 8 ohm speaker. The erase head is excited by the +6 volt source and switched into the circuit in record mode with SW1 -D.

For a weak sound symptom, clean up the tape head with alcohol and cleaning stick. Check C1, C3 and C7 for open conditions. Suspect a leaky C3, C5, C7 and IC-i for extreme distortion; don’t overlook a mushy speaker. If the sound is weak and distorted after head cleanup, suspect a defective IC-1. Take a resistance measurement to common ground of all IC terminals to locate a leaky capacitor or a change in bias resistors. Take an input signal test at pin 14 and output at terminal 10 with an external audio amp, to determine if IC4 is defective or there is connecting component failure.


The simple single-play or portable phonograph with changer might include 3 or 4 separate transistors. A Darlington transistor or IC might serve as AF or driver transistor to two small output transistors in push-pull operation. The mono-crystal cartridge feeds into a volume control and directly-coupled the amplified signal to the output transistors. A tone control is found in the directly-coupled input audio circuits. C5 (100 uF) couples the output audio to a higher-ohm impedance speaker.

FIG. 8. A simple phono crystal audio amplifier with only three audio transistors.

The crystal cartridge in this small record player might have a 0.5 to 3 volt output. A defective crystal cartridge might result from dropping the pickup arm or being left out in the sun too long, melting down the crystal cartridge. Dust and dirt collected from the record can cause a weak and mushy sound. Brush out the stylus with a small artists brush. A mushy- weak audio sound in the speaker indicates a defective crystal cartridge.

Besides a crystal cartridge, a leaky driver or output transistor can cause distortion in the speaker. A weak and distorted sound might result from leaky Q2 and open Q3 transistors. Suspect D1 and D2 with a low hum and distorted audio in the sound. Check each transistor with an in-circuit test for open or leaky conditions. Take critical voltage measurements upon each transistor. An open Q3 results in high dc voltage on the collector and emitter terminals of Q3.


The crystal phono input circuits in a large console or compact table model feed into phono input jacks. The stereo cartridge circuits are connected to a function switch that switches in a microphone, radio, tape, and phono input signal. Check the defective crystal cartridge for weak, intermittent, or distorted sound. One stereo channel might be dead or intermittent with a broken wire connection at the cartridge.

FIG. 9. Stereo cartridge circuit connected to a function switch.

The phono player might have a magnetic cartridge instead of a crystal cartridge for better frequency response. A crystal cartridge is a transducer made up of piezoelectric crystal material. The stylus or needle is directly connected to the crystal element which follows the groove of the record. The vibration or movement upon the crystal surface provides an output voltage that is amplified into music production. The crystal cartridge has a much higher output voltage than the magnetic pickup.

The magnetic pickup is called a variable-reluctance pickup where the stylus or needle causes a metal vane or armature to move within a magnetic field. The coils on each side of the metal piece are fed into a preamp or equalizer stage. Since the signal is so weak in a magnetic cartridge compared to the crystal cartridge, both stereo channels must be amplified with a directly-coupled transistor preamp or IC input circuit (FIG. 10).

FIG. 10. A single play stereo phonograph turntable.

A separate stereo preamp, equalizer or tone directly-coupled transistor circuit is coupled to the phono input jacks. Sometimes the inexpensive units have a preamp circuit that is switched in for the radio, tape and phono functions. The higher-powered output amplifier circuits usually contains a separate preamp stage for the magnetic stereo cartridge.

The magnetic input jacks are coupled with a electrolytic capacitor to the preamp Dual-IC. The same Compact combination receiver, tape, and phono circuits might have separate transistors as the preamp stage in the tape circuits of the high-powered output circuits. The weak magnetic phono audio is amplified by IC2 and capacity coupled to the function switch.

Check the signal in and out at pins 5 and 7 of the left channel and 3 and 1 of the right channel with an external audio amplifier. The audio signal might be quite weak at pins 3 and 5. Check for an open coil in the magnetic pickup with the low ohm scale of the DMM. Take critical voltage measurements on the supply terminal pin 8 (22.5v) of lC2 to deter mine if IC2 or the power source are defective.

FIG. 11. The magnetic phono pickup and preamp IC circuit.


The phono stereo preamp circuits might consist of two transistors that are directly-coupled together. The left and right channel phono jacks are coupled to the base of the first NPN transistor. The collector terminal of Q101 is tied directly to the base of Q102 in the left channel. The audio phono signal is amplified by Q101 and Q102, and coupled with a 1 uF electrolytic capacitor to the function switch SW-1. Both right and left channels are identical.

