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By HOMER L. DAVIDSON NEW LIFE FOR RADIO-CASSE'I*I'E PLAYERS Ghetto-blas.ters and portable cassette players can sound awful after a few years. Here's how to bring back the good sound. Everything with moving parts runs down with age. When it comes to electronic equipment, the sound gets fuzzy and distorted, power drops off and so on. After a while you might be considering dropping the whole lot in the bin and then lashing out on a new one. But it doesn't have to be. You can resurrect most portable audio gear without being a servicing genius. Of all the gear that suffers the ravages of time and use, the portable "ghetto-blaster" suffers the most because it's always in harm's way. If it's not getting bounced, dropped, or fried in the boot of a car, it's getting beach sand in the tape drive and rain in the speakers. A few simple adjustments and repairs are often all 6 SILICON CHIP it takes to make sluggish portables come back to life. And keep in mind that a few simple repairs early enough can prevent an expensive repair later on. With the cost of servicing the way it is nowadays, it is better to do preventative maintenance now rather than having to discard it later on because a repair job is too expensive to be worthwhile. Here are a few easy-to-make repairs that can be done by just about anyone who can tell one end of a soldering iron from the other. Dead - Nothing Perhaps the most common fault in a portable player is that it will appear to be dead when you turn it on. Inspect the dial drum (4) and the plastic pulleys (1, 2 3) if tuning problems are encountered. If necessary, ' restring the dial-cord assembly. Make sure that the dial pointer moves in the correct direction as you rotate the tuning knob. Check the power supply's silicon diodes when there's no sound, or no tape movement. When tested with an ohmmeter, a leaky or shorted diode will indicate a low resistance measurement in both directions. This is often after a period of weeks or days when it hasn't been used. This is where your powers as a detective come into play. To determine if a portable stereo cassette player is dead only when connected to the mains power, try switching over to battery operation. Suspect a defective mains cord or plug if the unit operates on batteries but not from the mains power. Try another cord, if handy. Sometimes the power cord of your electric shaver will fit the two-pin mains socket on the player. Don't force it though as you may damage the socket. Check the continuity of the mains cord by testing it with the ohmmeter function of your analog or digital multimeter. If the cord tests OK, check the mains onoff switch. Often these are slide switches of dubious quality and their contacts become sloppy after a few years. If the switch is OK check the silicon diodes used in the power supply. Look for breaks or burn marks. You may find two or four diodes wired as a full-wave or bridge rectifier circuit. On a normal diode a low resistance should be found in only one direction. If 'the diode also shows a reading in the other direction (ie, with the test leads reversed), remove one end from the printed circuit board and repeat the test. If it still shows a reading in the reverse direction, the diode is leaky and should be replaced. the battery compartment and terminals, and from the printed circuit board. It is important to remove every trace of electrolyte because it really can cause serious corrosion damage. If this corrosion is allowed to go too far, the player will have to be tossed out. Clean up each battery contact with emery paper until they are sparkling clean. Before installing new batteries, polish their end caps to make sure they are thoroughly clean. Make sure that the polarity of each battery is correct, as you insert it. If the player doesn't operate with a new set of batteries, check the total voltage at the battery terminals with your DMM when the batteries are under load - ie, when the radio or tape player is turned on. If the batteries are new and the voltage is seriously down on what you'd expect, you may have a more serious fault in the player. If the player works but doesn't sound brilliant, Dead When Battery-Powered Make sure that the radio plays when mainspowered before checking for poor batteries. One dead or several weak batteries can prevent proper operation; the sound may distort or the tape may run at a lower-than-normal speed. Inspect the battery terminals for dirty or corroded contact areas. Sometimes, when the batteries are left in the player for a long time they will leak and seriously corrode the battery terminals. If the batteries have leaked, thoroughly clean all traces of battery electrolyte from A dirty play/record or function switch can cause motorboating and erratic or intermittent operation. Spray contact-cleaning fluid inside the switch area and work the switch back and forth a few times to clean the contacts. MARCH 1988 7 around the pulleys and back to the drum (using masking tape to hold the dial cord on the pulleys until finished). Attach the free end of the cord to the small spring sticking out of the drum. Then pull on the cord to slightly stretch the spring before securing the cord with a knot. Place a dab of glue on each end of the dial cord to prevent it from unraveling after you're certain that the drum is rotating in the right direction. Erratic Operation A loud popping noise, motorboating, or intermittent or erratic operation may be caused by a dirty function switch. A loud squealing noise may be heard when in the record or play mode of a cassette recorder