I have an SQ80 that has lost resonance on one voice - I've swapped the CEMS and the problem stays with the same voice, so its not the chip. When I run the filter calibration routine one of the voices comes up zero instead of between 145-155.
I had the same problem it is the black capacitor below the voice. Unfortunately later was discovered that the capacitor from voice 3 (voice 6 was not working) was also the problem. Solution check which voice is not working and remove and measure the capacitor. If still not working check the other capacitors. On that SQ80 I already changed the other logic chips, but in the end these were not the problem.
FIXED - tuned out to be the SSM2300 that controls Q = pulled one out of an ESQ1 I have laying around and bingo! Will order one to replace the ESQ1 donor, 10$