My Pro-Ject DebutIII turntable started to show some irregular platter movement, the platter jittering heavily while rotating at a bizarre speed.
This is the Debut3 model incorporating the speed box controller and the phono preamp.
Once the speed box was opened up I started probing the power supply, hoping that the Atmel microcontroller on the board would not be the problem source. Well it was not, the 5V DC from the 7805 voltage regulator was all chopped up, so that was the reason for the strange movement. The speed select LED’s did not light up either.
I started by changing the capacitor behind the voltage regulator, this improved the dc signal already a bit but still choppy. Next I replaced the large input capacitor, which eventually solved the problem, a nice constant 5VDV at the output.
This made the uC all happy and generate the correct voltage level for the motor controller.
Which means that these 2 capacitors had just died. Strangely they were not shorting out the supply, just no more capacitance.
I can only speculate about the cause for these caps to break, but maybe they did not like being plugged in for the last 10 years or so?
On a side-note:
When I bought this turntable some years ago (second hand), it produced a constant hum on the preamp output. The culprit was a badly grounded shield which is covering the preamp pcb. I only re-soldered that shield which then fixed the hum.