Important Audio Tests and Adjustments

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Signal tracing the defective audio circuits by injecting a signal from the audio signal or function generator can help to locate a defective audio component. The external audio amplifier can locate the weak or distorted circuit in the audio stages. Using the oscilloscope as indicator can quickly locate a defective preamp or audio output circuit with wave forms.

Correct adjustments within the audio circuits or components can improve the sound of the audio amplifier. In this section, you will learn how to make your own test cartridges and cassettes for use in signal tracing the signal from input to output terminals of the audio amplifier.

FIG. 1. The function generator, power supply and frequency counter can quickly locate a defective stage in the audio amplifier.

SIGNAL INJECTION

Signal injection is a method of quick audio or RF troubleshooting in which a signal injector or generator is used to inject a signal into the audio circuits. The signal injection frequency might be 1 kHz, 3 kHz, and 10 kHz. Signal injection can be accomplished with the audio generator, audio oscillator, function and noise generator test instruments. The generator can inject a sine or square waveform into the audio circuits. The audio signal can be injected at the input or at any point in the audio circuits. Signal injection can quickly locate a defective stage or component, weak or distorted, and a dead audio circuit.

The audio signal must have an indicator that produces the audio tone, waveform, or analog meter movement. The oscilloscope can quickly identify a dead, distorted, clipped, or weak audio circuit with a signal input waveform. An external audio amplifier can locate a distorted, weak or dead audio signal in any audio stage or circuit.

A frequency counter can be used as an audio indicator and also in audio alignment procedures. The VTVM, FET-VOM and AC voltmeter can be used as a signal indicator and in alignment of balance and head azimuth adjustments. Of course, the audio indicator can also be the test speaker connected at the amplifier output terminals.

The audio, function and noise generator can be connected at the input terminals of the audio amplifier and the audio tone can be checked from stage to stage with the scope or external amplifier as an indicator. Simply check the signal from base to base of each transistor in the preamp, AF, driver and audio output circuits. If the preamp and audio output circuits are comprised of IC components, check the signal at the input and output terminals of every IC. When the signal is lost or diminished (weaker), you have located the defective circuit.

FIG. 2. Scope the signal in and out of the suspected audio IC.

The audio signal from the audio, function and noise generator can be injected at any point in the audio circuit to locate a defective component. Instead of moving the indicator at different points within the audio circuit, simply inject the audio signal. Likewise, go from base to base terminal of the audio transistors and input to output terminals of IC components. The most common indicator of the audio signal is the connected PM speaker. The sound of the signal generator is heard in the speaker until the defective circuit is located. The noise generator can be used in RF and IF circuits, as well as in audio injection Troubleshooting.

FIG. 3. Injecting the audio signal at the transistor base terminal can quickly locate the defective circuit with the speaker as an indicator.

Break the audio circuits down by starting signal injection at the volume control. Inject the audio 1 kHz signal into the left and right stereo circuits. In the RCA CTC146TV chassis, the sound was weak and sometimes the sound level could not be controlled. A 1-kHz audio signal was injected at the base terminal of the first audio amp (Q1201) and the sound appeared normal. Since the IF/SIF IC component fed into the audio circuits, U1001 was suspected of causing the weak sound.

All voltages appeared normal on U1001. A further check of the audio circuits indicated the sound was controlled by the Analog Interface Unit (U3300) at pin 30 of U1001. Perhaps, the audio symptom existed in the audio control circuits. The control volume was traced back to pin 37 of U3300. Here, diode CR3306 was found to be leaky and was replaced with a universal ECG-177 silicon switching diode. In another RCA CTC1 46 chassis, C3314 off of the leg of CR3306, produced no control of the volume. Replace both the CR3306 and C331 4 (0.22 uF) capacitor when either one is found to be defective. Check outside of the sound output circuits for possible sound problems.

FIG. 4. The weak sound symptom in an RCA CTC146 chassis was caused by CR3306 and C3314 off of pin 37 of AIU (U3300).

AUDIO SIGNAL TRACING

The external audio amp is used in signal tracing the distorted, weak, dead, or intermittent audio within the audio amplifier. The audio amplifier can have a PM speaker or head phones as an indicator at the output of the audio signal tracer. The audio signal can be traced from the preamp to the audio output terminals with the external audio amp. Again, the audio is signal traced starting at the volume control and proceeding either way to locate the defective stage or circuit.

