The seven most used pieces of test equipment on the audio service bench are the DMM, FET-VOM, dual-trace scope, semiconductor and capacitance tester (both LCR and ESR), audio generator, and isolation transformer. The DMM and FET-VOM take critical voltage, resistance and current tests, while the scope might be used as waveform and signal indicators. A semiconductor tester checks transistors, diodes, zener diodes, and signal diodes in and out of the circuits. The audio signal or function generator injects waveforms and signals into the defective audio circuits for signal tracing procedures ( FIG. 1). The audio chassis is plugged into the isolation transformer, so as not to damage test equipment or the electronic product.
Of course, some electronic technicians trace out audio circuits with an external audio amp and power supply. The more experienced electronic technicians might require only a few audio test instruments. Several small tools, test equipment and test parts such as speaker load resistors, test speakers, test clip-leads, test discs and cassettes are used to round-up the required audio test equipment. The audio oscillator and frequency counter, distortion, wow and flutter meter might be found on the audio specialist bench.
FIG. 1. The function generator provides a sine and square wave waveform in making audio tests of the amplifier.
THE ANALOG MULTITESTER
The new analog volt ohmmeter (VOM) should have a 20,000 to 50,000 ohms/volt DC sensitivity range. Some large VOM’s have 25 or more ranges of AC/DC volts, dc current, and resistance measurements. The deluxe VOM might have auto-ranging where you select the function and the meter sets the correct range. Other features might include LED range indicators, auto polarity continuity buzzer, milliamperes and micro amp ranges. The continuity buzzer lets you quickly spot problems like damaged connectors, broken wires, and blown fuses.
In the early days, the VOM was the standby test equipment of the electronic industry ( FIG. 2). The Simpson 260 was the work horse of yesterday. Remember, the VOM can load down the circuit being tested. The VOM can still make quick continuity tests and remain the audio indicator in alignment and adjustment procedures. If you don’t have a good VOM, choose the FET-VOM and DMM for greater accuracy.
FIG. 2. The analog (VOM) meter is handy in taking voltage and continuity test in audio circuits.
HIGH IMPEDANCE MULTIMETER
Many of the early audio tests and adjustments were made with the vacuum-tube-voltmeter (VTVM). Although, the VTVM is no longer manufactured, the FET-VOM can take its place. The FET-VOM provides high input impedance with a dual field-effect transistor (FET) circuit. The analog meter hand is easier to read than the digital multimeter in alignment procedures.
The analog FET-multitester might measure up to 1000 volts (AC/DC), 10 amps (DC) and 100 megohms resistance with accuracy and ease. It can also measure down to 20 milli volts (DC) and a few micro amps (DC). The input impedance might be above 10 megohms. Some deluxe models have a special audible continuity check function with a built-in buzzer. The buzzer will sound when the circuit continuity is approximately 300 ohms or less ( FIG. 3).
The FET-VOM might have other features such as a mirrored scale for accurate meter readings, overload protection for both meter and internal circuitry, zero center scale, and polarity reversing switch. Other audio tests might include frequency response (4.5kHz to 10kHz) and decibel (dB) measurements. Set the range selector to one of the ACV positions and it’s best to start at the highest range selection. For absolute dB measurements, the circuit impedance must be 600 ohms with a -10dB to +63dB on some FET-meters.
The FET-VOM can make very accurate resistance, voltage and current tests within the audio circuits. Don’t apply voltage to the test leads when the range selector is in the ohms position. When attempting to identify cathode and anode ends of semiconductor junctions, or the type of transistor (PNP or NPN), the actual polarity of the tester’s voltage is opposite of the tests lead colors. The red lead is the negative source and the black lead is positive. The FET-VOM and DMM test leads upon a diode are just the opposite of the VOM ( FIG. 4). The FET-VOM is ideal in making audio adjustments and alignment procedures.
FIG. 4. Notice the different polarity of test leads when taking a diode measurement with VOM or the FET-VOM and DMM.
FIG. 3. The FET-VOM is a brother to the famous VTVM with high impedance input and much more accurate measurements than the portable VOM.
TODAY’S CHOICE
The portable and bench-top digital multimeter is the most versatile, accurate and used test instrument upon the electronic service bench. The high-tech DMM has just the right combination of features for service technicians, hobbyists and students. The meter might have auto-ranging that lets you select DC volts, AC volts, resistance, DC current and the meter sets the correct range automatically.
