NOTES ON REPAIR OF AUDIO EQUIPMENT AND OTHER MISCELLANEOUS STUFF V1.00

• 1) About the Author & Copyright
• 2) Safety
• 3) Turntable (Changer) Maintenance
• 3.1) Speed Control in Turntables
• 3.2) Wow and Flutter with Turntable
• 3.3) Erratic Sound from Turntable
• 4) AC Adapter Basics
• 4.1) AC Adapter Testing
• 4.2) AC Adapter Repair
• 5) Walkman/Discman Power or Sound Intermittent
• 6) Automotive Power
• 7) Tape Players
• 7.1) One Channel Dead
• 7.2) Distorted Tape Recording
• 7.3) Erratic Auto-reverse
• 7.4) General Tape Speed Problems
• 7.5) Sudden Increase in Flutter on Tape Decks or Walkmans
• 8) Noisy Motors in Audio Equipment
• 9) Answering Machine Comments
• 9.1) It Picks Up then Hangs Up
• 9.2) It Does not Complete Cycle
• 10) Touch Tone Phone Doesn't Dial
• 11) Checking Phones, Answering Machines, and Modems

Author: Samuel M. Goldwasser
E-Mail: sam@stdavids.picker.com
Corrections/suggestions: [Feedback Form] [mailto]
Copyright (c) 1994, 1995
Version: 1.00
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1. This notice is included in its entirety at the beginning.
2. There is no charge except to cover the costs of copying.

IF IT IS LINE POWERED you MUST read the following Safety notes or you might

Notes on Safety V1.00 (7KB)

Usually there is a 'C-clip' or 'E-clip' which holds the platter (the thing that rotates) onto the spindle. It may be covered with a decorative piece which can be easily removed. The clip can be pried off (gently) with a small screwdriver (just don't lose it, though even this is not a biggie so long as you never turn the thing upside-down).

The platter can then be lifted straight up and off the spindle. You will see several things (this will vary depending on your particular unit):

1. A flat washer, sitting on a ball bearing race sitting on another flat washer (one or both of these washers may be missing. Also, the top one may stick to the platter when it is removed.) The ball bearings, shaft, washers, etc. should all be cleaned with degreaser and then lubed with a light grease.

2. Changer gears etc. These will have varying amounts of grease on them if it is not gummed up, leave them alone. Put a drop or two of light oil on the shafts. Inspect other linkages as well. If the grease is gummed up on the gears or sliding linkages, you will need to clean it off thoroughly with degreaser and then use a small amount of high quality grease suitable for delicate mechanisms. One cause of a changer failing to activate at the end of a record is gummed up grease.

3. Motor. Check to see if the motor shaft turns freely. If it does, don't do anything else. If it is tight, then you will need to carefully disassemble the motor and clean and lubricate the bearings at each end with light oil. Don't lose any of the various washers/spacers that may be present on the shaft as it is removed from the end pieces and make sure to put them back in exactly the location and order they were in originally.

4. Check the belts if any. If flabby or cracked, these will need replacing. Check the idler tire if any - if hard or cracked, it will need replacing. While you are here, clean the rubber parts with isopropyl alcohol and Q-tips or a lint free cloth and dry thoroughly.

Servo controlled turntables utilize a feedback technique which locks the platter speed to a stable reference - either the power line (50/60 Hz) or more commonly a crystal oscillator. Here is one example:

A Sony turntable I repaired used a magnetic stripe pattern on the inside of the platter which was sensed by a magnetic pickup. The resulting signal was phase locked to a stable reference and used to control a brushless DC direct drive motor. Speed would become erratic if (1) the magnetic pattern were damaged, (2) the pickup position was moved too far from the surface of the platter, (3) the Hall-effect sensors in the motor were bad, or (4) the control electronics went bad. In one case, it turned out that one of the Hall effect sensors had failed in the motor. This required disassembling the motor and replacing the sensor - $4 from Sony.

1. Bad belt or idler. Rubber 'rusts'. If it is old, then almost certainly the rubber parts have deteriorated and will need replacement. Unfortunately, replacement parts are not as readily available as they] once were. The places listed below may have some and there are many other sources but it is not as easy as one would like.

2. Dirty or worn spindle bearing. This will cause rumble. The bearing can be cleaned and relubed or replaced.

3. Dried up lubrication in motor, idler, or other rotating part.

4. Bad motor (not that likely) except for lubrication in which case the motor can be disassembled, cleaned, and lubed.

5. Physical damage to platter - something heavy was dropped on it upsetting the delicate balance.

1. At the pickup itself. There may be small press fit connectors at the cartridge. These sometimes become loose. Gently remove each one (one at a time! so that you do not mix up the wiring) and squeeze with a pair of tweezers or needle-nose pliers. Snap in cartridges may have dirty contacts the springiness may have disappeared.