When the left channel is weak or dead, signal trace the phono audio at the base of Q101 and the output at the collector terminal of Q102. Check the voltage on each transistor and compare to the schematic. If one transistor becomes open or leaky, the other directly coupled transistor voltage will also change. If a diagram is not handy, compare the voltage and resistance readings to the normal channel.

FIG. 12. A directly-coupled L & R phono preamp transistor circuit.

A leaky preamp transistor can cause weak and distorted music. The weak phono audio might result from an open coupling electrolytic capacitor or transistor. The intermittent preamp transistor might restore to normal when tested in circuit or voltage measurements are taken at each terminal. Simply replace the suspected open or intermittent transistor. Universal transistors can be replaced in the phono circuits without any problems.


The boom-box player with radio, cassette and CD functions might have a dual-output IC circuit. A special function switch applies each function to the output audio circuits. A muting system might be found in the input or output circuits. The left and right channels are switched to the headphones or external speaker terminals. Some of the larger boom- box players have additional left and right speaker output jacks.

The preamp circuits might contain transistors, IC or the preamp circuit can be included within one large IC output component. Bass and treble tone controls are usually located between the preamp and volume control. The right and left channels are coupled through an electrolytic capacitor (1 uF) from the volume control to the input terminal of IC204. The amplified output of IC204 is found at pins 2 and 11.

Suspect the audio output circuits, when the sound is intermittent, dead or weak within the CD, radio or cassette functions. A dead left or right channel might result from a defective IC204, 1000 uF electrolytic capacitor, or improper supply voltage at pins 1. The intermit tent left channel can result from a bad 1 uF or 1000 uF coupling capacitor and IC204. The weak right channel can be caused with open 1 uF, 1000 uF, and improper voltage at pins 10 and 11.

Check the switching contacts of earphone jack for noisy, erratic or cutting-out of sound with dirty switching contacts. A noisy or worn volume control can be silenced with cleaning fluid sprayed down inside the control terminals. Rotate the volume control back and forth to smooth out the noisy audio.


The portable CD player audio circuits begin at the DIA (digital-analog) converter stage. The DIA converter and audio amplifier might be included in one large IC or microprocessor.

In the earlier chassis, the audio amps after the D/A circuits might be separate transistors or IC components. The portable CD player is chock-full of SMD processors with gullwing terminals. The portable CD audio output components might consist of a separate head phone IC amp or a dual-IC audio amp. The audio amp can be attached to the line output and headphone jacks.

Scope the D/A converter audio output terminals when either channel is weak or dead. The external audio amp can be used to signal trace the audio from the D/A converter to the line output jacks. Suspect the headphone amp IC or improper voltage source when the head phones are weak, distorted or dead. A dirty or worn headphone jack can cause intermit tent or erratic sound in the headphones. Determine if a low frying noise in either channel is caused by the D/A converter, headphone or line amp IC with an external audio amplifier. Take critical voltage measurements on each IC terminal to determine if the IC is leaky or if there is a defective voltage regulator transistor in the power supply source.

FIG. 13. A dual-power output IC circuit in a boom-box player.

FIG. 14. The portable CD player line and headphone output circuits.


The deluxe car receiver-cassette player might have a normal reverse tape deck, audio muting, IC preamp or equalizer, transistor preamp, Dolby, switching, transistor AF amp, line output amp, and power output IC circuits. The dual-right and left cassette tape heads are switched into the input circuits of a preamp or equalizer dual-IC component. The tape heads are coupled to the input terminals with a 4.7 uF electrolytic capacitor. The output terminals of IC501 are pins 3 (left) and 6 (right). These input circuits are tied to the radio/tape function switch.

FIG. 15. The auto cassette input preamp and equalizer circuits.

Signal trace the head circuits at pin terminals 1 and 8 of IC501 with the external audio amp. Insert a test cassette for the audio signal. If signal is found at the input terminals and not at the output, suspect a defective IC501 or improper voltage source at pin 4. Check all voltages on each IC terminal to locate a leaky IC501. Suspect IC501 is leaky when the voltage at pin 4 is very low compared to the schematic. Remove pin 4 from the pc wiring with soldering iron and solder wick. Now take another voltage test at pc wiring. Replace leaky IC501 if the voltage rises above 5.5 volts.

The power output IC might amplify both right and left channels to the respective speaker terminals. An input switch can switch the preamp audio to the external amp or line output jacks. A separate line output IC amplifies the output jacks, while another power amplifier provides from 5 to 30 watts to several different speakers.

