Briarsfield Hi-Fi - Audio motors

The motors commonly used in audio grade gear are precision components, not the type of motor you'd find in the average power drill or electric fan. Audio is by its nature sensitive to distortions caused by the circuits which convey it and needs high quality components in order to reproduce an uncorrupted high quality duplicate of what is fed in.

Audio motors are generally high precision and quite delicate, overall they are very reliable but after 20 years plus it's inevitable that they will sometimes fail.

Some motors develop flatspots with age (sometimes the motor runs, sometimes it will start fine then stall during a tape, sometimes it won't restart at all unless you turn it 180°). Bearing in mind these motors weren't designed to be disassembled, it needs a lot of care and some small tools to avoid wrecking the internals. Depending on type of motor, they have either a centrifugal governor or an electronic speed control circuit, ageing grease or metal debris shorting sections of the commutator (the strip of metal on the armature where the brushes contact) will cause problems but both can be repaired.

I've stripped many speed controlled Pioneer and Matsushita (Technics) motors over the years, what often happens is that the commutator slowly gets bridged with brush material as they wear and the motor gradually gets shorted and stops working, the speed control circuit then sometimes overheats and burns out the components around it.

Sometimes, the commutator gets coated with oxides and as the brushes gradually lose contact the motor loses power and eventually fails, again usually overheating the speed control circuit and doing more damage. Again, with some care this can be rebuilt with a handful of new components. I cringe now when I think about the days long ago when I used to trash dud motors and replace them with new generics as many people do.

The image to the left is of a pair of common Pioneer capstan motors, also used in single motor Pioneer decks - despite the difference in part numbers they are virtually identical. Motor on the left has gone short circuit and overheated. Components on brown PCB are speed control, heat damage evident on left hand motor. Despite the damage, both these motors can be rebuilt and work reliably again.

Most audio cassette motors in vintage Pioneer and Technics decks are 12v brush motors. There are some which came with 6v or 9v motors but the status quo is 12v. Of these, you then have two broad types: heavier reel drive motors which have a mechanical speed governor and also speed controlled capstan motors. The really top end models (CT-F1250 etc) had servo controlled 'pancake' motors driving the capstans, these don't have brushes so don't suffer from the problems that the majority do.

The electronically controlled motors which are normally used to drive capstans or as the lone motor in lower end decks have what is basically a precision voltage regulator inside the outer case, the voltage which gets to the motor (and hence the motor speed) is then adjusted via a carbon pot through the hole in the back of the case. This voltage is usually somewhere from 7-12v for speeds somewhere between 2000-3000 RPM for the average cassette deck. There is also a phenomenon called 'drift' which is the amount tape speed will vary over the course of a tape, this can be caused in part by a poor quality motor or a varying supply voltage (sometimes because of a fault in the speed controller itself) but has other causes too relating to the tape transport mechanics. These motors usually have precious metal brushes and bronze bearings and hence are very quiet, both aurally and electrically.

The second type are those with a mechanical speed governor which are used to drive the reels on most 2 motor decks.

These have a speed governor attached to the armature (the part of the motor which spins), the contacts are sprung to keep them closed but they also have weights attached to the contact leaves and at a predetermined speed the contacts will open due to the lateral force of the spinning armature and hence cut the power and then close again as the motor begins to slow - the result is actually quite smooth. These motors have carbon brushes hence are more robust and produce a higher torque which makes them ideal for heavier work such as driving the tape reels. Some decks have these motors solely for rewind/fast forward, the tape reels being driven in playback mode from the (quieter) main capstan motor.


Pioneer reel motor	Close up of the reel motor. The sky blue paint is covering the screws which attach the weights to the govenor

The third common type of motor is a brushless servo controlled motor, a similar idea to those generally used on direct drive turntables, these are generally used as capstan motors on high end cassette decks and also appear in some reel to reel machines too. These are virtually silent and very reliable as there are no moving electrical parts and if driven by a good circuit they are also very stable with regards to speed.

There are usually three or more field coils attached to a plate and an armature containing a permanent magnet which gets rotated by the magnetic force when the coils are powered. These motors are usually wider and shorter and get the term 'pancake' motors because of this. The downside to these is that they need alternating current (AC) to actually spin and hence need more complex driving circuits. They lack the instant starting torque of a good brush motor although this rarely causes a problem in a well designed deck.


Close-up of field coils on brushless servo motor on a Sony K60. The two components near the top are 'hall effect' sensors for speed feedback - they work in a similar way to the ABS sensors on a modern car.	Same idea once more but this time on a Pioneer CT-F1250. Rotor (containing magnet/flywheel) to left, field coils in centre, speed governor coil unit to right