2. At the RCA plugs under the turntable which connect to the tone-arm. Depending on your design and problem, you may need to simply clean with contact cleaner or squeeze the metal shell or center contact.

3. At the Receiver/Amp. Same as (2) above.

4. Sometimes the cables themselves will develop broken wires at one end or the other. Easiest is to try a different set of cables.

The most common problems are due to failure of the output cable due to flexing at either the adapter or output plug end. See section below on repair procedure.

1. AC Transformer. All wall warts are often called transformers. However, only if the output is stated to be 'AC' is the device simply a transformer. These typically put out anywhere from 3 to 20 VAC or more at 50 mA to 3 A or more. The most common range from 6-15 VAC at less than an Amp. Typically, the regulation is very poor so that an adapter rated at 12 VAC will typically put out 14 VAC with no load and drop to less than 12 VAC at rated load. To gain agency approval, these need to be protected internally so that there is no fire hazard even if the output is shorted. There may be a fuse or thermal fuse internally located (and inaccessible).

   If the output tested inside the adapter (assuming that you can get it open without total destruction - it is secured with screws and is not glued or you are skilled with a hacksaw - measures 0 or very low with no load but plugged into a live outlet, either the transformer has failed or the internal fuse had blown. In either case, it is probably easier to just buy a new adapter but sometimes these can be repaired. Occasionally, it will be as simple as a bad connection inside the adapter. Check the fine wires connected to the AC plug as well as the output connections. There may be a thermal fuse buried under the outer layers of the transformer which may have blown. These can be replaced but locating one may prove quite a challenge.

2. DC Power Pack. In addition to a step down transformer, these include at the very least a rectifier and filter capacitor. There may be additional regulation but most often there is none. Thus, while the output is DC, the powered equipment will almost always include an electronic regulation.

   As above, you may find bad connections or a blown fuse or thermal fuse inside the adapter but the most common problems are with the cable.

3. Switching Power Supply. These are complete low power AC-DC converters using a high frequency inverter. Most common applications are laptop computers and camcorders. The output(s) will be fairly well regulated and these will often accept universal power - 90-250 V AC or DC.

   Again, cable problems predominate but failures of the switching power supply components are also possible. If the output is dead and you have eliminated the cable as a possible problem or the output is cycling on and off at approximately a 1 second rate, then some part of the switching power supply may be bad. In the first case, it could be a blown fuse, bad startup resistor, shorted/open semiconductors, bad controller, or other components. If the output is cycling, it could be a shorted diode or capacitor, or a bad controller. See the Notes on the Switchmode Power Supplies (35KB) for repair info.

The most common problem is one or both conductors breaking internally at one of the ends due to continuous bending and stretching.

Make sure the outlet is live - try plugging in a lamp.

Make sure any voltage selector switch is set to the correct position. Move it back and forth a couple of times to make sure the contacts are clean.

If the voltage readings check out for now, then wiggle the cord as above in any case to make sure the internal wiring is intact - it may be intermittent.

Although it is possible for the adapter to fail in peculiar ways, a satisfactory voltage test should indicate that the adapter is functioning correctly.

The most common problem (and the only one we will deal with here) is the case of a broken wire internal to the cable at either the wall wart or device end due to excessive flexing of the cable.

Usually, the point of the break is just at the end of the rubber cable guard. If you flex the cable, you will probably see that it bends more easily here than elsewhere due to the broken inner conductor. If you are reasonably dextrous, you can cut the cable at this point, strip the wires back far enough to get to the good copper, and solder the ends together. Insulate completely with several layers of electrical tape. Make sure you do not interchange the two wires for DC output adapters! (They are usually marked somehow either with a stripe on the insulator, a thread inside with one of the conductors, or copper and silver colored conductors. Before you cut, make a note of the proper hookup just to be sure. Verify polarity after the repair with a voltmeter.

The same procedure can be followed if the break is at the device plug end but you may be able to buy a replacement plug which has solder or screw terminals rather than attempting to salvage the old one.

Once the repair is complete, test for correct voltage and polarity before connecting the powered equipment.

This repair may not be pretty, but it will work fine, is safe, and will last a long time if done carefully.

If the adapter can be opened - it is assembled with screws rather than being glued together - then you can run the good part of the cable inside and solder directly to the internal terminals. Again, verify the polarity before you plug in your expensive equipment.

WARNING: If this is a switching power supply type of adapter, there are dangerous voltages present inside in addition to the actual line connections. Do not touch any parts of the internal circuitry when plugged in and make sure the large filter capacitor is discharged (test with a voltmeter) before touching or doing any work on the circuit board. For more info on switching power supply repair, refer to the Notes on the Switchmode Power Supplies (35KB).