Insert the electronic products accord into an isolation transformer for test equipment and product protection. When the audio is weak in the AM radio, tune in a local radio station as the signal source. Insert a test cassette when signal tracing the distorted sound in the cassette player. A test disc can be used when signal tracing the audio output circuits in the CD player.

Clip an audio or function generator to the input terminals of the high-wattage amplifier for a signal source and signal trace the audio circuits with the external audio amp. Rotate the signal generator to 1 kHz frequency for audio tests.

Go from base to base of each succeeding transistor from preamp to output circuits, to check the audio signal with the external amp ( FIG. 5). Clip the ground terminal to common ground and place a test probe on each base terminal. Check the audio signal at the volume control. When the defective stage is in the output circuits, began at the volume control and check each output transistor. Test for audio signal at the input and output terminals of a suspected IC with the external amp.

FIG. 5. Troubleshooting the transistor audio circuits with the external audio amplifier.

When the audio signal becomes weak, intermittent, or distorted within the external audio amp, you have located the defective circuit. Remember, the audio signal should become stronger as you proceed toward the speaker terminals. Simply turn down the audio at the external audio amplifier. The external amplifier can monitor the defective stage for weak and distorted reception. You can pin-point the weak audio component with the external audio amp. Take critical voltage and resistance measurements at the defective stage or circuit. Then, test each suspected transistor with in-circuit transistor tests.

The audio was distorted in the PM speaker of an RCA CTC145TV chassis. Sometimes a touch-up of the quadrature sound coil (L1201) off of IF/SIF IC (U1001) can cure the extremely distorted audio. The audio signal within the external audio amp was normal on output pin 28 of U1001 and base of Q1201 ( FIG. 6). The audio was normal on the base of Q1202 and Q1203, but distorted at the speaker terminal. The audio was distorted on both sides of C1207 (100 uF). No doubt, Q1202 and Q1203 were defective. Q1202 tested open with an in-circuit transistor test. The collector voltage had increased to 19.7 volts. Both output transistors were replaced, Q1202 with a SK3854, and Q1203 with a SK3867A universal replacement transistor.

FIG. 6. Open Q1202 caused extreme distortion in the RCA CTC145 audio output circuits.

IC AUDIO AMP

The external audio amplifier for chasing down the lost, distorted or weak audio in the audio circuits, does not have to be a complicated or high-powered audio circuit. The audio amp can be constructed around a LM386-1 audio amp IC. To acquire the highest wattage output, choose either a LM386-1 or LM386-3 IC ( FIG. 7). Turn the volume down (R1) as the external amp tests each AF or driver transistor. Mount C4 as close to pin 4 as possible to prevent oscillations in the speaker. The external amp components are listed in the part lists:

  • IC1 LM386-1 or LM386-3 audio IC
  • C1 0.1 uF 250 volt capacitor
  • C2 1 uF 35 volt electrolytic
  • C3 10 uF 35 volt electrolytic
  • C4 220 uF 35 volt electrolytic
  • C5, C6 0.1 uF 50 volt ceramic capacitor
  • C7 100 uF 35 volt electrolytic
  • R1 10K ohm audio taper control
  • R2 560 ohm 1/4 watt carbon resistor
  • R3 4.5K ohm 1/4 watt carbon resistor
  • R4 10 ohm 1/4 watt carbon resistor
  • Spkr: 4 inch 8 ohm PM speaker
  • Batt.: 9 volt energizer battery
  • Sw1: SPST on rear of R1
  • Misc.: PC board, 8 prong IC socket, hookup wire, cabinet, solder, etc.

FIG. 7. The small external audio amp was built around the LM386-1 or LM386-3 low-powered amplifier.

TEST CASSETTES

There are several different test cassettes and discs used in cassette and CD players for proper adjustments. The azimuth adjustment of the tape head should either be a 6.3 kHz or 10 kHz test cassette for proper alignment. The scope, ac voltmeter and frequency counter can be used as the indicator across a 4 or 8 ohm load at the speaker terminals. The bias, record gain and record/playback sensitivity adjustments should be made with a 1 kHz test cassette.