A portable DMM can quickly check diodes and transistors on a diode-transistor test. Auto polarity gives you valid readings if you connect the test leads in reverse. These small DMM’s might have manual override, continuity buzzer, data hold to freeze display and audible overload-alert features. A continuity buzzer lets you quick-check fuses, cords, and connections; its audible indication frees your eyes for faster testing.
Today the digital multimeter (DMM) has actually replaced the VOM ( FIG. 5). The B & K TEST-BENCH DMM, model 388-HD, can make most of the electronic tests in servicing audio products. This meter can check transistors, diodes, frequency, current, logic, capacitance, current, and voltage. The diode test can accurately test diodes for open, leaky or shorted conditions. Also, the diode test can check transistor junctions for open, leaky, shorted and high resistance measurements. A separate transistor socket is located above the positive test probe to plug the PNP or NPN transistor for a hFE test.
This DMM has three different frequency ranges: 2 kHz, 20 kHz and 200 kHz. The current range is from 200 uA to 20A in five different ranges. Small capacitors can be checked from 2 nF to 20 uF with a separate CX socket. The ac voltages vary from 200 mV up to 1000 volts. Seven resistance ranges vary from 200 ohms to 2000 megohms. The ac voltage range measures up to 750 volts.
The portable DMM is ideal when taking the different tests upon audio components ( FIG. 6). Besides critical voltage measurements upon diodes and transistors, transistors can be tested with the diode test or in a separate transistor socket. The frequency range can be used to indicate the audio amp frequency range and cassette speed problems with the frequency-counter test.
FIG. 5. The digital multimeter is today’s workhorse and can take accurate voltage, resistance and current measurements in audio circuits.
The cassette tape head azimuth adjustments can be made with the low ac voltage range or frequency-counter tests. Remove one terminal from the circuit when testing unknown capacitors, high-ohm resistors and diodes.
THE DUAL-TRACE SCOPE
The dual-trace scope is ideal in taking waveforms or alignment in audio stereo channels. The scope can also check the stability of the amplifier and hum in the ac power supply. Select an oscilloscope with at least a 40 to 100 MHz bandwidth for audio signal measurements. Some of the features might be dual channel, dual trace, dual time base, delayed sweep, auto triggering, x-y operation, scale illumination, and intensity control ( FIG. 7).
FIG. 7. The oscilloscope provides visual waveforms of weak and distorted circuits.
FIG. 6. The digital multimeter (DMM) can check capacitors, diodes and transistors besides the regular voltage measurements.
The vertical features might include a sensitivity of 2mV/div-5V/div in 11 calibrated steps of 1-2-5 sequence. A bandwidth from DC to 40 Hz or 100 MHz (-3 dB), AC to 10 Hz to 100 MHz (-3 dB) with a rise time less than 3.5 ns. The maximum input voltage between 250 and 400 volts p-p (DC to peak AC) at 1 kHz or less. The input impedance should be 1 megohms with polarity inversion. The different modes might include CH-1, CH-2, ALT, CHOP and ADD.
The horizontal deflection features might include display modes of A, ALT, B, BTRIG’D, and X-Y. Delay line jitter better than 1:10,000 with sweep magnification of 10 times. The sweep time base should be at least .1 m sec/div to .5 sec/div in 20 calibrated steps. A maximum in put voltage should be from 30 to 250V DC to peak AC.
The sine/square wave generator and oscilloscope can locate weak and distorted stages of the electronic audio circuits. The scope is also ideal when making alignment and adjustments procedures. A dual-trace scope can be used to check out the stereo circuits at the same time. Inject an audio signal from generator or test cassette into both stereo channels (FIG. 8). Go from stage to stage and locate the weak sound or distorted circuit. The dual-trace scope can be used in AM/FM/MPX, compact-disc signal tracing, and offset adjustments.
FIG. 8. Both square waveforms are quite normal in each stereo channel of a suspected audio circuit.
ISOLATION TRANSFORMER
The isolation transformer provides isolation from the AC power line when servicing a “hot” chassis. The transformer eliminates a shock hazard and prevents damage to the test equipment, technician, and electronic chassis. An isolating transformer might have a 1:1 turn ratio for isolating equipment from direct connection to the power line.
The variable-isolation transformer can make the intermittent chassis act up by applying a variable power line voltage. Some variable transformers provide a variable AC voltage from 90 to 140 volts. Plug the intermittent audio chassis into the variable transformer and lower the ac line voltage. Often the intermittent will act up on either low or higher then normal-applied AC voltage.
IMPORTANT AUDIO TEST EQUIPMENT
All audio chassis that operates from the power line should be plugged into the isolation transformer before attaching any test equipment ( FIG. 9). If not, the fuse might blow with extensive damage to the silicon diodes within the electronic power supply. Some times expensive test equipment is damaged and the technician can receive a power line shock between the audio chassis and test equipment; it’s time to think. Be very careful out there.
FIG. 9. The electronic chassis should be plugged into the isolation transformer before attaching test equipment.
NOISE GENERATOR
The noise generator is just as effective in RF front-end circuits as the sound circuits. A noise generator produces a noise signal for signal tracing the audio circuits. The hand-held noise generator in the early radio days was to go from the RF to the audio stages to locate a defective circuit or component. The noise generator can quickly locate a defective stage within the AM/FM/MPX receiver circuits.
TONE GENERATOR
The tone or 1 kHz audio generator can be used like the noise generator to quickly locate a dead or weak stage within the audio circuits. The weak audio circuit in a cassette player might result from a packed-oxide tape head, leaky or open transistors and IC, weak battery and defective electrolytic coupling capacitor. A dead stereo channel can be caused by defective transistor or IC, open coupling capacitor, and improper voltage source. A tone generator can be purchased or easily constructed for less than a ten dollar bill.
The tone generator circuit is shown in FIG. 10. The tone generator is constructed around a low voltage LM386 IC. The frequency can be changed from 1 kHz to 10 kHz by switching R1 to 4.7K ohms. The output frequency is lowered (kHz) with R1 at 56K ohms. Although, the frequency might not be on the nose at 1 kHz frequency, it’s close enough for signal injection of sound circuits. The tone generator can be housed in a small metal box.
FIG. 10. The circuits of a 1 kHz portable tone generator.
Select a general-purpose IC PC board so the IC socket can straddle the PC board terminals. Tie each part of the circuit into the correct board holes and solder underneath the board. All parts are mounted on top and hand wired with wire terminals and hookup wire beneath the PC board. LED1 is a pilot light indicator to indicate when the generator is operating. C5 should have a 1000 volt rating to prevent damage to the tone generator, if higher voltages are encountered.
PART LIST:
SEMICONDUCTORS:
- IC - LM 386 low voltage audio amp IC
- LED1 - 2 volt chassis mount LED
CAPACITORS:
- C1, C3, C4 - 0.1 uF 50 volt ceramic capacitors
- C2 - 10 .uF 15 volt ceramic capacitor
- C5 -0.1 p 1000 volt ceramic capacitor
RESISTORS:
- R1 - 56K ohm 1/2 watt fixed resistor
- R2, R5 - 1K ohm 1/2 watt fixed resistor
- R3 - 10K ohm 1/2 watt fixed resistor
- R4 - 10K ohm linear control with SPST switch
SW1 - SPST switch on rear of R4
misc - 276-150A Radio Shack general purpose IC board; 8 pin IC socket, red and black banana jacks, 9 volt battery, 9 volt battery clip, knobs, cabinet base, etc.
IMPORTANT AUDIO TEST EQUIPMENT
SEMICONDUCTOR TESTER
The semiconductor tester should automatically identify transistor leads, NPN or PNP types, leakage test, dynamic gain test, and checks Bi-Polar and FET transistors. A good semiconduction tester should check the transistor in or out of the sound circuits. Some expensive semiconduction testers have an in-circuit “go-no-go” and a good and bad scale. The low- priced transistor-diode tester might not have an analog meter.
The LED transistor and diode tester might have LED’s that indicate a normal, open or leaky transistor or diode. Some LED testers will test transistors in-circuit provided the base biasing resistance is greater than 100 ohms. Pull the power cord from the electronic audio product before testing transistors or diodes in the circuit. Shut off the battery power before taking the transistor tests. Remove one end of suspected diode from the board to make an accurate diode test.
FIG. 11. The LED semiconductor tester checks transistors in and out of the circuit.
CHECKING CAPACITORS
The standard digital-multimeter might include a capacity range from 2 NF to 20 NF in 4 or 5 steps. Choose a capacitance meter that accurately measured the value of any capacitor between 0.1 PF to 2000 uF. The extended range capacity tester might test capacitors from 0.1 pF to 20,000 1 in a different range. Some of these capacity meters test the suspected capacitor in or out of the circuit. Always discharge the electrolytic capacitor before testing. The LCR meter can accurately test capacitors in and out of the circuit ( FIG. 12).
The ohmmeter range can check the fixed capacitor for leakage and charging qualities. Check the capacitor out of the circuit for leakage. Place the two ohmmeter probes on the electrolytic and watch the numbers charge up and discharge. Reverse the test probes and again the good electrolytic will charge up. The numbers will increase until the over range sign appears in the LCD. Rotate the ohmmeter range to 200K ohms when testing a low capacitance of 4.7 or 10 uF to acquire the charging effect with the DMM. The analog FET VOM meter hand will charge up and down with the same tests.
FIG. 12. The LCR meter can check small capacitors
and transistors in the circuit. Shown above is the LCR40 from Peak Electronics
( UK).
A capacitor tester called the is an extremely fast and reliable tester that measures the equivalent series resistance (ESR) on capacitors of 1uF and larger. These accurate tests are made in-circuit, eliminating the need to remove the capacitor for accurate tests.
NOTE: Standard capacitor 'LCR" meters cannot detect any change in ESR, therefore they miss defective capacitors leading to time consuming “tough dog” repairs. Therefore, use an ESR tester -- such as the Atlas ESR Plus (ESR70) made by Peak shown below (it can be used in the circuit.)
http://www.peakelec.co.uk/acatalog/jz_esr70.html
above: Atlas
ESR Plus (ESR70) meter from Peak
THE SIGNAL CHASER
The audio signal tracer is nothing more than an external amplifier that picks up the signal at any spot in the defective amplifier. The audio signal tracer troubleshoots audio circuits from input to the speaker. The commercial signal tracer might include a 1 kHz injection signal. The audio signal chaser might have earphones or a speaker as indicator. The commercial amplifier speakers might be used as a signal chaser.
Check the audio circuits by the number. Start at the volume control and if audio signal is present, proceed towards the speaker. Move to the front end of the audio circuits by checking the output at preamp IC or transistor. The audio signal within a cassette player can be traced with the external amplifier from the tape head to the output speaker ( FIG. 13). Compare the audio signal at any point with the normal audio channel in stereo circuits.
The external amplifier tracer can be made from an audio amplifier kit, external amp or build one yourself. The amplifier should be able to drive a small 8 ohm speaker. An amplifier with 1 to 5 watts is preferred. This amp should be powered by batteries or if operated from the power line should be isolated with a power transformer. A separate volume control should be included to control the increased audio as it’s traced through the circuit.
FIG. 13. Signal trace the audio circuits by the number with the external audio amp. IMPORTANT AUDIO TEST EQUIPMENT; LEFT CHANNEL SPEAKER; RIGHT CHANNEL SPEAKER
AUDIO GENERATOR
The hand-held audio generator is ideal for field and bench service of audio equipment, stereo decks, car stereo and cassette players. The portable generator has a wide range of frequency, sine and squarewave output, sync output, and a continuously variable 20 dB fixed output. The frequency range varies from 20 Hz to 150 kHz in 40 to 50 steps. The sine wave output voltage of 1 .2V and a squarewave of 8V maximum under no load, with a 600 ohm output impedance.
The bench audio generator operates from the ac power line which generates sine and squarewave forms. The frequency range might vary from 10 Hz to 1 MHz in five ranges. The sine wave frequency varies in X1, X10, X100, X1K, and X10K ranges, with a frequency response of 10 Hz to 1-mHz and a + or - .5 dB distortion. The output voltage is around 20 volt peak to peak with no load. The squarewave signal is the same with 10 volt p-p output and no load. These audio signal generators might have external synchronization with an output impedance of 600 ohms.
The audio oscillator test instrument might have a frequency counter in the same cabinet. The features might include low distortion, TTL output, sine and square wave output, 1 MHz frequency counter and external sync input. The frequency range might vary from 10 Hz to 200 kHz in 7 or 8 steps with a standard 600 ohm output impedance. The enclosed frequency counter might have a display of four digits with a reading accuracy of .01% + or - 1 count.