Suspect the 4.7 uF coupling capacitor or IC502 when the left line output channel is weak, dead or distorted. Check the supply voltage at pin 8 of IC502 for a leaky line output IC. Check SW5-3 and IC502 with a noisy right line output sound at iC502 when both line output jacks are dead or distorted. Check the audio at pin 2 with the external audio amp when the left channel is noisy or weak.

Take voltage measurements on all IC504 terminals with a dead symptom in both speaker channels. Spray each transistor or IC with coolant when one channel is intermittent. Apply several coats of coolant before going to the next component. Likewise, spray electrolytic coupling capacitors for weak or intermittent symptoms.

FIG. 17. Spray each intermittent transistor, IC or audio component with coolant.

FIG. 16. A deluxe auto-cassette receiver line and power output circuits.


The auto CD player might have a few more audio circuits than those found in the portable CD player. Signal trace the audio from pin 17 and 18 of D/A converter ( IC301), with the external amp or scope. Check the IC circuits where the audio stops. The left and right channels can be signal traced from the D/A converter to the line output jacks. Most auto CD player audio symptoms can be repaired with signal tracing methods and critical volt age measurements.

Signal trace IC301 and IC401 for noisy or garbled audio. Check the D/A converter (IC301) for distorted audio. No audio in the left channel might result from a defective low pass filter (LPF) circuit. Suspect a voltage regulator IC with a defective right channel and normal left channel.

FIG. 18. A block diagram of the auto CD player audio line output circuits.

Check IC401 with a frying or static noise in both channels. Suspect a dead, noisy or intermittent music with an open or defective electrolytic coupling capacitor. Solder all pin terminal connections upon IC301 and lC401 for intermittent, noisy or weak sound symptoms.


A dirty earphone jack or connecting wires can cause intermittent or erratic music in the headphones. Clean around the ground jack input area for erratic audio. Check the cord for breakage where it enters the earphone or at the male plug. A dirty male plug might not make a good connection where it enters the headphone jack.

Check the audio output components when there is no audio from either the speakers or headphones. The mono headphone has only two connections at the male plug. A stereo male plug has a common ground, left and right stereo connections. The low impedance headphones might have an impedance from 8 to 55 ohms, while the high impedance phones might be 1000 to 2000 ohms.

The stereo headphone jack might switch the speakers out of the audio circuit and place the headphones in the output circuit. A small 100 or 220 ohm resistor is placed in series with the audio output to lower the level for headphone reception ( FIG. 19). Of course, the volume control should be lowered when the headphones are plugged into the output jack.

FIG. 19. The stereo headphone output jack coupled to the PM speaker circuits.

Clip the RX1 ohm range of the DMM across the tip and ground area of the earphone plug for continuity tests. Both right and left resistance measurements should be quite close. An 8 ohm impedance headphone might have a 7.5 ohm measurement on the DMM. Flex the cord while clipped to the DMM. Notice if the resistance changes, indicating a break within the headphone cord.

Clean up the male plug with cleaning fluid for erratic or intermittent reception. Spray cleaning fluid down inside the headphone jack and work the plug in and out to help clean up the contacts. Check for a cord break at the earphone plug. Most electronic stores stock male headphone plug replacements. Remove the rubber headphone cover or cap and check for a break in the cord or a torn-loose wire connection. The small, low impedance head phones might have miniature 8 ohm pm speakers inside the covers.


The headphone circuits within the portable CD player might have a separate IC amplifier or be contained within the line output circuits. The headphone amplifier connects directly to the line output jack of each stereo channel. A separate volume control might be used to control the audio to each stereo headphone. The input and output audio is coupled through an electrolytic capacitor at the dual-headphone IC amplifier. Some headphone circuits have mute transistors at the input or output amp circuits.

A defective volume control might cause no sound, intermittent or erratic music with a worn control. A leaky coupling capacitor (C11) and IC11 can cause distortion in the head phones. The leaky or open Id 1 might cause distortion in one or both stereo channels. C113 can cause weak, intermittent and no sound in the headphones. A dead right channel might result from a defective IC11, with a normal left channel.

FIG. 20. A portable CD player headphone output circuits.


In the most simple cassette players, the recording and playback circuits are the same. Sometimes only the preamp circuits are used for playback and recording circuits. In larger or deluxe recording circuits, a separate transistor or ICs are used for only recording features. The stereo tape heads are switched into the input amplifier circuits for playback, while in recording mode the tape heads are switched into the output circuits.

