TROUBLESHOOTING PREAMP and AF (audio frequency) CIRCUITS

The sound amplifier stage provides a boost of audio signal at the input of an audio amplifier circuit. Often, the preamp stage is the first electronic amplifier (analog) connected to a tape head in the cassette player, phonograph, or an audio amplifier ( FIG. 1). In the early tube audio circuits, the preamp tube was the first audio circuit connected to the microphone in the PA system. The preamp circuit usually consists of a voltage amplifier.

FIG. 1. A block diagram showing the location of the preamp stage in the cassette player.

You might find two transistors or one IC that provides amplification of the weak cassette tape or microphone audio signal. In the AM-FM-MPX radio-cassette circuits, the radio signal is switched directly into the audio circuits while the cassette sound is picked up off of the moving tape, amplified by the preamp circuits, before being switched into the audio circuits. Just about every consumer electronic product has a transistor or IC preamp audio circuit.

The preamp stage might amplify the weak signal of a phono cartridge, before it’s switched into the AF audio circuits. A magnetic phono cartridge signal is very low and a separate audio stage must amplify the weak signal of the moving coils before it’s coupled to the preamp or AF audio circuits. The preamp circuits amplify the very weak audio signals to the AF or audio driver circuits of the audio amplifier.

THE SECOND BEST

The audio frequency (AF) amplifier might be the first stage that amplifies the radio, phono graph or cassette preamp circuits. The AF amplifier operates within the 20 Hz to 20 kHz frequency range. This audio frequency amplifier can be the second audio stage within the cassette player. You might find more than one or two AF transistor or IC circuits within the large stereo amplifiers. Often, the AF circuits amplify the audio signal and are coupled to the driver transistor within a push-pull output amp circuit.

The audio signal from the radio or cassette circuits is coupled into AF transistor (Q210) by a 4.7 uF electrolytic C210, R212, R213, and R210 provide forward bias for transistor Q210. The different voltage upon the base and emitter terminals to ground should have a forward bias of 0.6 volts on an NPN transistor. A 0.6 volt DMM measurement from base to emitter is the forward bias voltage. When a correct forward bias voltage is found, nine times out often, the transistor is normal. An in-circuit forward bias voltage test can quickly determine if the transistor is open, leaky or normal.

FIG. 2. A critical forward bias voltage measurement (0.6V) between emitter and base terminal of a silicon transistor.

The audio input signal is applied to the base terminal of Q210 by capacitor C210, amplified and capacity coupled to the audio driver stage with C211 (4.7 uF) capacitor. R215 (5.6k) serves as a collector load resistor. The supply voltage (1 3.4v) from the low voltage power source is connected to R215 and R211.

DIRECTLY-COUPLED FOR LIFE

Two preamp or AF transistors directly-coupled to one another can provide more audio gain with less electronic components in the audio circuits. A direct-coupled amplifier is where the output of the first transistor is wired directly to the input of the following stage. The collector terminal is wired-direct to the base terminal of the second audio transistor. This type of amplifier circuit has a wide frequency response and can handle either ac or dc signals. There are no capacitors or resistors between the collector of the first audio amp and the second transistor ( FIG. 3).

TROUBLESHOOTING PREAMP AND AF CIRCUITS

Although FIG. 4 is an AF audio circuit, an identical preamp audio circuit with R136 is eliminated from the circuit. Notice that the collector terminal of Q102 is wired directly to the base terminal of Q103. The preamp or AF input transistor supply voltage (1 .85v) is very low to prevent pickup noise. A critical voltage measurement from base of Q103 to ground and from emitter of Q103 to ground should equal the forward bias voltage. This voltage difference should be quite close to 0.6 or 0.7 volts for a normal NPN transistor.

FIG. 4. The directly-coupled Q102 from collector to bass terminal of Q103.

A preamp directly-coupled amplifier circuit is usually found in the tape, cassette, or phono input circuits. The AF directly-coupled stage is found in the radio, cassette and other amplifier circuits. You might find a volume and tone control circuit after the AF direct-coupled amplifier. Several different AF circuits might follow the directly-coupled circuit within the higher-powered amplifiers.