The tape speed adjustment can be checked with a 1 kHz or 3 kHz test cassette, with the frequency counter as an indicator. The playback sensitivity adjustment, Dolby NA level adjustment, and bias current adjustment should be made according to the manufacturer’s service literature. The flutter meter might use a 3 kHz and 3.15 kHz test cassette for wow or flutter tests.

ROLL YOUR OWN TEST CASSETTE

Test cassettes with different frequencies can be used in Troubleshooting the audio circuits, cassette azimuth and cassette speed adjustments. The most common test cassettes used are the 1 kHz, 3 kHz and 10 kHz. You can make your own test tapes with the audio signal generator and cassette recorder.

Simply inject the 1 kHz audio signal into the input jack of cassette recorder and record the 1 kHz signal on a 1/2 hour cassette. Keep the output control of the generator at a normal volume level. Likewise, record the 3 kHz and 10 kHz audio signals on a separate blank cassette. These audio cassette tapes can be used in Troubleshooting and adjustments on the cassette player.

The commercial microcassette alignment tape might have a 15 minute alignment cassette of a 1 kHz audio tone recorded at -4dB level. The commercial reference level cassette might have a frequency of 6.3 kHz, 8 kHz, and 10 kHz. A commercial cassette speed tape might operate at 50 Hz, 3.15 kHz, and 3 kHz. The commercial frequency response cassette might have a wide range of frequencies between 315 Hz, 1 kHz, 6.3 kHz, 10 kHz, 14 kHz, and 18 kHz.

TROUBLESHOOTING WITH TEST CASSETTES

The different test cassettes can be used to signal trace the audio circuits within the cassette player. When the cassette player has intermittent, weak and distorted sound problems, insert a 3 kHz or 10 kHz test cassette into the cassette recorder. Use a scope or external audio amplifier as an indicator ( FIG. 8). Start at the tape head terminals and trace the audio signal through the preamp and AF circuits, if the audio defects are not heard at the volume control terminals. The audio cassette signal can be traced clear through to the audio speaker terminals.

FIG. 8. Insert a test cassette and check the various waveforms throughout with the scope as the indicator.

When the cassette signal is missing, weak or distorted, take critical voltage and resistance measurements on the components in that stage. Suspect the AF transistor when the signal is weak at the collector and normal at the base terminal. Poor component soldered terminals on diodes or regulator transistors can produce an intermittent or dead voltage source.

The cassette audio signal and scope can locate an open or leaky transistor and IC in the audio output circuits. A defective speaker relay circuit can be located with the test cassette and scope as indicator.

VOLTAGE INJECTION

When the voltage source is derived from another source in a battery operated audio amplifier, radio, auto receiver, TV and special audio circuits, an external working voltage can be injected from a separate power supply. For instance, in the TV chassis, most sound circuits are powered by a voltage source developed in the flyback transformer windings. Thus, the horizontal circuits must operate before any secondary voltage is developed in the horizontal input transformer. A defective component in the sound circuits might shutdown the TV horizontal circuits.

FIG. 9. The external power supply can provide a voltage source for a defective audio circuit.

The sound circuits can be tested by injecting an external voltage from the power supply. Check the schematic for the supply voltage terminal source operating the output IC or transistors; this is usually the highest working voltage applied to the component. In a Panasonic model CTL-1 030R T\ the supply voltage from the flyback circuits was a +15 volts. By injecting the supply source at R21 0 (10 ohm) resistor on pin 9 of IC201 ,the audio circuits should now be alive.

Inject the 1 kHz audio signal from the signal tracer, signal or function generator at the C209 (4.7 uF) electrolytic at input terminal 2 of IC1201. With a voltage source injected at pin 7 of IC201, the audio should be heard in the loud speaker. If no sound, inject the audio signal at output pin 8 of IC201. A low audio tone should be heard in the speaker, indicating IC201 or components tied to it are defective.

Take a voltage and resistance test on each pin terminal of IC201 to locate a leaky electrolytic capacitor. Replace defective IC201 when the voltage and resistance measurements were quite normal to those on the schematic.