The electret or condenser microphone is switched into the preamp circuits of IC401 with the tape/radio switch and record/play switch S203 in record mode. The microphone audio is amplified through IC401 and switched back to the record/play (RIP) head so the audio can be recorded on the cassette tape. The bias oscillator is switched into the tape head circuit by S2-1 for high-fidelity recordings.

FIG. 21. A portable cassette recording circuit.

Often, when problems occur in the recording circuits, the same conditions exist in the same playback circuits. If the cassette player operates normally in the play mode and not in the record mode, check for dirty or poor switching contacts. First, Clean up the function RIP switches. Clean up the tape heads with alcohol and a cleaning stick. Check the microphone circuits by inserting another external microphone.

Now check the bias oscillator circuits for no, poor or distorted recording. Scope the output of the tape head in record mode. A sine waveform should be noted when the bias oscillator circuits are performing. The jumbled-noisy recording might be caused by a defective erase head or circuit. Both the RIP and erase heads are excited with the bias oscillator, when the bias oscillator circuits are switched in the recording mode.


A bias oscillator circuit provides a 30 to 100 kHz frequency to the tape heads to erase and bias the system for linear recordings. The bias oscillator circuit operates only when in record mode. Some cassette recorders switch in the bias oscillator circuits with a positive voltage source or grounds the entire oscillator circuit. A separate waveform is sent to the erase head to erase the previous recordings and to both RIP heads for high-fidelity and linear recordings.

The bias oscillator circuit might contain one or two different transistors. A bias oscillator circuit might not be found in inexpensive cassette recorders. When a supply voltage is switched into the bias oscillator, the circuit provides a waveform to the erase and tape heads ( FIG. 22).

FIG. 22. A bias oscillator waveform at the tape heads.

Check the bias oscillator circuits with no sound or jumbled recordings. Scope the ungrounded erase and RIP heads for a waveform in record mode. Go directly to the bias oscillator circuits when no waveform is located on the tape heads. Suspect a dirty record switch or power source with low or no voltage at the bias oscillator circuits. Test the bias transistor with in-circuit diode tests of the DMM or transistor tester.

Take a continuity measurement of T507, if the bias transistor is normal. Sometimes the oscillator transformer might have voltage going into it and no voltage to the collector terminal, indicating an open connection or winding. Take a voltage measurement upon the collector terminal of bias transistor for open or leaky conditions. Check and shunt the small bypass capacitors for leakage, shutting down the bias oscillator circuits. Solder up all bias oscillator connections for intermittent recordings. Monitor the tape head windings when the recordings appear intermittent or erratic.

FIG. 23. The bias oscillator circuit connected to RIP tape and erase heads.


The double-cassette recorder might appear in a portable boom-box AMIFMIMPX receiver or integrated stereo component systems. Two different sets of stereo tape heads are found with one set to record and playback, while the other set is used for only playback features. Tape head 1 is connected directly into a preamp IC and tape head 2 is switched into the record and playback modes. The erase head and RIP tape head (2) are excited with a transistor bias oscillator circuit.

Cleanup the tape heads for packed oxide and RIP switch with dirty contacts. A good clean-up might solve many tape sound and speed problems. The dead right channel can result from an open tape head winding. Check and compare each stereo head resistance with the other normal channel. The tape head should be okay if quite close to the other head resistance measurement. A high resistance measurement can be caused by a poor internal tape head connection. Intermittent and erratic playback can be caused by a poorly soldered or loose cable wire head connection. Push on the head terminals with a pencil eraser and notice if the resistance changes, indicating a poor internal connection.

Check tape head (2) for intermittent recording and playback modes. Make sure the RIP function switch is clean and not worn. Solder all function switch contacts on the pc board. Notice if the tape head (2) is switched into the preamp IC at different pin terminals than playback head (1). Tape head number 2 is switched into the record mode from a separate record IC. The record or playback audio signal can be traced from the tape heads through IC5 with the scope or external audio amp.

FIG. 24. The double playback and RIP heads are switched into a preamp IC circuit.

The leaky preamp lC5 can cause a dead, weak or distorted record or playback symptom. Take critical voltage measurements on all IC terminals. Check the supply voltage at pin 12 to determine if the IC is leaky or shorted. When the recorded audio is found at the input terminals of IC5 and not at terminals 13 and 14, suspect a defective IC. Sometimes the IC voltages test close or are only off a fraction of a volt compared to the schematic or the other channel and still the IC might be open. Simply replace the suspected IC when voltages are close and no audio is found at either output terminal.