FIG. 3. Locate the power output transistors upon the heat sink and the preamp transistor circuits are nearby.

The defective preamp or AF transistor might become leaky, open or intermittent. If the first or second amp transistor becomes leaky or open in a directly-coupled amp circuit, the voltage will also change on the other transistor. When Q102 becomes leaky, the base and emitter voltage goes to zero volts with only .003 volts on the base terminal of Q104 ( FIG. 5). This voltage should measure 1.7 volts with a normal transistor. You will find the voltages are also lower upon the emitter and collector terminals of 0104.

FIG. 5. Notice the low voltages on all preamp transistor terminals when Q102 becomes leaky.

When the first direct-coupled transistor goes open, the measured voltage on the emitter and base terminals will decrease to about one half of the original normal voltage. Since the transistor opens up, the collector voltage will almost increase in voltage equal to the supply voltage for the directly-coupled transistors; this also increases the dc voltage to the second directly-coupled transistor. Sometimes when the meter probe touches the base or emitter terminal, the intermittent transistor might return to normal operation. At other times, the suspected transistor might test good out of the circuit and open up under load.

DUAL-IC PREAMP CIRCUITS

Instead of two preamp or AF transistors directly coupled, you might find one dual IC per forming audio amplification to both stereo circuits. The IC preamp circuit might contain more than one transistor in each stereo channel within the IC component. You will find the IC preamp and AF circuits within the latest auto receiver, cassette players, receivers and amplifiers.

The auto receiver with a cassette player might have a single IC preamp stage with switched input tape heads. The stereo tape heads are switched into the input terminals of IC1O1 with S5-1 and S5-2 ( FIG. 6). C102 and C202 couple the tape audio to the input terminals 1 and 8 of IC1 01. The amplifier output audio signal is found on terminal pins 3 and 6. C203 and C204 connect the preamp audio to the volume and tone controls. Pin 4 of IC1 01 is the supply voltage terminal.

FIG. 6. A typical preamp stereo IC circuit within the auto stereo receiver.

The defective preamp or AF IC might become leaky, shorted or open. Extreme distortion with weak tape audio is heard with a leaky or shorted IC. An open front-end IC might have no or weak audio reception. When the IC becomes leaky or shorted the supply voltage will decrease in voltage with a slight change in voltage on the other IC terminals. The voltages on all IC terminals might match those on the schematic when the suspected IC opens up. Critical voltage measurements on each IC terminal and an input-output signal tracing method can locate the defective IC component.

MICROPHONE INPUT AUDIO CIRCUITS

The microphone input circuits might be found within the cassette player, phono amplifier, sing-a-long amp, compact disc, boom-box players, and PA systems. The stereo micro phone input jack in a high-powered amp may be switched into the AF audio circuits after the preamp stage. Usually the mike jack is connected ahead of the volume control in the same amplifier that amplifies the tape and receiver circuits.

The built-in microphone of the cassette player is switched out of the circuit when the auxiliary or external microphone is used ( FIG. 7). Here, the built-in auxiliary jack (J2) and external microphone jack (J1) are switched into the AF or preamp circuit of the cassette player. SW1 -A places either the tape head or built-in microphone into the front-end circuits of a low-powered amplifier.

The built-in microphone is usually a condenser or electret type with a low dc voltage applied to the microphone. The built-in mike signal is coupled with C101, through R101 to the AUX switching terminals, and to the shorting jack terminal of J1 and switched by SW1-A. SW1 -A switches in either the tape head or microphone audio signal. Then the audio signal is amplified by two preamp or AF transistor stages before the volume control.

FIG. 7. The built-in microphone audio passes through the external mic (J1) before being switched to the base of Q101.

In the early monaural cassette player, the built-in microphone, auxiliary J1, and external mic J2 are switched in the same manner to SW1 -D and coupled to C103. C103 couples the audio signal to pin 14 of IC101. The entire tape and microphone signal is amplified with a single IC chip (FIG. 8). Pin 1 is the ground terminal while pin 9 is the voltage supply terminal.

FIG. 8. IC101 amplifies the tape head and microphone audio signal.

