I have been trying to get a 40-year-old turntable back to life, with relative success until now. Everything works fine but for an intermittent issue with the motor.
The motor has two modes (for 33 1/3 and 45 rpm playbacks) which work fine most of the time.
Unfortunately, the motor sometimes jump to "full speed" for a period of time. It also happens on startup regularly. After a while (sometimes several minutes) it gets back to normal speed. When it goes wacky like this, it disregards completely any change to the switch or variable resistors that are meant to tweak speed.
I have checked the resistors and variables resistors along the way, which all give correct readings.
What is the purpose of the D947 transistor in front of the FG servo motor ? Can it be the culprit ?
Is this a common problem for old servo motors ? Can it be caused by anything mechanical in the frequency generator / tachometer internal parts of the motor ?
Service manual schematics. Unfortunately not many details on the motor:
Actual motor and its control circuit: