Fullscreen HTML5Med Res (??MB)HTML4

Don't spend big dollars getting your radiocassette player fixed. You can do the job yourself. The most common cassette player problems are found in this article. By HOMER L. DAVIDSON The cassette music comes to a sudden halt with no rewind or fast-forward movement. So you push the FM button only to hear distorted music as you move into another lane on your way to work. In disgust, you flip the radio to AM to get the local traffic report but the radio still sounds dreadful. You try to adjust the volume control but that only produces loud scratching noises and the sound cuts in and out. Nothing is going right this morning so you eject the cassette and some of the tape is left in the machine. What a mess! Incidents like that happen every day to someone but "Why me?" you exclaim. You want to kick it or hit it with a hammer except both are out of reach. Enough is enough - this weekend, you are going to whip the tape player out and work it over. Yes, just about anyone with nimble fingers and a little patience can keep an auto cassette player in tip-top shape (with a little help from this article). Where to start Like the electronic technician, you must consider the symptoms, isolate the probable section and then locate the component(s) on the schematic diagram and in the player itself. You should have a schematic diagram to locate the suspected components. Check the service instructions as a small diagram may come 6 SILICON CHIP in the package. Of course, you may need a magnifying glass to see all the wiring close up. If you have a few tools and an analog or digital multimeter, you are in business. Cleaning up the tape head may solve distorted or RF FM FM FET RF RF AM AM CONV TO IF TO AM DETECTOR AM +8.9V Fig.I: check the AM-FM slide or pushbutton switch with erratic AM or FM. It may simply need cleaning. Often, the AM-FM switch provides DC voltage to the front-end components. DEFECTIVE MOTOR TAPE REEL DIRTY ANO SLUGISH INTERMITTENT BOARD CONNECTIONS PRESSURE KEEPER BENT DIRTY CAPSTAN LEADS PRESSURE ROLLER DIRTY BROKEN TAPEHEAD LEADS This photo shows the parts that can cause problems in a typical auto cassette player. You should check out each of the parts in turn as described in the article. weak cassette music. Wiping off the motor belt and pinch roller may cure some speed problems. No rotation may result from a broken belt or defective cassette motor. Intermittent cassette music may be caused by broken tape head connections. Distorted and weak music may be solved by making accurate voltage measurements around the suspected output transistors or IC components. But hang on a bit - let's take things one step at a time before we fall flat on our faces. We'll look at each of the common problems and see how they are solved. A good clean up of the tape head and surrounding parts may save a few tapes and cure many tape speed problems. Be sure to remove all the oxide from the small gap areas of the tape head. down movement of the tape. To cure this problem, remove the dry capstan/flywheel, clean up the bearing area, and apply a dash of light phono grease or vaseline. Often, the motor belt and capstan are found under the cassette assembly. You will have to remove the bottom cover to clean up the moving components. In other cases, the whole cassette assembly must be removed to get at the motor belt. Always mark down any wire leads or components you must remove to get the assembly free for clean up. Erratic FM If the AM or FM reception appears intermittent or erratic after switching from one to the other, suspect a dirty AM-FM switch. In some radio-cassette players the selection is made with a pushbutton or slide type switch, while in the later digital models the bands may Clean up time Sometimes a good clean up with a cotton bud stick and alcohol can solve a lot of problems. First, clean off the front of the tape head to prevent weak, dead or distorted music. Get all the oxide off the small gap areas of the tape head. Those areas are very small and can easily be plugged with stubborn oxide dust. Next, wipe off the motor drive belt with a clean cloth moistened with alcohol. Inspect the rubber belt for loose or cracked areas. Clean off the motor pulley and capstan flywheel with a cotton bud. If everything looks OK, whirl the flywheel with your fingers and check if it rotates freely. Incidentally, when we mention alcohol in this article, we're talking about isopropyl alcohol which can be obtained from your local pharmacist. As a second preference, you could use methylated spirits (preferably obtained from your pharmacist) although it does tend to leave a residue. Sometimes a gummed up flywheel bearing slows Clean up that noisy volume control with cleaning fluid and an extension tube poked down inside the control body. Rotate the control shaft after spraying to clean up the dirty contacts. JANUARY 1989 7 S2 h.__TO RADIO _}. LEOl .,. ~ LE + 220l .,. Fig.2: check the cassette switch and isolation resistor when no voltage is found at the motor terminals. Replace the motor if tape movement is erratic or intermittent. Dead A broken or dislodged belt, or a defective cassette motor, can halt tape rotation. Check the price of a new motor before ordering one to make sure it's worth it. If a replacement motor is very expensive, you may be better off buying a new player. be changed by pushing in the tuning knob. Often, the AM-FM switch applies voltage to the respective circuits (see Fig.1). Whatever switching system is used, the switch contacts must be cleaned. Often, you can quickly tell if the switch is dirty by moving the switch button after switching to the correct program. If the switch contacts are dirty, the sound will cut in and out. To clean the switch contacts, spray cleaning fluid (eg, WD40) down inside the AM-FM switch assembly. In most cases, you will have to remove the top cover to get at the switch assembly. After spraying, work the switch back and forth to help clean up the contacts. While you're at it, clean up all the other pushbuttons and switches located within the player. Keep the spray nozzle away from the motor belt and capstan/flywheel assembly; the chemicals in the spray can damage them. Rough volume control While you have the cleaning spray can out, give the volume control a squirt or two. After the radio has been used for several years the volume control seems to wear and get dirty at the comfortable volume setting. Sometimes the sound may jump up and down with a noisy control. The life of the control may be extended for many months or years with a good clean up. Remove the top cover and place the plastic nozzle from the spray can into the opening of the control where the contacts come out. Note that the volume control cannot be cleaned from the outside of the radio, by squirting cleaning fluid into the shaft area. Give each section of the volume control a couple of squirts, then rotate the volume-control shaft. Do that at least twice for each control section. Remember, there are two volume-control sections for the stereo output channels. Likewise, clean up the balance and tone controls while you are at it. If that doesn't do the job, replace the component with an equivalent part. 8 SILICON CHIP no tape action Improper or no tape rotation may be caused by a stretched motor belt, a broken belt or a dead motor. If the belt is loose, it can come off when the tape player is started up. Or, after several years use, the belt can crack and break. Usually, the broken belt is found loose inside the player mechanism. Slow tape speed may also be caused by the presence of oil on the belt. Remove the motor drive belt and clean both sides with alcohol and cloth. One method the electronic technician applies in checking out a loose belt is to hold the capstan and belt while the motor is rotating. If the motor pulley turns inside the belt, the belt is too loose. If the motor ceases rotation, you may assume the belt is not slipping. A sure sign that the belt is slipping is a shiny surface on the motor pulley. If the motor appears to be dead, it may be defective or there may be no voltage at the motor terminals. Measure the voltage across the motor terminals (12-14 volts). If the voltage on the motor terminals is incorrect, check the tape motor switch, the isolation resistor and the power supply components (see Fig.2). If the correct voltage is found and the motor refuses to rotate, check for continuity between the motor terminals using your multimeter set to the low-ohm scale. All resistance measurements should be made without voltage applied to the player. The reading should be a Critical voltage and resistance measurements on transistor and IC components may indicate a leaky or open circuit component. Cl - 51+ LEFT TAPE HEAD I ..._CHECK FDR BROKEN TAPE HEAD lOOpFI CONNECTIONS ./ .,. 21 ICl PREAMP .,__ _ __. .,. Fig.3: intermittent music may result from broken tapehead leads. You may find individual small wires or two shielded cables connected to the tape head. They fray from head movement. fraction of an ohm and should vary only slightly as you rotate the motor's shaft. In some cases, the motor may operate intermittently. When power is applied, the player may start up but at other times it's dead. To investigate this problem, remove the motor belt, apply power to the player and spin the motor pulley. The motor will either take off or pull slightly as it is rotated. Replace the motor if it plays normally after rotating the motor pulley or is intermittent. It probably has open-circuit commutator sections. Intermittent tape head Suspect broken wire connections to the tape head if the sound is intermittent. The sound may cut in and out on either or both channels. Often, one channel may only have a hissing sound while the other channel is normal. A loud rushing noise indicates that the tape head is open in that channel. Check the tape head for broken wire connections (Fig.3). You may find separate wires or two shielded cable wires to the tape head. Double check each soldered connection. The outside shield may be used as the common return wire. Solder all wires to the head using a low wattage soldering iron. Make sure no flexible wire ends touch the adjoining connection. Improper rewind Improper rewind or fast forward may result from oil or residue on the motor drive belt. Poor pressure of the idler wheel can also cause improper rewind and fast forward. Check the idler wheel for sluggish operation or an uneven rubber drive area. Clean off the motor pulley, belt and flywheel for slow or uneven fast forward. A good clean up and lubrication of the idler pulley and motor drive areas solves most erratic or slow rewind and fast forward operations. Clean off each spindle or turntable drive pulley with alcohol and a cotton bud. A drop of light oil on each pulley bearing may help. Do not over lubricate or let oil drip on the rubber pulleys or drive belt. minutes with tape spilling out, the tape may wind around the pinch roller and capstan drive shaft. If enough tape spills out, it's difficult to remove the ejected cassette. Sometimes the take-up reel will stop for a moment and then start up again, spilling out tape. If this happens, check the take-up reel drive pulley and belt. Clean up the drive pulley and belt with alcohol or replace the belt if it's loose. You should also remove the take-up spindle and inspect the bearing for gummed up grease or dry areas. A drop of light oil on the spindle bearing is enough. Clean off any excess oxide on the capstan drive and all tape guides. Do not overlook a defective cassette as the cause of tape spillage. Try another one. Critical voltage tests Accurate voltage and resistance measurements around the motor, transistors and IC components will often quickly track down any faults. Suspect the power supply or a leaky (or shorted) component if low or no voltage is found on the supply pins of a device. For example, low voltage on the collector terminal of a transistor may indicate a leaky component which should be replaced. A transistor may also be defective if there is no bias voltage between its base and emitter terminals. An open transistor is indicated if there is no voltage across its emitter resistor. Likewise, low voltage on the supply pin of an IC may indicate a faulty IC or an incorrectly functioning power supply. Often, comparing voltage and resistance measurements between the two audio channels will help to locate a defective component. Accurate resistance measurements from the terminal connections of transistors and ICs to ground can also help isolate a defective stage. Distortion in one channel Since most distorted sound problems are found in the audio output stages, try to locate the suspected +12.1v------------~ 2700 470 _ _ _ _ __.-.:.11+ F---ro SPEAKER 1 Spits out tape Spilling out of tape may be caused by an erratic or stopped take-up reel assembly, or by an uneven pinch roller. Check the pinch roller for sticky or packed oxide. It's usually a good idea to clean off the rubber pinch roller when cleaning up the tape head. Inspect the pinch roller for excessive tape down around the bearing support. If the player operates for a few Fig.4: after locating a leaky transistor, check each bias resistor for correct value. Make sure both transistors test OK and check the replacement before installation. JANUARY 1989 9 TAPE DECK TUNER ASSEMBLY MOTOR RADIO BOARD VOLUME CONTROL AND PREAMP BOARD OUTPUT SECTION This photograph shows the general layout inside a typical radio-cassette player. The output transistors or audio power ICs will be mounted on a heatsink or bolted to the chassis. output transistors or audio power IC on the chassis. These components will either be bolted to the metal chassis or to separate heatsinks. Note that an audio power IC stage may be part of a single package containing a stereo pair or may be a separate component. First, take voltage measurements on each transistor terminal. Be sure to write them down on the schematic with a pencil. Determine from the voltage measurements if either of the output transistors is open circuit or leaky. Remove any suspect transistors and test them in a transistor checker. Most DMMs these days have an in-built transistor test function. Double check all the bias resistors while the transistors are out of the circuit. Often, with a leaky output transistor, you will find a burned bias resistor. You should also take accurate resistance measurements of each bias diode. A leaky bias diode may produce slight distortion in one of the audio channels. Do not overlook an open-circuit speaker coupling capacitor as the cause of dead or really weak audio. Dead channel A fault in a dead audio channel is usually easy to locate. If both channels are dead, first check the power supply output. If this is OK, check the voltages on the supply pins of the audio power ICs (or on the collectors of the output transistors). A low voltage on the supply terminal may indicate a leaky IC or power transistor. If only one channel is faulty, compare its voltage readings with the good channel. In some circuits, you will find a preamp driver IC ahead of the output IC so be sure to check this as well. Sometimes, you will find very little difference in 10 SILICON CHIP , CAPSTA N FLYWHEELS Rapid changes in the direction of the cassette player may be caused by dirty contacts on the prongs and commutator. Clean off both with alcohol and a cleaning stick. voltage measurements between the two channels, even though one IC is defective. Before replacing a suspect IC, measure the resistance from each terminal to ground. Compare those measurements with the functioning audio output IC. Sometimes the meter readout will keep changing, which could indicate charging or discharging of a capacitor in the circuit. Leave the probe set on the same terminal until the reading steadies. If the resistance measurement is way off from the good channel, then the IC or an associated component is open or leaky. Changes direction Suspect a loose belt or dirty commutator when the cassette player rapidly keeps changing directions. If the belt is OK, look for a two or three-prong component placed on a rotating commutator, on top or underneath the chassis. The long prongs are silverplated and eventually become corroded. You can often restore correct operation by cleaning the prongs (or contacts) and the rotating commutator with alcohol. Conclusion Keeping your auto cassette player in tip-top shape may not be as difficult as you imagine. Of course, you may not be able to repair all the faults that can occur in a cassette player. But if you can only do the clean up procedure, you will have saved quite a few dollars. So next time that auto cassette player gives up, give it your best shot. You might just be able to nurse it back to health and save a few dollars. ~ This article first appeared in Hands-On Electronics, USA (September 1988); reprinted with permission.