TALK TO ME

The dynamic microphone has a small coil attached to a moving diaphragm that moves freely over a magnet, somewhat like the construction of a speaker. The coil moves in the magnetic field generating the ac output voltage ( FIG. 9). This output voltage is very low. The dynamic microphone is a popular microphone with great frequency response and used in large PA systems, churches and broadcast stations. The dynamic mike has a low-impedance and must be matched to the input sound circuits. The typical dynamic mike might have a frequency response of 60 Hz to 15 kHz and a 500 to 600 ohm impedance.

FIG. 9. Taking a continuity test of the dynamic mike and cord with the ohmmeter.

The crystal mike employs a piezoelectric effect somewhat like the crystal phono cartridge. When sound pressure is applied to the diaphragm, this places mechanical stress upon the crystal that generates a low voltage. A crystal microphone has a high impedance output and can operate directly into the audio circuit. The crystal element is made from Rochelle salt and can be damaged when dropped on a hard floor. The output level is fairly high with a fair frequency response.

A condenser microphone has two small plates that are separated. The front plate is a flexible diaphragm while the other is rigid. The movement between the two plates changes the capacity between them. The condenser mike has an excellent frequency response with low distortion. A dc voltage must be applied to the microphone elements.

The electret microphone is constructed like the condenser mike with an external voltage applied. The electret microphone might operate from 2 to 10v DC ( FIG. 10). When sound waves strike the electret mike, an output voltage is generated. The electret micro phone must have an outside dc voltage applied before it will operate. The electret con denser microphone is unidirectional with a frequency response from 50 Hz to 14 kHz. The condenser and electret mikes are found in cassette players, sing-along amps, and camcorders.

The different microphones can be repaired by replacing the internal elements. Simply replace the low-cost mike, instead of repairing it. The high-priced dynamic microphone can be sent to the factory for element replacement. Most problems related to the microphones are damaged cords, broken or worn plugs, and damaged internal elements when dropped on a hard surface. The ceramic and electret microphone can be damaged by children poking sharp objects into the built-in microphones. Replace the defective built-in mike with original part number.

FIG. 10. A typical electret microphone circuit with external voltage applied.

RECORD PLAYER MAGNETIC PREAMP CIRCUITS

The typical early phono players and changers contained a crystal cartridge with sapphire and diamond stylus or needles. A crystal cartridge might develop a higher output voltage than the magnetic cartridge. Very little voltage is developed with the magnetic cartridge which must have an additional preamp circuit to boost the audio signal from the record. A variable-reluctance phono pickup is called a magnetic cartridge.

The magnetic cartridge has a high frequency response compared to the crystal cartridge. The stylus is attached to a piece of magnetic material that moves between the two coils, producing ac voltage. Since the variable-reluctance cartridge output voltage is so low, a preamp stage is needed to boost the signal to a typical AF circuit ( FIG. 11).

FIG. 11. The magnetic phono cartridge signal is amplified by two preamp transistors.

Check the resistance of each coil of the stereo phono cartridge to determine if the cartridge or preamp stages are defective. The resistance should be under 200 ohms and both coils should measure the same resistance or within a few ohms. Notice the dc voltage on TR1 is very low to prevent additional pickup noise. Perform a transistor and voltage in- circuit test to locate the defective component.

DIRTY TALKING FUNCTION SWITCHES

You will find some type of function switch in just about every electronic product that pro vides more than one function. The deluxe receiver might have a rotary function switch to provide AM-FM radio, phono, cassette, and recording features. A boom-box AM/FM/MPX receiver and cassette player might switch to the different functions with a sliding type switch. Likewise the auto receiver might have a function switch that removes the AM-FM radio reception when a cassette is placed into the compartment. The small portable radio might have an AM-FM switch that switches in the various reception bands.

Since most switching contacts contain a silver-type switching connection, the contacts tarnish and become dirty. A dirty AM and FM switch might result in no FM with normal AM reception or vice versa. The dirty or poor contacts within the sliding bar switch of many contacts might produce a loud howling noise and erratic operation. The cassette player may not record with a dirty record/play switch. A bad switch might cause a dead-no sound symptom.

You might locate a broken plastic shaft that engages the RIP switch when the cassette player plays but no record. Garbled and distorted audio might result from a dirty radio- cassette switch. A record/play switch which is not fully engaged might cause the cassette- recorder to erratically record. A loud rushing or hissing noise on one stereo channel might be caused by a defective function switch. A no left stereo channel might be caused by a defective channel selector switch.

Clean up the function switch contacts with a silicone based cleaning fluid. Place the plastic tube down inside the switching area ( FIG. 12). Work the function switch back and forth to help clean up the contacts. Check for poorly soldered contacts if the switch is still erratic. Resolder all switching contacts on the PCB. Replace the defective function switch with worn or broken contacts.

FIG. 12. Spray cleaning fluid down inside the boom-box function switch to clean up the dirty contracts.

LOUD AND CLEAR PROBLEMS

After several years of usage the volume control might become worn and cause noise in the speaker, when rotated. Sometimes the audio will cut in and out with a defective volume control. The right channel of the amp might be dead with a poorly soldered connection on the volume control. The left channel might be dead with an open volume control. One channel might be intermittent, and again, have no volume change with a defective volume control. The radio section might be dead with an internal short within the volume control. No volume change will be noted with a broken ground connection or open volume control. Replace the volume control when the volume cuts up and down.

Clean up the noisy volume control by spraying cleaning fluid down into the open slot where the terminals come out of the control. Rotate the control up and down to clean up the carbon control. Resolder all terminal connections with an intermittent or erratic volume control.

Replace the volume control with worn contacts or open condition. While special type volume controls must be replaced with the exact part number, the most common controls can be replaced with universal controls. Choose the correct resistance and correct taper. Replace the audio volume control with a known audio taper.

BALANCING ACT

When one channel becomes weaker than the other channel, the balance control must be rotated towards the normal channel to provide equal volume out of the stereo speakers. The left channel might be louder than the right when the balance control is at zero, indicating a weak right channel. The stereo channels won’t balance up when one audio channel is a lot weaker than the other.

The equal adjustment of the audio signal in a stereo amplifier might have a separate balance control or individual volume control to balance the audio in the speakers. The balance control is usually connected ahead of the volume and tone controls. The stereo signal from the left and right AF stages are applied to the outside terminals while the center (wiper) terminal is grounded ( FIG. 13).

When the amplifier stereo channels won’t balance, suspect a weak audio channel. Connect a sine wave signal to the stereo auxiliary, phone, or function switch input. Clip both input terminals to both L and R channels. Check the waveforms at the high end of the volume control with the scope ( FIG. 14). Both audio channels should be equal on the dual-gated oscilloscopes. Another method is to check the audio sine wave at each end of the balance control with the balance control at zero or balanced point on the dial. Al though, the balance control can balance a weak channel, the very weak channel should be signal traced and repaired.

FIG. 14. The bottom waveform indicates the right channel is weak compared to the left stereo channel.

A defective balance control might not balance the stereo channels with an open or defective control. Check both terminals of the control for broken wire connections. Inspect all wiring connections. Make sure the center terminal is grounded. Check for correct resistance of the control. Replace the control, if open or erratic, with a linear audio taper.

TREBLE AND BASS CIRCUITS

The treble and bass controls might be located in the same circuits, after the volume control. Some larger audio amplifiers might have a tone amp circuit with the volume control in the base circuits and the bass and treble controls in the collector output circuits. Notice the balance control is located across the volume control center terminals. The controlled- volume is amplified by 0102 and 0202, while the output signal is taken from the bass control ( FIG. 15).

FIG. 15. Q102 and Q202 are separate tone amps with the bass and treble controls in the collector circuits.

The defective bass or treble controls might become noisy or erratic. Clean up each ganged-control with cleaning fluid. If the control is worn or broken replace with the original part number.

DEFECTIVE TAPE HEAD CIRCUITS

The dirty tape head in the cassette player might cause weak or distorted music. A heavily- packed tape head with oxide dust might be dead or produce a weak and distorted signal. Broken wires on the tape head might result in a loud rushing or howling noise in the speakers ( FIG. 16). A loud rushing noise with no music might result from a broken ground wire in the tape head. A weak right channel and normal left channel might be caused by a packed-oxide tape head. A worn tape head can cause a missing of the high notes of the recorded music.

