Shot an email off to Rainer last night and I had a reply this morning:
From Rainer:
I think the culprit is the 4051 voltage mux that routes the input
voltage to the ADC. Might also be the signal conditioner sitting behind,
in worst case the DOC itself (which also houses the A/D converter).There might also be a chance that the 4051 addressing fails so that it
doesn’t get activated or only routes the unconnected input.
PSU issues can be ruled out, otherwise the sound would be not working
or sounding distorted
After reading that I started poking around in the Q-chip/4051 area of the main board (to the right, under the key bed). I pressed gently on the main board in that area and the SQ-80 came back to life, however the controllers wouldn’t hit their max values. Some further wiggling around and everything started working correctly, but after a while the issues would return. Another press and it was working again…
…sounds like this may be a bad connection/cold solder joint or broken trace on the main board right around the DOC / 4051 chips.
This might involve pulling the main board and giving it a close examination from the back side, I’ll keep this thread updated with what I find.
-C