CRITICAL WAVEFORMS

The audio stereo circuits within the TV chassis or high-powered amplifier can locate the weak and distorted audio stage or circuits. By injecting a sine or square waveform audio signal into the input of both left and right stereo circuits, the oscilloscope probe can quickly locate the defective stage. Connect a speaker or dummy load to both stereo output speaker terminals. Besides the speaker load, a PM speaker can be connected to each audio channel to indicate what channel is defective. A weak stereo channel does have a lower audio sound than the normal channel.

FIG. 10. A dc voltage from external power supply located a leaky IC1201 in the Panasonic TV sound output circuits.

Keep the input audio signal as low as possible to prevent clipping or over-driving the audio circuits. Use the dual-trace scope to test both audio channels at the same time. Clip the scope probes to both stereo balance or volume controls. Notice if one channel is weaker than the other. Proceed to the AF or driver circuits if the sine waveform at the controls have the same waveform and amplitude. Check the waveform of the defective channel against the good audio channel.

FIG. 11. The scope waveforms in the right channel were quite weak with a square wave in the audio amplifier.

The stereo sound can be checked stage by stage in each stereo channel. Proceed slowly and deliberately to trace both audio channels. When the audio waveform changes in form and amplitude, the defective part is close at hand. The defective waveform might be weak (lower amplitude), have rounded corners, or a different shape than the normal audio channel. Make sure each scope test is on the same point in each stereo circuit.

Take critical voltage measurements on a transistor or IC component that indicates an improper waveform. Compare these voltage measurements with the same spot in the normal channel. If a schematic is not available, check the voltages in the normal circuit and compare with the defective measurements. The audio channels in the stereo amplifier are much easier and quick to service, since comparison voltages, resistance and wave forms can be measured without a schematic.

TAPE HEAD PROBLEMS

The defective tape head might have an open winding, poor internal solder connections, or torn wires from the tape head terminals. When the volume is wide open with only a rushing noise in the speaker, suspect an open head or torn wires from the head terminals. The cassette audio might be intermittent with poor internal connections. Sometimes moving the tape head while the player is operating will cause the sound to cut in and out. The sound can be intermittent within the auto cassette player in reverse mode with broken head wire terminal leads.

The tape head might not have any sound in record or playback modes when the tape head is moved backwards or out of the line of the tape path. Sometimes the mounting screws on the tape head can work loose and let the tape head swing out of line. In players where the RIP head is pivoted downward, the head may not reach its full position in play mode; thus, no sound from the tape. At other times, the tape head might come loose from the welded support causing no audio in the speaker ( FIG. 12).

FIG. 12. Check the tape head terminals for poor connection and to see if they have moved out of the line of rotating tape.

Check the face of the tape head when the recordings become tinny or contain a high- pitched music sound. Look for a worn front area of the tape head. This worn condition usually occurs after many years of operation.

The tape head can be magnetized with slight distortion and a loss of high frequency response in the speakers; demagnetize the tape head. The typical tape head resistance might be from 200 to 850 ohms. The erase head resistance might be from 200 to 1000 ohms. Both stereo tape head windings should be quite similar in a resistance measurement.

DEMAGNETIZE THE TAPE HEAD

When servicing the cassette player audio circuits, don’t use magnetized tools near the tape head, as the head can easily be magnetized. With normal use, the head will retain a small amount of residual magnetism, resulting in increased noise and distortion, and loss of high frequency response. Long hours of tape playing can magnetize the tape heads, causing high-end frequency response and increased distortion, resulting in overall poor performance.

There are many different kinds of demagnetizer cassettes and tools on the market. A modern cassette demagnetizer automatically demagnetizes the record and playback heads of all standard decks and restores the best performance. This system quickly and conveniently restores audio performance, displaying a red light when the system is operating.

FIG. 13. The demagnetizer cleaner cassette is self-powered while the demagnetizing cassette receiver power is from the cigarette lighter.

Insert the demagnetizer like a regular cassette. Be sure the front side faces up. Set the cassette player to “play” mode. Notice if the red light is on. Eject the demagnetizer cassette after 10 seconds of operation. The demagnetizer should be used after 20 to 30 hours of playing time. Clean up the tape heads with an audio cassette cleaner or alcohol and cleaning stick. Keep all test cassettes away from the demagnetizer cassette when operating.