Replace the defective tape head when it won’t record in the left or right channel. A weak and rushing noise recording might result from a defective tape head. Sometimes the dead recording will come to life when pressure is applied from an insulated tool on the tape head. Moving the tape head might remove the distorted recording on the right channel. A jumbled recording might be caused by a broken ground wire broken on the erase head. A packed oxide on the tape head might result in a weak record on the right channel and no record on the left channel, and on it goes.

Most weak and distorted cassette music can be cured by simply cleaning up the tape head. Clean up the front of the tape head with alcohol and cleaning stick. Remove stub born oxide with an insulated plastic tool. An old toothbrush dipped in alcohol can clean up the packed-oxide tape head.

Check the tape head for broken wires or poorly soldered connections. The suspected tape head can be checked by clipping the external amp across the tape head terminals. Mea sure the continuity of the tape head. The typical cassette heads have a resistance from 200 to 830 ohms. A defective stereo channel might have a different resistance measurement than the other tape head winding ( Fig. 17). Both stereo tape head measurements should be within a few ohms of one another. For instance, if one head measured 235 ohms and the other 230 ohms, the stereo head is normal. If one side measured 315 ohms and the other 380 ohms, replace the defective tape head.

FIG. 16. Check for broken tape head wires with a loud rushing sound.

FIG. 17. The normal stereo tape heads resistance should be within a few ohms of each other.

A SNAKE IN THE FRONT-END CIRCUITS

Hum and noise in the front-end circuits, including the preamp or AF, might be caused by a leaky transistor. A defective or dried-up decoupling capacitor within the voltage source of the preamp or AF circuits can cause no sound with only a hum noise. A poor ground in the front-end circuit can cause a low hum symptom. Motor-boating sound in the speaker can also be caused by an open decoupling electrolytic capacitor, defective transistor or IC. The noisy sound condition that goes dead might result from an intermittent AF transistor.

A loud rushing or hissing noise might be caused by broken tape wires on the tape head. An open tape head might cause a rushing sound in the speakers. A no volume with a rushing sound results in a defective tape head. The loud howling noise can be caused by a broken tape head connection. Suspect a noisy preamp transistor when the noise disappears by turning the volume control down. Check each electrolytic coupling capacitor in the front-end circuits for a noisy channel. Double check ceramic and electrolytic capacitors in the emitter circuit for a frying noise in the sound. The intermittent noisy channel can be caused by poor board or soldered connections on the volume control.

Signal trace the noisy audio to the preamp or AF circuits with the external audio amplifier. After locating the noisy stage, determine if the transistor or IC is making the noisy condition. Apply several coats of coolant to the semiconductor devices. Shunt electrolytic and bypass capacitors with a good known value. Replace the suspected transistor or IC preamp when all other tests fail.

ALL IN ONE

The stereo preamp or AF circuits might be included in one IC component. Within the auto- cassette receiver the preamp tape head circuits are found in IC201. When a cassette is inserted into the auto radio, the tape switch applies a dc voltage to the preamp IC circuit. The left tape head is coupled to pin 1 of IC201 with C203 (1 uF). Likewise, the right tape head audio is coupled to pin 8 with C204 (1 uF).

C209 and C210 (4.7 uF) couple the output signal to a fixed diode to the volume and tone controls ( FIG. 18). D203 and D204 prevent the radio AM and FM signal from entering the preamp circuits. Diodes D206 and D205 apply the radio input signal, while D201 and D202 connect the FM stereo audio signal to the audio amplifier circuits. Instead of a large function switch, the fixed diodes provide AM, FM and tape music to be switched to the audio circuits without any moving parts.

FIG. 18. IC101 amplifies the stereo tape heads and is switched in the output with fixed diodes in the auto receiver.

