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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.

My questions:

  • 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 ?

Thanks !

Service manual schematics. Unfortunately not many details on the motor:

Service manual schematics

Actual motor and its control circuit:

FG Servo motor and its control circuit

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    \$\begingroup\$ We can see that the motor part number is PXM-081, but searching for service information would be a lot easier if you told us the exact model of the turntable itself. \$\endgroup\$ – Dave Tweed May 19 at 11:45
  • \$\begingroup\$ Sorry about that. The turntable is a Pioneer PL-100. Obviously parts have been discontinued. I have not been able to find details or a data sheet on the motor itself, but I came across other brands using similar references (4H2RPA). I cannot find what the naming means, however. \$\endgroup\$ – Julien Vivenot May 19 at 11:56
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    \$\begingroup\$ Does the power-supply voltage remain constant? if 12 volts and 60mA, this motor is less than 1 watt. There is no heat sinking on that regulation transistor (the Darlington), which may have become intermittent. \$\endgroup\$ – analogsystemsrf May 19 at 12:13
  • \$\begingroup\$ @analogsystemsrf I am waiting for it to fail again to confirm the supply voltage during failures. I’ll let you know. Regarding the lack of heat sink on the transistor : there is a large plated area connected to it on the back of the PCB, would that be for heat sinking ? \$\endgroup\$ – Julien Vivenot May 19 at 12:44
  • \$\begingroup\$ @analogsystemsrf Supply voltage goes from 13.6V when working properly in either 33 or 45rpm to 13.1V when it goes crazy. What would you make of this ? \$\endgroup\$ – Julien Vivenot May 19 at 13:14

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