A cassette demagnetizer might operate directly from the cigarette lighter on the car dash board. Insert the cassette demagnetizer into the player. Place cassette deck into “play” mode. Plug the cord of the demagnetizer into the cigarette lighter. Withdraw the demagnetizer from the player after 10 seconds of operation. Place recorded tapes at least 3 feet away from demagnetizing operation to avoid accidental erasure of the recordings. Always clean the erase and tape heads after repairs, and then demagnetize the tape heads.

HEAD AZIMUTH ADJUSTMENT

Make sure the tape head is clean. Insert a music tape into the cassette player and select a side of the cassette which contains piano or violin strings. Rotate the tone control to the high position. Check the side of the tape head which has the adjustment screw with a spring on one side of the tape head ( FIG. 14). Insert a screwdriver blade to rotate or turn the azimuth adjustment screw. Sometimes a hole is found in the top cover of the auto- cassette player for azimuth adjustment.

FIG. 14. Adjust the azimuth screw with a tension spring for maximum sound in the speaker or on AC meter.

Turn the screwdriver left or right until the high frequency reproduction is the clearest. If readjusting the azimuth does not reproduce the high frequencies, check for a worn magnetic head, defective component in the audio circuits and a magnetized magnetic tape head. Demagnetize the tape head and try again.

The head azimuth screw can be accurately adjusted by inserting a 6.3 kHz or 10 kHz test cassette into the player. Connect a 4 or 8 ohm dummy resistance load across the head phone jack, line output, or the speaker terminals. Connect a high sensitivity AC voltmeter or FET-VOM across the dummy load. Playback the test tape with the volume at the proper level. Adjust the azimuth screw for maximum output reading on the meter. After completing the alignment, lock the screw with a screw lock, glue or paint.

FIG. 15. Insert a 10 kHz test cassette with 8 ohm load across headphone jack and ac voltmeter for correct azimuth alignment.

SPEED TESTS

The tape speed in the cassette player might run slow or too fast. The cassette motor might contain a wow or flutter sound in the speaker with oil spots on the belt. Slow speeds can be caused by a stretched or loose motor drive belt. When the motor speed changes, the cassette player results in poor music reproduction. Clean up all belts, pulleys, dry bearings and capstan. If the motor is still running slow, make a tape speed adjustment.

A cassette motor might have a separate speed adjustment within a speed regulator circuit or on the rear bell of the motor. Play a standard test tape (3 kHz) and adjust the variable resistor on the motor or speed circuit for correct speed. Connect a frequency counter to the headphone jack, line output, or speaker terminals with a 4 or 8 ohm load resistor. Adjust the motor tape speed VR control to obtain an output of 3 kHz with a + or -10 Hz on the frequency counter ( FIG. 16). The speed regulator circuit might have a normal and a high speed adjustment. Use a 3 kHz cassette for normal speed and the 6.3 kHz cassette for the high speed adjustment.

FIG. 16. Check the tape motor speed with 3 kHz cassette and frequency meter connected to 8 ohm load resistor.

PHONO SPEED TESTS

The phonograph speed can be checked by a speed-strobe disk that is placed upon the turntable of the record player. The different 33, 45 and 78 RPM speeds will stand still with a strobe, neon or fluorescent light above the turntable. The electronic technician’s fluorescent bench light is ideal. When the speed stripes or bars stand still at any given speed, the turntable speed is normal. When the stripes or bars slowly goes backwards, the turntable is running slow. Likewise when the speed is too fast, the bars and stripes are slowly moving ahead. Check all three speeds with the strobe disk.

Adjust the motor speed control for the bars and stripes to stand still under .a strobe or fluorescent light ( FIG. 17). When no speed adjustments are found in the early models, the slow speed indicates a worn idler wheel, dry motor bearings or slippage of the belt on the rim of the turntable. A good cleanup of the idler, motor pulley and bearings might solve the slow speed problem. If not, replace the idler wheel, rubber tire or motor drive belt. Resurfacing the turntable rim with liquid rosin might prevent idler slippage.

FIG. 17. Adjust the speed control until the lines or bars standstill with overhead fluorescent light.