WEAK PREAMP CIRCUITS

Check and shunt electrolytic capacitors within the preamp circuits for weak audio symptoms. The leaky AF transistor can cause weak reception. A weak audio stage can be caused by a leaky preamp or AF transistor or IC. An open preamp transistor might result in no audio signal in that channel. A very weak right channel with a normal left channel can be caused by a defective preamp IC. The weak and intermittent sound symptom might be caused by a defective preamp transistor. Shunt electrolytic capacitors in the emitter cir cults for weak sound in AF circuits. Don’t overlook a dirty tape head in the cassette player for a weak audio channel. A weak preamp circuit might result from an improper voltage supply source.

Insert a 1 kHz test cassette to test the preamp and AF circuits of the radio-cassette player. Signal trace the preamp circuits with the scope or external audio amp. Clip the ground wire of the audio amp to common chassis ground. Start at the tape head and work back to the volume control to determine what stage produces the weak audio signal. Proceed through the circuit by placing the amp probe on each side of the coupling capacitors.

You can quickly signal trace the preamp circuits by taking critical waveforms through the preamp circuits. Start at the volume control and work towards the tape heads in the preamp or AF audio circuits. The test cassette should show a dual waveform on the scope if the preamp stages are normal. Check the weak waveform against the normal channel at the volume control, coupling capacitors, collector and base terminals to locate the weak component.

FIG. 19. Both stereo preamp waveforms are fairly equal in amplitude indicating a normal preamp circuit.

After locating the weak stage, check each transistor with in-circuit transistor tests. Mea sure the bias resistors and check the resistance with the schematic or color code of the resistor. Sometimes a resistor might overheat and change the color bands on the resistor. Remove one end of the resistor for a correct measurement. An increase in resistance of a bias or base resistor can cause a weak audio signal with some distortion.

DEAD-NO AUDIO IN PREAMP OR AF CIRCUITS

The dead chassis with no audio is much easier to service than a weak or intermittent one. No audio in the preamp or AF circuits can be signal-traced with an external amplifier or scope. A quick method to check the front-end circuits is with a clicking test. Turn the volume control wide open and listen for a hum sound. Touch the tape head ungrounded wire, tape head coupling capacitor, or base of the first preamp transistor with a small screwdriver blade. A clicking noise or a loud hum should be heard in the speaker. No click, no hum, no operation.

A dead preamp circuit can be located with an injected audio signal at the input auxiliary jack, tape head or phono input jack. Clip a 1 kHz sine or square wave from the function generator to the input and common ground; tie both stereo input channels together. Scope the input and output of the preamp or AF circuits to determine where the waveform stops.

Likewise, insert a 1 kHz or 3 kHz test tape in the .cassette player and signal trace with the external audio amp. Go from tape head to electrolytic coupling capacitor, base of first preamp transistor or input IC terminal, and trace the missing signal through the preamp circuits. If no signal is found at the volume control, the dead audio component must be from tape head through the preamp and AF audio circuits.

The no audio symptom might be caused by an open preamp or AF transistor. A leaky preamp transistor can cause no audio sound. An open or poor electrolytic coupling capacitor can cause a dead audio problem. A dirty or worn input switching terminal can cause a no sound symptom. A burned or open emitter resistor can cause a no audio condition. The broken ground connection of the tape head in the cassette player might produce a dead chassis.

The dead front-end circuits can be caused by an open preamp IC with a normal voltage source. Improper or no voltage applied to the preamp or AF circuits can produce a dead chassis. The dead preamp stage can be caused by an open voltage regulator transistor in the low voltage power supply source. When both stereo channels are dead, suspect a leaky preamp IC or shorted decoupling capacitor in the voltage source feeding the preamp circuits. A leaky AF transistor might result in no audio, with hum in the speaker. A dead receiver with low hum noise can result from a leaky decoupling capacitor in the voltage source of the preamp circuits.

DISTORTED PREAMP CIRCUITS

Although audio distortion is found in the output circuits, leaky components in the preamp can cause distortion. A leaky preamp transistor or electrolytic coupling capacitor can cause distorted audio. The weak and distorted audio symptom might result from a leaky preamp IC. Suspect a leaky AF transistor for a weak and distorted audio problem. Check for a bad or dirty radio-cassette switch with distorted and garbled music. Slight distortion in the left channel might be caused with an open AF transistor and when tested in-circuit, the transistor was normal.