if the function switch is dirty. To clean the switch, spray contact-cleaning fluid on the switch's contacts, then move the switch back and forth a few times to clean the contacts. Do the same with other dirty switch contacts, or a dirty (noisy) volume control. Slow speeds can be caused by a dry or worn rubber pressure roller (3), gummed spindles or drive motor (4, 5, 6), very dirty heads (1, 2), or a dry or gummed counter (7). maybe the batteries are due for replacement. Most portables will perform reasonably well provided the battery voltage is at least 75% of the nominal value. For example, if your portable has four 1.5V cells giving 6V, you can expect it to give reasonable sound down to about 4.5 volts. Below that it probably will still work but it will sound yuck. Replace the batteries. While on the subject of batteries, it is a good idea to use alkaline cells instead of carbon zinc batteries. Alkaline cells give longer life and their generally lower internal impedance can help the portable's amplifiers to deliver better sound quality. Keep The Dial Cord On Track Check for a broken dial cord, or a dial cord that's slipped off a pulley, when stations can't be tuned in or the dial pointer won't move. Remove the back or front cover to get at the dial-cord assembly. Most of the larger portable players can be inspected internally by removing several Phillips-head screws located in the back cover. Inspect the dial cord for a break or slippage. Sometimes the plastic dial-cord pulleys break loose from their plastic bearings and allow the dial cord to lie loose. Repair the pulley's plastic bearing by substituting a metal screw (use a small screw that fits inside the pulley area). To do that, first make a sketch of the dial-cord stringing, then remove the dial cord. While holding the damaged pulley in the correct position, apply heat from a soldering iron. When the plastic softens, press the screw into the plastic and let the assembly cool. Then restring the dial cord. To restring a broken dial cord, draw a rough sketch showing where the cord passes around each pulley and the drum that's attached to the tuning capacitor. Select a piece of dial cord 30cm longer than necessary and tie one end to the drum. Then route the cord 8 SILICON CHIP Slow Speeds Incorrect tape speed, or no tape movement at all, can be caused by tape that's wrapped itself around the , capstan drive shaft, or an old, dry capstan pressure roller, or a defective motor. Inspect for tape wrappage around the pressure roller or its bearing. Also inspect the capstan by rotating it with your fingers. Notice if the capstan/flywheel seems to drag, indicating dry or gummed bearings. Suspect a defective motor if the capstan appears to be free. Rotate the motor pulley with the cassette player turned on. Sometimes, a dead motor starts if this is done, indicating that its operation is intermittent. Measure the voltage at the motor's power supply terminals if the motor appears to be dead. No voltage indicates poor wire connections or a dirty off-on switch. If the motor proves defective, replace it with a unit having the exact same part number. A Good Cleaning A dirty tape head may produce weak volume, or a A dirty tape head can cause weak sound, distortion, or a noise in one or both channels. Check the erase head for excess oxide that may prevent full erasure of a previous recording. dead or noisy channel. Use a cotton bud saturated in alcohol to clean the record/replay head, erase head, capstan, and pressure roller. Deposited tape oxide appears brown or shiny-black in colour, so remove anything that resembles those colours from the pressure roller and tape-head surfaces. (Those parts usually can be cleaned through the front loading area). Clean off the oxide each week if the portable is operated continuously. A cleaning cassette can also be used to help keep the tape path clean. When the plastic cover is removed to make other repairs, clean up all oxide from the chassis and mechanical areas with a cleaning stick soaked in alcohol. A Loud Rushing Noise Suspect a broken tape-head lead, its connection, or an open tape head when a loud rushing noise is heard on either channel of the cassette player. Often, the tape-head wires will break off right at the connections. After you make the repair, make sure the tapehead cable is flexible and moves freely with the tape head. Check the tape-head windings for continuity by switching your DMM to the 2k-ohm range. Compare the measurements with the resistance readings of a tape head known to he in good condition. Usually, a stereo tape head will have four connections for two separate windings. Compare both stereo windings. They should be quite close in resistance. An infinite resistance reading indicates an open winding. A very low or zero resistance measurement may indicate leaky or shorted turns. Check the resistance between the head's high terminal and the chassis ground because it's possible to find leakage between the metal shell and a tape-head winding. Push or pull the tape-head terminals with the DMM leads attached. Sometimes an intermittent connection will show up when you do this. The continuity of the erase head may be tested in the same manner. Tape Spillout A loud, rough noise may be produced by an open-circuit tape head (1, 2) winding or because the wires have broken loose from the tape head terminals. Turn the volume up to determine if the noise is in one or both speakers. Often the dead or defective channel is indicated by the VU meters (if so equipped). Recordings that sound jumbled can be caused by a defective or dirty erase