If it is a normal adapter, then the only danger when open are direct connections to the AC plug. Stay clear when it is plugged in.

The connections can be easily resoldered but you will need to open up the case using jeweler's screwdrivers and great care. Use a low wattage iron and make sure you do not introduce any solder bridges. Try not to lose any of the micro-screws.

Furthermore, there is essentially unlimited current available from the battery (cigarette lighter) - 20 A or more. This will instantly turn your expensive CD player to toast should you get the connections wrong. No amount of internal protection can protect equipment from fools.

My recommendation for laptop computers is to use a commercially available DC-AC inverter with the laptop's normal AC power pack. This is not the most efficient but is the safest and should maintain the laptop's warranty should something go wrong. For CD players and other audio equipment, only use approved automotive adapters.

To determine if the playback circuitry is working, gain access to the terminals on the playback head - a metal cased little cube near the center of the tape side of the cassette. There should be four wires coming from it. While the machine is supposed to be playing, touch the end of a jeweler's screwdriver gently to each of the four terminals in turn. When you touch the good channel, you should hear a buzz from the appropriate speaker. If you touch one terminal and get a buzz from the 'dead' channel, then it is possible that the head is bad for that channel. If you can touch two different terminals and get a buzz in the bad channel for both, the it is likely that the ground connection to the input preamp has fallen off. If you do not get anything from the bad channel, then there is likely an electronic problem in that channel. Bad connections aside, the most common problem area would be the audio power amp - bad IC or capacitor.

If record is the problem with very distorted sound, this may be a sign of a bad bias oscillator or switching circuit or record switch. The bias is an ultrasonic signal that is impressed on the tape along with the input signal. Without it, the sound will be highly distorted. In effect, it is a linearizing signal.

First, check that the record switch is clean - it may have many contacts and may have collected a lot of crud. If behavior changes with each activation of the record switch, get some contact or tuner cleaner spray and use the extension tube to spray inside the switch (with the power off), put the switch through its paces several times and allow to dry before powering it up.

If it is a portable subject to abuse, check for bad connections as well.

Beyond this, you can try to measure the signal going to the record heads while in record mode. You should be able to see a high frequency signal in addition to the input signal. If it is absent, then you need to trace back to its source and at this point will probably need a schematic.

I had one from a Toyota where the plastic drive gear which included the magnet and was part of the reel split and was getting stuck at the broken tooth causing a reverse and eventually eating the tape. It was $9 for that little plastic gear.

Others are entirely mechanical and if there is a lack of lubrication, dirt, or broken parts they may start acting erratically.

Although there could be an electronic fault, carefully examine the mechanism for obvious or subtle problems before breaking out the 'scope.

The following methods are use for auto-reverse:

1. Optical looking for the clear leader on the cassette. Better tape decks use this for sensing at the end so that the reverse occurs just quickly at the end of the tape rather than waiting for the leader to go by and a second or two for the tape to stop.

2. Totally mechanical - a lever arm presses against the tape and when the tension increases with the reel stopped, it trips a mechanism to reverse.

3. Optical sensors on reel rotation.

4. Magnetic sensors on reel rotation - either hall effect devices or simple reed switches.

Belts and idler tires are readily available from places like MCM Electronics.

Did this problem start suddenly or was this a tape recorder you found buried under an inch thick layer of dust in an attic?

If the latter, then there could very well be multiple mechanical problems due to deteriorated rubber parts - replace then or toss it.

Fast play could be an indication of a hard deteriorated pinch roller.

Clean and lubricate the mechanism. Check for dry or tight bearings.

Is there any pattern to the problems - like with respect to the start and end of cassettes?

If the tape speed has suddenly become excessive:

1. Mechanical. If you had a recent tape eating episode, there may be a wad of tape wrapped around the capstan. Remove it. Alternatively, the pinch roller may not be fully engaging against the capstan and the takeup reel is simply pulling the tape through without any speed control. Clean the mechanism, check for tired belts and springs.

2. Electrical. The motor speed control is not working. This may be either a mechanical governor inside the motor or a voltage regulator or other electronic control often also inside the motor. In the latter case, you may be able to disassemble the motor and repair it. One possibility is that the series regulator has decided to turn into a short circuit. This may be external or internal to the motor.

Is it slow and steady - no more wow and flutter than normal? Or slow and erratic indicating that (1) the speed regulator is faulty, (2) some bearings may need oil, (3) the pinch roller is glazed.

If the mechanics seem ok, then check for electronic problems with the motor or regulator. Sometimes there is a trimpot for speed adjustment inside or external to the motor.