FIG. 20. When the sine or square wave form stops, you have located the defective stage.

A change in resistance or burned bias resistors of the preamp or AF circuits can cause a distorted symptom. The right channel in a tape player can be distorted with a defective tape head. A dirty tape head can cause distortion in the recorded sound. Replace the preamp or AF transistor for a microphonic-distorted noise. Apply several coats of coolant on each transistor to locate the one that produces the microphonic sound. Most distortion problems in the audio amplifier are found in the audio output circuits.

REPAIRING THE PREAMP RECORDING CIRCUITS

The preamp recording circuits within the preamp stages are the same in the play mode of the portable, boom-box or auto receiver cassette player. These preamp circuits might be two directly coupled transistors or one IC component. You will find two identical preamp circuits within the stereo channels. In early portable cassette players all of the audio transistors operated in the play and record circuits, while in other players the preamp circuits provided recording features.

The first preamp IC might include both play and recording operations. You might find several different switches engaged in the input and output circuits of the RIP functions. When SW1 -1 is switched to record mode, the built-in mike and external microphone jack is switched into the input circuit of IC1101 ( FIG. 21). The microphone sound is amplified by the IC and comes out of pin 13 and coupled to another switch that returns the recorder audio to the left channel RIP tape head. The recorded message is applied by the tape head to the recording cassette. The right channel recording circuit is identical to the left channel of IC1101.

No recording in the left channel might be caused by a detective tape head or packed tape gap. When a cassette player operates in play mode but not in record, this indicates a broken plastic shaft engaging the R/P switch assembly. Intermittent recording can result

FIG. 21. A typical left channel RIP tape head.

from a poor tape head connection or dirty switching contacts. Check for a defective electrolytic coupling capacitor where it ties into the PC board with intermittent recording.

A recording with only a rushing sound might indicate a defective tape head. The record intermittent and warbling sound can be caused by a dirty record switch assembly. A defective function switch might produce intermittent recording and play modes. When one recorded channel is weak and the other won’t record, check for a packed oxide-tape head. The loud noise in one channel, when recorded, can be repaired with a cleanup of the record/play switch.

The jumbled recording can be caused with a defective erase head circuit or a ground wire oft of the erase head. A noisy recording can be caused by a defective built-in microphone. Poor erasing of the erase circuit can cause a messed up recording. Scope the erase head signal from the erase head bias oscillator and check for poor shielding of the erase head circuits. No erasing of previous recordings might be caused by a poor ground connection in the erase bias circuits.

Make sure the playback functions are normal before attempting to service the recording circuits. Clean up the tape head, erase head and RIP switches. Scope the waveform from the recording oscillator at the left and right tape heads. Check for a dc voltage switched into the erase head on low-priced cassette players. If no voltage is found, check for a dirty recording switch or open voltage dropping resistor. Repair the recording bias-oscillator stage when no scope waveform is located at either recording tape head. When the play back mode is normal, the preamp and AF RIP circuits are good. Suspect defective switching and erase head problems with normal playback circuits.

SERVICING TWO HEADS IN ONE

The dual-tape cassette recorder usually has two different tape head compartments where one cassette is used only for playback and the other for record and playback modes. The record/playback tape heads can record from the built-in microphones or from the other set of playback tape head sections ( FIG. 22). The record/play compartment is to the left and play only to the right side of stereo cassette player.

FIG. 22. The cassette compartment on the right plays only with no recording features.

You will find one erase head in the record/play cassette and one play head in the play only mode cassette player. One tape motor might rotate both cassette departments with a large motor belt. Some portable dual-boom-box players might have a separate motor to rotate the tape in each tape deck. The stereo play tape heads are connected directly into the preamp IC circuits while the stereo record/play heads are switched in play/record modes.

A block diagram of the dual-deck cassette player is shown in FIG. 23 Tape head-i contains playback stereo tape heads and tape head-2 has both RIP tape heads. The left and right built-in microphones are coupled to IC102 amplifier and to a buffer amp IC before the stereo signal is applied to the Dolby B/C NR IC111. The recording output signal from IC111 is applied to the record amp Q201. The recording signal is amplified by 0201 and switched to the RIP heads of tape(2).