FUNCTION GENERATOR
The function generator generates several different waveforms at a variable frequency ( FIG. 14). The multi wave generator might appear in a 2 MHz, 5 MHz, and 20 MHz sweep function requiring either a standard function generator or a sweep generator. Some sweep generators include a 10 MHz frequency counter. Most sweep generators have a six digit LED display. A function generator might produce a square, triangle, sine, TTL, CMOS, and also pulse, ramp and sweep outputs. The 2-MHz sweep function generator might have a frequency range from .2 Hz to 2 MHz in seven ranges. The frequency ranges are: 2 Hz to 2 Hz at 1,2 Hz to 20 Hz at 10, 2O0Hz to 200 Hz at 100, .2 kHz to 20 kHz at 1K, 2 kHz to 20 kHz at 10K, 20 kHz to 200 kHz at 100K, .2 MHz to 2 MHz at 1 megohm.
FIG. 14. The audio or function generator injects sine and square waveforms into the defective audio circuits.
The frequency counter included within the function sweep generator might have a time base oscillation frequency of 10 MHz. The gate time: 10sec, 1sec, .1sec, and .01sec. A frequency range from 5 Hz to 10 MHz, a maximum input voltage of 42V peak, and an input impedance of 1 megohms (-20 dB), 500K ohm (0dB). The function generator can be used to inject different waveforms within the audio circuits to locate weak or distorted circuits and for audio test procedures.
EXTERNAL POWER SOURCES
The external power supply might provide DC voltage to the auto CD and cassette player, high wattage amplifier and for injecting voltage to eliminate intermittent problems within the electronic chassis. The 1 OA and 1 5A regulated power supply might provide voltage to the car radio and cassette player, while a 30 or 40 amp power supply might be needed to operate those high-powered amp circuits. The 30 amp power supply is ideal for servicing or powering car stereo products.
The 10A regulated power supply has a 13.8 VDC regulated output for most auto radios. The higher amp power supply might have a variable voltage from 1 to 15V DC with a voltage fluctuation of less than 1%. The 40 amp power supply might have a meter to read the output voltage and current drawn by the electronic product.
The variable single, double or triple output DC power supply might have an output voltage from 0 - 1 8V DC, 0 - 30V DC, 0 - 60V DC. A triple output dc supply might have a fixed 5 volts, 0-24 volts, or two 0 -30V DC supplies rated at3 amps. The laboratory VDC power supply has an adjustable output voltage with separate voltage and current meters. This power supply might also have fine and coarse voltage adjustments for accurate voltage settings ( FIG. 15). Connect the external voltage source for a steady and accurate voltage in making alignment and adjustments upon battery operated products.
IMPORTANT AUDIO TEST EQUIPMENT
FIG. 15. The external dc power supply provides a steady voltage to amplifiers for the auto and battery operated products upon the service bench.
TEST SPEAKERS
The test speaker should have the correct output impedance connected to the amplifier for a perfect match. Since the common speaker impedance is 8 ohms, just about any size will do. The test speakers should be mounted into the service bench or speaker enclosures. A pair of compact-shelf speakers are ideal where they can be moved and clipped to the audio chassis. A 10 or 12 inch pair of woofer speakers that have an average power of 50/ 100 watts might be required when servicing high-powered stereo amplifiers.
Most speaker enclosures have RCA-type plugs to plug into the amplifier. Some audio chassis have lug to lug, pin connectors, or tinned wire ends ( FIG. 16). A pair of alligator clips on each speaker might make the quick connections. Make sure the connections are good and tight. Check the volume control setting before firing up the amplifier. Don’t accidentally apply too much power to the test speakers so the voice coils are damaged at once.
FIG. 16. The output of the high-powered AM/FM/MPX receiver might have lug, plug or tinned wire speaker connectors.
LOAD IT DOWN
Attach the speaker before attempting to service any audio amplifier. The amplifier must have a load attached before the volume control is rotated or the amplifier output circuits can be damaged. High-powered dummy loads are required for servicing PA systems and high-powered auto or car amps. Sometimes the audio output circuits become defective and place a DC voltage upon the speakers voice coil destroying the speaker. Always check the speaker terminals or jacks for a DC voltage before attaching any PM speaker.
