Customer brought in this 25 year old unit stating that it no longer works properly – turntable starts up and then stops a few seconds later. Switched over to a spare alternative and the turntable works, so wanted me to look at this unit.
Looked inside and immediately obvious that two capacitors were bulging and dried out so these were replaced temporarily (didn’t get a photo of the dead ones – one measured about 3nF the other about 15uF – should be 470uF!) but then noticed that all the 22uF ones looked bad too, first the board needed releasing so easiest is to desolder the three wires from the transformer:
Removed and replaced the affected electrolytics with quality 22uF Rubicon and Panasonic 470uF/63V caps which are 10000 hour rated at 105C.
Some of the 22uF reached a magnificent peak of 40pF in value, most were less (1/1,000,000th of rated value), not seen this much in burnt out, dried up caps in a while!
After replacement it was decided to scope the output waveforms, and glad of doing so – all was not right.
45RPM speed was ‘ok’, if a little clipped on one channel, but the waveform for the 33RPM speed was completely out of whack, and was unstable, fluctuating in one channel (the yellow trace). Though the mode selection switch was demonstrably a little dirty, cleaning it didn’t have the full effect desired.
Scoped around and could see the input to the amplifier (green trace) was also distorted so this wasn’t an amp issue.
This circuit works on the basis of two crystal oscillators, which high frequency square waves are then bitshifted / divided down by a 14 bit divider and then shaped into sine waves using what is believe to be a 6 pole butterworth sallen-key low pass filter built around 2 four channel opamps, before treating the output as a 2 channel low frequency audio signal offset by 90 degrees to be amplified by 2x LM1875T 20W power amplifiers to provide power to a 2 phase 24v turntable motor.
There was a bad joint on the input to one of the opamps causing this problem, once sorted, all was stable again and beautiful sine waves…
The output waveform levels were then adjusted for each channel and respective speed (the 4 adjustment pots on the board) to 19.5V RMS (was clipping at just over 21V RMS)
Frequencies 50.009Hz for 33rpm and 67.667Hz for 45rpm were measured.
Cleaned up the case inside of expelled capacitor vapour/spatter, and spruced up the outside too before returning to customer who is grateful to have it all working again. Advised against leaving the power on 24/7 as unit does get quite warm (although probably will last another 25 years or longer even if so!).