A variety of techniques are used to regulate the record/playback speed:

1. Mechanical governor inside motor - centrifugal contacts open at correct speed reducing current to motor. If speed is too low, than springs could have weakened or contacts could be bad - open. If speed is too high, contacts may be welded closed. There may be a resistor and/or capacitor across the contacts. An open resistor could conceivably cause unstable speed fluctuations. A capacitor may be present to reduce electrical noise.

2. Voltage regulator inside motor case or external to motor. The regulator or transistor may be faulty. If power for the motor seems to come directly from an unregulated supply, check across the motor terminals with an ohmmeter. A low reading which is identical in both directions would indicate a direct connection to the motor brushes with no internal regulator. A high reading or one that is different in each direction indicates an internal electronic regulator - or you could just use your eyeballs to determine if there are any electronics inside the motor. These can be disassembled and bad parts replaced. There may be an access hole on the motor for an adjustment. Alternatively, you could remove the guts and install an external regulator using an LM317 or similar part.

3. Active regulator with tachometer feedback from motor winding - there would be 4 wires instead of two coming out of the motor - 2 for power and 2 for tach. Control circuitry could be bad or the tach output could be dead (speed too high).

4. If an optical strobe disk is located on the motor shaft, then it may be part of a speed control circuit. If it is on one of the reels - probably the takeup reel - then it simply operates the (electronic) tape counter or signals the controller that the takeup reel is turning - to catch tape spills.

1. Batteries (where appropriate). Almost dead batteries will greatly increase flutter.

2. Tired belts - loose flabby belts will produce varying, probably slow, speed as well.

3. Dirt or goo on pulleys. Sometimes a glob of stuff gets stuck to a pulley and produces a periodic variation in speed. I picked one up at a garage sale that had this problem. I thought it was a bad motor until a careful examination revealed that the belt was jumping a mm on each rotation of an idler pulley.

4. Lack of lubrication - a dry or worn bearing may result in a variety of speed problems.

5. Bad speed regulator - either mechanical or electronic.

6. Bad power supply.

7. Bad tape. Don't overlook this obvious possibility, try another one.

With a little care you should be able to determine the normal RPM of the motor knowing the tape speed (1-7/8" inches per second for a cassette). Simple measurements of pulley and capstan diameter ratios.

One motor I repaired with an internal regulator dropped the 12V to around 7V for the motor. I replaced the internal regulator with an LM317 chip.

MCM Electronics, Dalbani, and Premium Parts stock a variety of generic replacement motors for tape decks, Walkman, boom-boxes, and CD players.

Many non-mechanical problems with answering machines are related to the circuitry connected to the phone line. This is subject to the high on-hook and ringing voltage and possible voltage spikes due to lightning, etc.

Testing of the components on the phone line side of the coupling transformer is a worthwhile exercise and may reveal a shorted semiconductor or capacitor.

With endless loops outgoing message cassettes, the metal strip that is used to sense the beginning can wear or become dirty. Try a new cassettes or clean it.

1. If it has a 'telco' and a 'phone' connector verify that you are plugged into the 'telco'. Otherwise, it may hang itself up. Who knows. If someone else attempted a repair, these jacks could even have been replaced interchanged.

2. Measure voltage on the relay coil. If it actually disappears when the relay cuts out, then something is telling the relay to turn off. If it is just reduced, then there may be a power problem. If it is relatively stable, then the relay may be bad.

3. Test components near the telephone connection for shorts/opens. Parts connected to the telephone line get abused by the ringing voltage and other transients. Maybe you will get lucky and find a fried part.

4. If you can identify the power supply outputs, verify their voltages if possible. Check the 'wall wart' if it uses one for proper output.

5. Make sure that the tape mechanisms have completed their cycles. While unlikely, it is possible that the logic gets confused if one of the tape units has not reset itself due to a mechanical fault like a bad belt.

6. As usual with cheaply made consumer stuff (as well as cheaply made expensive industrial stuff), check for bad connections.

For endless loop outgoing cassettes make sure that the metal sense strip is not worn off and that the sensor is making good contact.

Newer electronic phones will utilize either polarity. The older AT&T battlewagons will only dial when hooked up with the correct polarity. This does not affect conversation, ring, or rotary phones.

Some units extract power from the phone line and the rectifiers can go bad.

In a modem, for example, there will be various diodes and transistors for routing the incoming and outgoing signals, or for protection and these can fail.

If you have signal problems - a modem will try to dial out but not make its way to the phone line, testing on each side of the coupling transformer with a scope or Hi-Z headphones should be able to determine if the problem is on the logic or phone line side of the device.