Since many of the latest audio amplifiers have high wattage output systems, a test-speaker dummy load must be able to withstand up to 100 watts or more. Several 50 or 100 watt resistors can be placed in series or paralleled to acquire the correct resistance and watt age. For instance, two 50 watt 8 ohm resistors can be paralleled to achieve a 100 watt resistance to match a 4 ohm impedance. Simply connect two 150 watt at 4 ohms in series to match an 8 ohm 150 watt dummy load ( FIG. 17). A variable load resistor is de signed for testing non-inductive load for bench testing amplifiers from 0-8 ohm at 90 watts and.0-18 ohms at 225 watts. Adjust the wiper ring for correct resistance upon the high wattage load resistor.
FIG. 17. The various connections of power resistors for correct wattage and impedance dummy load resistors.
Since the full wattage of a high-powered amplifier is never turned full on while being repaired, a bank of 20 or 25 watt resistors can be added for a 100 watt output. Often the electronic technician keeps the volume turned down low as possible while trying to locate the weak or distorted circuit. A 100 to 250 watt 8 ohm dummy load resistor on each stereo channel can provide adequate loading of the most powerful amplifiers.
TEST CASSETTES, TEST DISCS and TEST SIGNAL FILES (WAV, MP3, etc.)
There are several different cassette alignment tapes available for azimuth, frequency response, and speed adjustments for the micro cassette and standard cassette players. Besides these adjustments, the cassettes can be used for signal tracing the various audio circuits. A 15 minute micro cassette alignment test cassette consists of a 1 kHz audio tone recorded at a level of -4dB can be used for azimuth tape head adjustment. The 30 minute alignment cassette tapes include a full 20-20,000 Hz sweeps, as well as 14 other stereo tones recorded at -10dB. These alignment cassettes might include three different cassettes at normal bias, high bias, and metal bias types.
IMPORTANT AUDIO TEST EQUIPMENT
The deluxe BFC41 reference and frequency response cassette combines in one cassette all reference levels needed to set up and balance cassette recorders and decks. Part 1 provides a reference level of 315 Hz (0dB), and a 315 Hz (DE10dB). Part 2 provides the azimuth alignment of kHz, 8 kHz, and 10 kHz (DE10dB). Part B is the speed adjustment cassette with frequency at 50 Hz, 3.15 kHz, and 3 kHz (DE15dB). Part 4 provides a frequency response test of 315 Hz, 31.5 Hz, 40 Hz, 63 Hz, 125 Hz, 250Hz, 500 Hz, 1 kHz, 4 kHz, 6.3 kHz, 8 kHz, 10 kHz, 12.5 kHz, 14 kHz, 16 kHz, 18 kHz, 315 Hz, (DE20dB) at fifteen second recording of each frequency. Part 1, Part 2, and Part 3 have a 30, 90, 120 second recording, respectively.
The BSC21 tape path mirror cassette makes it quick and easy to reliably check the way the tape moves through the recorder. Part 1 is a see-through lead-in path for optical check-up of heads and tape guiding parts. Part 2 is a normal FE tape running for approximately 10 minutes.
The DMC100 torque meter cassette, provides accurate torque meter measurement. The special feature of this cassette is a 5-sided display. By this method it’s possible to read off either side of the cassette without having to remove cassette. This is particularly useful with the car cassette recorders where only one side of the cassette can be seen. This cassette measures the torque at normal, fast forward and rewind speed. Calibration is in gcm from 0 to 100.
The WFC1 wow and flutter cassette is recorded on both sides of the tape with 3000 and 3150 Hz respectively and so is fully compatible with all wow and flutter meters. Although these cassette alignment tapes are quite expensive compared to the 1 kHz at $19.95 and the audio alignment cassettes at $24.95, they are required if you are servicing many different cassette players. These alignment cassettes can be purchased at: MCM Electronics
TAPE HEAD CLEANERS
There are many different kinds of cassette tape head cleaners on the market. The automatic cassette cleaner has a gear driven cleaner with one wet and one dry pad that thoroughly cleans the tape head. The Recoton deluxe head cleaning system uses a wet process cleaning system to maximize audio quality. A dishwasher System 11Th head cleaner cleans heads and prevents “eaten” tapes. The cleaner removes sticky residue from capstans and pinch rollers. A total cassette care kit might include a combination demagnetizer and wet process head cleaner for restoring audio-quality playback and recording.
