Actually, these aren't synchronous motors. They're induction motors. The
speed is mostly controlled by the power frequency but it isn't absolutely
locked to it. That's why you don't find motors like these in high quality
record players (or tape recorders, either).

Right again, Jim, I don't think I ever owned an audio device with a
synchronous
motor. In Vietnam I serviced and rebuilt tape decks that had some sort of
motors that had adjustable speed motors (brushless if I'm not mistaken)
that I adjusted while playing a "master" tape of a popular recording.
Most of the repairs were done by carefully honing out the grooves in
worn
pick-up and record heads. Probably not accepted practice, but, hey
it certainly put a lot of tape players back in business at the Danang
Airfield tape library back in service!

Old Chief Lynn