Notice the stereo play tape heads are connected directly to the microphone amps while the RIP heads (2) are switched in and out of the input circuits. The different tape deck circuits are switched into the amplifier circuit with an RIP switch, supplying a dc voltage source from the regulated power supply.

Service the dual-cassette player like any cassette player. If the playback only tape deck (1) is normal and no RIP in deck-2, you know the playback circuits of preamp IC1O1 and id ii are normal. Clean up the RIP switching contacts and tape heads (2). When tape head (2) does not play back or record, suspect the tape head (2) RIP switch assembly and record amp. Inject a 1 kHz or 3 kHz signal at the input of Q201 (record amp) and scope the record amp RIP switches, and tape head. Check for a bias oscillator signal at the record tape heads (2) with the oscilloscope.

Suspect one of the built-in microphones when one channel is dead in the record mode.

Switch microphones to determine if the mikes are normal. To check the microphone amps (IC102), inject a 1 kHz audio signal at one of the microphone jacks and check the output of FIG. 23. A block diagram of a two-deck cassette player left channel record/play circuits.

IC102, IC103, and IC111 with the external amplifier. The entire record and playback circuits can be checked by injecting an audio signal and using the scope or external amp as an indicator. When the audio signal stops, take critical voltage and resistance measurements.

The early auto radio AM and FM stereo circuits are switched into the transistor preamp or AF audio circuits. When the cassette player is included within the auto receiver, the RIP stereo heads might be switched into the preamp transistor or IC circuits. The early IC preamp circuits might have a single IC for each stereo circuit or one dual IC for both left and right tape head circuits ( FIG. 24).

FIG. 24. In the early auto receivers the cassette player has separate preamp IC’s.

After tape head and switching cleanup, take critical voltage measurements on each terminal of IC101 and IC202. Determine if the left or right channel is defective. Suspect a leaky IC if the voltage is low at pin 9 of either the left or right IC. Remove pin 9 from the PCB with solder wick and iron. If the supply voltage returns to normal, replace the leaky preamp IC. Suspect a voltage regulator transistor when the voltage is still low at pin 9.

Insert a musical cassette and signal trace the playback signal from tape head to the input terminal with the external audio amp. For instance, if the right channel is weak and distorted, start at the tape head, check both sides of Q102, and then on pin 2 of IC202. If the playback (PB) signal is normal check the audio at output pin 6. Then proceed to both sides of C104 (10uF) coupling capacitor. Most problems within the auto cassette preamp circuits are dirty tape heads and RIP switching contacts. Intermittent or no music can be caused by loose or torn wires on the tape heads.

TUBE AF CIRCUITS

In the early audio circuits you might find a triode as the preamp or AF amplifier tube. The preamp stages in a musical instrument PA system can be a dual-triode vacuum tube. The various triode tubes used were the 12AX7, 12AY7, 12AT7, and 7025. The volume control might be found between the two different triode elements. The bass and treble controls might be ahead of the volume control.

FIG. 25. The early tube preamp circuits might have a dual-purpose tube with the different controls between triode sections.

Check the preamp tube in a tube tester or sub a good tube in the preamp socket with a dead, weak or microphonic sound. Take critical voltage measurements of the tube elements. Notice that the voltages found on the plate socket terminals might range from 150 to 350 volts. Make sure the voltmeter is able to measure these high dc voltages. Clip the black meter lead to chassis ground and the red probe to take voltage measurements.

A leaky or gassy tube, or an increase in grid or plate load resistors, might cause a weak and distorted sound in the speaker. Check for a burned cathode resistor with a leaky or shorted preamp tube. The preamp tube might become weak, the cathode bypass electrolytic might become open or dried-up, and cause a weak sound symptom. Excessive hum can be caused by a defective decoupling or filter capacitor. The leaky coupling capacitor might cause a weak-distorted sound. A low pickup hum might be caused by an increase in resistance of a grid resistor or poor grounds. Spray cleaning fluid down into the different controls when a scratching or intermittent volume is noted.

. ===