Repairing Cassette Capstan & Turntable DC Motors

I recently attempted to fix a cheap Philips micro hi-fi system from the early ‘2000s. One of its many faults was a noisy cassette motor. The system was ultimately scrapped (more on that Here) but never one to miss an opportunity to extract every last piece of content from a piece of equipment, I detailed a complete disassembly of the cassette motor and give a primer on cassette motor basics herein.

Motor With Top Plate On

Most cassette capstan motors are counterclockwise (CCW) 2400RPM motors operating at 6, 9 or 12 volts. Most smaller portable players and Walkmans use a 3 volt motor that is constructed differently so if you’re repairing one of those this article probably won’t apply. But virtually any boombox or hi-fi cassette deck from the early ‘70s and later will use a similar motor.

The speed and direction of a typical brushed DC motor is proportional to the amount and polarity of the voltage applied. This is not so for the type of motor used in a cassette deck, or the similar motors found in many turntables as most of them contain circuitry to govern their speed or maintain constant torque under load. Even the simplest cassette motors, where the speed is mechanically governed by an adjustable unbalanced cam assembly usually have at least 1 capacitor across the input which will pop if you reverse the input power polarity. Better motors have an electronic speed control and a driver chip which in some cases maintains torque but in others is imply there as a form of overload protection to stop the motor burning out if the mechanism stalls. These chips will also be damaged if the polarity is reversed.

It is for this reason that you can’t simply take a clockwise motor and reverse the polarity to use it to drive a capstan in the correct direction. Clockwise motors can be modified to spin in a counterclockwise direction, but it’s not usually worth the hassle as there are plenty of genuine Matsushita, Sankyo and Mabuchi motors available usually as new old stock. Some of the Chinese clones aren’t bad either though some, it has to be said, are awful, so it’s luck of the draw.

Another type of motor you’ll encounter, usually in full-logic cassette mechanisms has four wires and no rear speed adjustment. The speed is set externally within the unit’s circuitry by altering the resistance between the two middle wires. This allows the motor to operate at any number of different speeds depending on the application. Typically these motors are used to facilitate high-speed dubbing in fancier decks, but may also be used to facilitate pitch controls or to electronically govern the speed during winding and tape seeking operations in mechanisms where there is only a single motor.

, Most cassette decks are at least 30 years old, and some are approaching their fifth or sixth decade of service. None of these mechanisms, and especially the motors, were designed to last for such a long time. That was fine when parts are widely available, but not so now as time and demand deplete the supply and prices rise as a result.

The common issue with these motors as I observed in the Philips system is excessive noise. Usually a grinding sound, but sometimes they squeal or develop horrible wow and flutter. You can quieten these significantly and improve speed stability by adding a drop of oil beneath the pulley and into the top bearing. I use a 100% synthetic motor oil designed for quiet-running turntable bearings but a drop of sewing machine oil works just as well. This reduces the noise to an acceptable level. Incidentally the same is true for the spindle and tracking motors in a CD player, especially mechanisms by Panasonic which typically fail in this way, causing the player to skip or fail to read at all.

But if something is worth doing, it’s worth overdoing – and with care these motors can be disassembled to access the bottom bearing too, so here goes.

Motor With Prying Tool

A small flat screwdriver inserted between the bottom plate and the circuit board, just above where the wires join the motor allows you to lift the bottom plate off. This should be done with a lifting motion only. Don’t lever downward on the circuit board itself, or you’ll snap it and ruin the motor. Avoid pushing the screwdriver in any further than you need to, and apply small amounts of consistent force as the plate gradually comes away.

Also note the location of the speed hole. If’s wise to mark the plate and motor body in some way to help you re-align them during assembly. If you forget, you can poke a screwdriver through the hole in the plate, through the foam pad underneath, and through the hole in the circuit board to keep things aligned as you press the plate back in. Note too that dual-speed turntable motors or the motors found in cheap cassette decks with high-speed dubbing will typically have a pair of speed holes.

Motor With Foam Buffer Fitted

With the plate off, we see a foam disc preventing the contacts on the bottom of the circuit board making contact with the metal plate.

Motor Control Board Fitted

With that out of the way the circuit board and its brushes lift out in their plastic carrier, exposing the bottom of the rotor and thus the bottom bearing. The board and its carrier can be difficult to remove. I normally take 3 small flat screwdriver bits, or a couple of guitar picks, and slide them to either side. You need to apply even leverage, upwards only, or you risk distorting or breaking the brushes.

Motor Armature Board

Clean the brushes and bearing with a dab of isopropyl alcohol or a non-lubricating contact cleaner. Then add a drop of oil to the central bearing, or onto the shaft protruding from the rotor, and set the board back into place, rotating the motor shaft as you press the board down into position to correctly set the brushes.

Inside Motor Case

Replace the foam pad and align the speed hole in the base plate. Press the base firmly until it is fully seated, and check that the motor spins freely.

Some motors are harder to disassemble than others. Some have a blob of solder bonding the rear plate to the circuit board for grounding. Some are pressed together so tightly that opening them without causing damage is impossible. But with due care and a steady hand many can be repaired with success.

By Ashley

I founded Audio Appraisal a few years ago and continue to regularly update it with fresh content. An avid vinyl collector and coffee addict, I can often be found at a workbench tinkering with a faulty electronic device, tweaking a turntable to extract the last bit of detail from those tiny grooves in the plastic stuff, or relaxing in front of the hi-fi with a good album. A musician, occasional producer and sound engineer, other hobbies include software programming, web development, long walks and occasional DIY. Follow @ashleycox2

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