DEMAGNETIZE THAT TAPE HEAD
The demagnetizer and head cleaner might operate from a 1.5 drycell (SR-44, S-76, RU 76, MS-76). After long hours of tape playing the audio heads can become magnetized causing a loss of high-end frequency response and increased distortion, resulting in over all poor performance ( FIG. 18). Insert cassette demagnetizer with front face up. Place machine in play mode. After a few seconds, eject demagnetizer. These head cleaners and demagnetizers can be purchased at many different hardware and mall stores.
FIG. 18. A self-powered tape head cleaner and demagnetizer cassette.
The electronic technician often cleans up the cassette tape head before attempting to service the cassette player. Use either a tape head cleaning kit or felt-tip cleaning sticks and alcohol. An old toothbrush with alcohol can remove the oxide packed tape head. Then, clean up with cleaning pads or sticks. Clean up all tape guides and paths with the top cover removed. Before buttoning up the cassette player, demagnetize the tape head with cassette or head demagnetizer with a curved tip to get at those hard to reach tape heads.
ADDITIONAL TEST EQUIPMENT
KEEP COUNTING:
The frequency counter might be included in a hand-held battery operated test instrument or within the digital multimeter. The B&k 388-HO DMM has a 2 kHz, 20 kHz and 200 kHz frequency range. A frequency counter might be included within a function generator or audio oscillator test instrument. The frequency counter counts signal cycles or pulses in a frequency measurement. The frequency counter can check frequency, speed, head alignment, and overall frequency response of cassette players and amplifiers.
The 100 MHz frequency counter might have selectable attenuation, low pass filter, select able kHz/MHz, terminated BNC input connector, gate indicator light, and eight-digit LED display. A counter might have a frequency range from 5 Hz to 100 MHz with a + or - time base accuracy + or - 1 count. The time base is a crystal controlled oscillator. The frequency counter might have a low pass filter of 100 kHz and a 1 megohms impedance.
FIG. 19. The frequency counter can check speed, head alignment and frequency response in the cassette player.
DISTORTION METER
The distortion meter is designed to measure total distortion at any frequency between 20 Hz and 20 kHz. The meter might also act as a level meter allowing simultaneous measurements of both signal levels and distortion. The distortion meter is usually connected to the line or amplifier output of the amplifier ( Fig. 20)
FIG. 20. The distortion meter can check the output distortion of the audio amplifier circuits. Scope --> Distortion Meter
The distortion meter might have auto ranging signal level and a distortion measurement from 0.1% to 100%. The meter may measure signal levels from 1 mV RMS (root measure square) to 300V in 12 ranges. X and V outputs allow the technician to observe the wave forms of the input signal and the total harmonics on an oscilloscope. The distortion measurement has an input level of 100 mV RMS to 300V RMS with residual distortion, including hum/noise less than .03%. The signal voltage frequency response of 20 Hz to 200 kHz with an output impedance of 600 ohms.
WOW AND FLUTTER METER
The wow and flutter meter is ideal when servicing any record/playback equipment such as cassette players, reel to reel, VCR’s and VTR’s. The meter should be compatible with JIS, NAD, CCIR, and DIN standards. The wow and flutter meter might have a four digit frequency counter with a center frequency of 3 kHz and 3.5 kHz. The tester should have a scope and recorder output for observation and recording. The input level should be from 5 mV to 10Vrms.
The wow and flutter center frequency should be 3 kHz (CCIR, NAB, JIS) with a 3.15 kHz (DIN). The wow and flutter tape speed range is from 2.7 kHz to 3.5 kHz. The frequency range of the frequency counter might be from 10 Hz to 999.9 kHz. The distortion factor of output drift and outputs to the scope and recorder should be less than 2%.
A FINAL NOTE
The test instrument is no better than the electronic technician that takes the critical measurements. Some electronic technicians with a great deal of knowledge and experience use only a few test instruments everyday. Some repairmen only use one or two per day. The most often used test equipment is the DMM, semiconductor, and oscilloscope. The required test equipment depends upon what part of the electronic entertainment field the technician wants to specialize in. Although, the wow and flutter meter and distortion test instruments are quite expensive, the sound specialist technician might be required to use them in servicing high-powered and expensive audio equipment.
. ===