The erase head removes the recorded music from the tape. The erase head might contain an erase magnet or a coil with erase current. The erase head is always mounted ahead of the RIP tape head to remove the previous recordings. The cassette could not be used over and over again, to record music or information, without erasing the previous recording with the erase head. A low-priced tape recorder might have a dc voltage from batteries or power supply to provide a magnetic field to erase the magnetic tape, instead of a bias oscillator circuit.

The bias oscillator circuits provide a magnetic erase current to the erase head and also excites the RIP tape heads. The dc voltage is switched to the erase head to demagnetize the tape in record mode. Likewise, a dc voltage is applied to the bias oscillator circuits when making a recording.

The erase head is usually mounted ahead of the RIP head within the tape path. In some cassette players, the erase head might be pivoted out of the way in playback and dubbing modes. The erase head is mounted on a swivel type assembly and in record mode is moved up alongside the RIP tape head. When in play mode, the erase head is pivoted down away from the tape path.

FIG. 25. The erase head might operate from a dc source or bias oscillator circuit.

The defective erase head might appear open or may not touch the magnetic tape to erase the previous recording. Inspect the mounting screws for improper mounting of the erase head. Check the continuity of the erase head on the 2K ohm scale of the DMM. Often the erase head is normal with a continuity measurement. Check for a dc voltage across the erase head winding in record mode with a dc voltage excited erase head circuit. No voltage might be caused by dirty or poor recording switch contacts or low voltage from a voltage regulator transistor.

Scope the erase head for a bias waveform on the ungrounded head terminal in a bias oscillator circuit. Check the recording switch or defective bias oscillator circuit with no waveform.


The microphones found within the cassette recorder are usually the electret-condenser type. Two identical microphones are found in a stereo cassette recorder. An external microphone self-shorting jack is located in larger players. The electret microphone is made from a dielectric disc and when sound waves strike the disc, a small voltage is developed. Most electret mikes operate with a low external dc voltage (1.5-10 volts DC). The dc voltage is provided from batteries or a dc source with a voltage dropping resistor to provide correct operating voltage.

The electret microphone is capacity coupled to the self-shorting external mike jack and switched into the input terminal of the preamp IC. The small microphone can be checked by inserting another microphone in the external jack J101. If the subbed microphone operates normally, you know the amplifier circuits are good. The preamp circuits are normal if the playback cassette can be heard. An infinite measurement should be found across this type of microphone.

First clean the RIP function and tape/radio switches with cleaning fluid. Clean up the external mic jack (J1 Q1). Check the dc voltage across the microphone terminals in record mode. No voltage might indicate an open switch, poor voltage source or an increase in résistance of the voltage dropping resistor. Shunt the coupling capacitor (47 uF) with another electrolytic capacitor. Sometimes sharp objects might be stuck into the mike holes, damaging the microphone element.


The early tube intercom might have 2 or 3 tubes. Most solid-state intercoms have simple circuits with three or more transistors or one large IC. Most failures in tube intercoms are weak or shorted tubes. Check the tubes in a tube tester or substitute another one.

FIG. 27. The early tube intercom unit operating from the AC power line.

A dried-up filter capacitor in the tube intercom can cause a constant hum in the speaker. Shunting the electrolytic capacitor with a known filter capacitor can solve the loud hum problem. Sometimes a dirty talk-listen switch produces noisy, static or no sound when in talk position. Clean all switches with cleaning spray.

Check all transistors in the solid-state intercom with a transistor tester. Place a connecting speaker close to the main station and notice the loud feedback-sound when the amp is normal. Suspect a break in the connecting wires with no sound in the remote unit. Check all wire connections (FIG. 28). To locate broken intercom wires, twist the wire ends together at the remote and take a continuity measurement with the ohmmeter at the main amp.

FIG. 26. The left channel electret microphone input switched into preamp Id.

FIG. 28. A simple intercom unit with one IC as audio amplifier.

Remove the top cover from the intercom unit and take critical voltage tests on transistors or IC components. In most cases, no intercom voltages or schematics are found, and you must Troubleshoot the unit without a schematic. Clean up the function switch with cleaning spray for noisy or erratic conversations. Remember the small intercom unit might have only a few transistors or a couple of IC parts. Signal trace with a 1 kHz injected signal at the input terminals and scope the waveforms through the audio circuits.

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Updated: Saturday, 2014-12-27 1:23 PST