A commercial speed-strobe digital turntable speed readout uses digital graphics instead of hypnotic bars or stripes. The optical disc can check any strobe set speed with unsurpassed accuracy. The digital speed readout can check the 16, 35, 45 and 78 RPM turn table speed (model #32-10345). Order from:

MCM Electronics

PLAYBACK CASSETTE SENSITIVITY ADJUSTMENT

Always follow the manufacturer’s alignment procedures when making playback sensitivity, line-out Dolby NR level, bias current, playback gain and record gain adjustments. The playback sensitivity adjustment might also be called playback gain adjustment. These are usually located with preset controls after the preamp tape head transistors or IC components.

A Dolby NR calibration tape (400 Hz), 200 pwb m/m), (MTT-150 test cassette) might be used with a VTVM or FET-VOM and oscilloscope connected to the Dolby output test points. Adjust V304 and V404 for a certain manufacturer 580 to 590 mVs ( FIG. 18). In a Sanyo MX72OK cassette player the Dolby output adjustment was 580 mV. This play back adjustment actually selects a certain volume of sound to be placed in the Dolby and output circuits.

TAPE HEAD BIAS ADJUSTMENT

Although the small tape head within the portable cassette player might not have a separate control to adjust the bias current on the tape head, insert a 100 ohm resistor in series with the grounded head terminal. To make sure the bias oscillator is operating, take a waveform upon the ungrounded tape head terminal in record mode. A current test at the tape head can be made by taking a voltage measurement across the 100 ohm resistor.

FIG. 18. Block diagram of the playback sensitivity or gain adjustment (VR304) in a Sanyo MX720K cassette portable.

Place the cassette player in record mode. Measure the voltage across the 100 ohm resistor with a VTVM, FET-VOM or DMM. The tape head current should be between 5 to 65 mVs. Some players have test points at the tape head terminals for these type of tests. Simply adjust the bias controls from the bias oscillator for correct manufacturers mV measurement.

In larger cassette players that have Dolby circuits, connect the audio signal generator to the AUX input terminal. Connect the AC voltmeter to the output test points of Dolby IC. Load the deck with a normal tape and set the player in the recording mode. Adjust the input level with attenuator control until the meter reads an average 30 mV with a 1 kHz and 12.5 kHz signal from the generator. Record both audio signals on the tape.

FIG. 19. Adjust for correct head bias in each stereo channel.

Playback the recorded signals and adjust VR602 for the left channel, so that the difference in output of the 1 kHz and 12.5 kHz signals become +1 dB - 0 dB. Adjust control VR602 in the same manner for the right channel. Follow the manufacturer’s exact head bias procedures found in the service literature.

Follow the manufacturer’s alignment procedures for cassette amp alignment, output level, record/playback (R/PB) and the various mechanical alignments. The other special alignment procedures are level meter, metal and chrome tape, Dolby level, normal recording levels, Dolby noise reduction check and erase current test alignments. You may find more required adjustments within the deluxe cassette deck than the common cassette player used everyday.

BIAS CONTROL ADJUSTMENTS

The high-powered solid-state or tube amplifier might have correct bias adjustments within the audio output circuits. The bias adjustment on the push-pull output tubes is to adjust the negative bias from the low voltage power supply applied to the grid circuits of the output tubes. An improper adjustment of the bias control can develop noise and distortion in the output circuits.

For instance, the correct bias adjustment of two EL-34 tubes in push-pull operation is -32 volts on pin 5, with a plate voltage on pin 3 of 415 dc volts. A 6550 tube with a +530 dc plate voltage should have a bias of -66 volts for proper operation.

The voltage measurement on the grid terminals should be made with the VTVM or FET VOM. Improper bias adjustment can cause extreme distortion. These screwdriver type bias controls can produce a noisy or erratic sound in the speaker with dirty wiping contacts. A weak-distorted sound might result from a poor wiping contact of the bias control. Sometimes, just spraying the control with cleaning fluid can remove the noisy condition. Follow each manufacturers correct bias and level control adjustment procedures.