head. First, remove (clean) the oxide from the erase head and then try making another recording. Suspect an open erase head or circuit if the previous recording isn't erased. Make sure that you check the wiring connections at the rear of the erase head. Checking The Record/Play Head The record/play tape head can be checked with a DMM. First, inspect the tape head's connections, then make a low-ohm continuity test of the tape head. Tape drag or spillout can be caused by a dirty or worn pressure roller, an erratic or slow take-up reel, or a defective cassette. Most tape spillout problems are caused by erratic operation or non-rotation of the take-up reel. While drag causes wow and flutter - or gurgles [as if the tape were playing under water) spillage allows the tape to come out of the cassette and foul any of the rotating mechanisms. Whatever it gets into, the tape outside the cassette gets damaged permanently. Often, erratic operation is caused by dirt that has become sticky over a period of time. Clean up both reel spindles with alcohol and a cotton bud. Also, check the hub for gummed or dry bearings. To inspect the bearing area, remove the "C" washer at the top and pull the hub off. A single drop of oil on the bearing might help. Also, notice whether the take-up reel rotates smoothly. Other checks In some models, a spider ring inside the plastic turntable is rotated to a higher speed to apply more pressure to the take-up reel. Rotate the spider ring for greater pressure and check the take-up adjustment. Don't forget to check the rubber pressure roller for wear, or for any sticky substance that can cause tape MARCH 1988 9 spillout. A binding tape-counter assembly can cause tape drag (slower speed). Most important, don't assume that the cassette's shell is good because it looks good. A slight warp, unnoticed by a quick glance, can cause tape drag, spillout, or total lock-up. Intermittent AM or FM Go directly to the AM-FM selector switch when either band is erratic or intermittent. Often, a dirty AM-FM selector switch prevents FM stations from being tuned. Locate the switch and spray contactcleaner fluid inside the contact area. Spray into the end area of a slide-type band switch. Move the switch back and forth to work the cleaner into the contacts. Also, inspect the switch's terminal wires for broken or poorly-soldered connections. Intermittent Sound Intermittent sound can be caused by a defective speaker, earphone jack, or amplifier section. First, check for the intermittent sound on both radio and tape operation. If the sound is intermittent on both, suspect a defective speaker or amplifier. Connect an external speaker to determine if the radio's own speaker is intermittent. Suspect a defective earphone or jack cable when the sound is intermittent only for earphone operation. Flex the earphone cable to test for loose or broken wires. Inspect the male plug to check for possible poor contacts or broken internal connections. Check the female jack for poor wire connections. A bent, dirty, or shorting earphone jack can prevent speaker operation. Distorted Sound Weak and distorted sound may result from a faulty Tape pulling or spillout may be caused by a jammed or slow take-up reel or turntable. You should also suspect a broken belt or a drive pulley that isn't engaging the take-up reel. 10 SILICON CHIP If the sound is noisy or distorted, check for foreign objects poked into the speaker's cone (1 and 2). Also, check the speaker's terminals for poorly soldered connections. Always replace a defective speaker with one that's the same size and impedance. audio power amplifier integrated circuit or the speaker. Determine if distortion is only from one speaker channel. Inspect the body of the integrated circuit for indications of overheating. Keep in mind that a problem in the audio power amplifier can disable the entire audio section because the preamplifier, booster and output stage might be part of a single integrated circuit. Comparison voltage measurements can isolate a leaky or open power amplifier IC. If the radio or tape player uses two of the same kind of integrated circuit, measure the voltage on the same terminals of similar devices for comparative voltage measurements. If the voltages vary by a few volts or more, suspect a faulty IC. A channel may appear dead, weak, noisy or distorted if its speaker is defective. Exchange the wires of the suspected speaker with those of the other channel. You can even connect another speaker across the suspected one with test leads to determine if the speaker, earphone jack contacts, and the amplifier are working. In fact, a speaker with leads can be used to signal trace the sound from the audio power amplifier's output capacitor to the dead speaker. A distorted or noisy speaker may have foreign objects poked into, or adjacent to, the speaker's cone. Inspect the speaker's terminals for poorly soldered connections. Replace the defective unit with one that's the same size and has the same impedance. Before you replace the cover, check the dial-cord movement, clean the tape heads and switches, and possibly replace the batteries. Make certain the each lever or pushbutton of the cassette player is working. Clean up the whole cabinet with a common household window spray after replacing the covers. Replace all the knobs. If you have the equipment pressurised air in cans or from a compressor - spray a jet of air into the corners and brush out the dirt with a small paint brush. ·! c Adapted by arrangement from an original article which appeared in HANDS ON ELECTRONICS. Copyright (c) Gemsback Publications, USA.