In the early high-priced solid-state directly-coupled audio output circuits, the bias adjustment control was found in the base of the driver amp. The VTVM or FET-VOM was connected across the emitter resistor and output terminal of TR71 4. VR704 was adjusted for a low mV measurement on the voltmeter. The Panasonic SA-6200 (100 watt) amplifier was adjusted for 3.7 mV on the meter. Follow the manufacturer’s bias adjustment procedures for correct bias and level control adjustments.

FIG. 20. Adjust VR704 to provide the manufacturer’s audio bias adjustment (MV) upon the emitter resistor of 0713 in the early high-powered amplifier.

TYPICAL FREQUENCY RESPONSE TESTS

The frequency response of the deluxe high-powered amplifier might be 20 Hz to 20 kHz. The typical frequency response of any amplifier can be checked by connecting the audio signal generator to the input and ground terminal. Set the volume and tone controls at mid-range. Plug the amplifier into the isolation power transformer. Begin the audio generator at 20 Hz and rotate through the 20 kHz range.

If the audio tapers off at either end of the frequency sound spectrum, in the speaker, either your ears cannot respond to the sound or the amplifier has reached the end of the frequency test. Don’t overdrive the generator output to show clipping on the oscilloscope. A dual-channel AC or audio voltmeter can be used as an indicator in stereo circuits and response tests.

Clip an 8 ohm 100 watt load resistor to the left speaker terminals and check the stereo amplifier frequency response of the left channel. Connect the signal generator to the input terminals of the left channel. Now clip a frequency counter across the load resistor ( FIG. 21). Start at the lower frequency and notice where the meter hand begins a roll off reading.

FREQUENCY COUNTER

FIG. 21. Typical hookup for a frequency response test with frequency counter, ac multimeter or scope as an indicator.

Let’s say that the lower frequency begins to measure at 40 Hz. Rotate the audio generator up past the 20 kHz frequency and notice where the audio signal tapers or rolls oft. If the meter hand falls rapidly at 18 kHz, the typical frequency response measurement is 40 Hz to 18 kHz. Of course, most men cannot hear above 11 kHz, while women can hear above 15 kHz. Follow the manufacturer’s frequency response test if one is found in the service literature.

WOW AND FLUTTER TESTS

The wow and flutter conditions of a cassette player might occur during playback or record modes. The short change of sound might be called flutter, while the longer duration of sound might be a wow condition. The wow and flutter sounds might not be heard by some technicians, while those with better hearing can hear these changes of sound.

Wow and flutter conditions within the tape deck can be located with a test tape and a wow and flutter test instrument. The frequency counter can be used to monitor the test tape during playback modes. Insert a 3 kHz test tape and connect the frequency counter or audio meter to a fixed load at the speaker terminals. Simply notice the deviation in frequency of the counter or meter as the tone test tape is played on the cassette player. A greater deviation in frequency (3 to 10 kHz) of the played test tape indicates a wow or flutter condition.

The wow and flutter test instrument is required in a service establishment that repairs cassette and VCR machines. A crystal-controlled oscillator inside the test instrument provides the most accurate 3kHz and 3.15 kHz frequency for wow and flutter tests. A pre recorded tone test tape or a recorded signal from the wow and flutter test instrument can be used in making wow and flutter tests.

Connect the output of the cassette player to the input terminals of the wow and flutter meter. The oscillator output of wow and flutter meter is connected to the record input terminals of the cassette player. Follow the test instrument and manufacturer’s operation procedures to check the wow and flutter measurements of the cassette player.

FIG. 22. Connect the oscillator output terminals to the record input and output of cassette player to input on wow and flutter meter.

LEVEL ADJUSTMENTS

Audio level adjustments should be made in the amplifier sections that don’t have a balance control in the stereo circuits. Insert a 1 kHz cassette and adjust the VU meters to 0 VU, if a VU meter is included within the cassette player or amplifier. Adjust the right and left level adjustments at the same voltage. Follow the manufacturer’s level procedure if a schematic is handy.

The stereo amplifier with left and right line output jacks can be adjusted for the same voltage reading. Insert a 1 kHz cassette or connect a 1 kHz audio signal to the audio input terminals. Connect a VTVM, DMM or ac meter to the line output jacks. Adjust each level control to read according to the manufacturer’s specifications. Most line output level volt age adjustments are from 400 mV to 1 volt.

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Updated: Saturday, 2014-12-27 1:44 PST