Odyssey, board B
This board has most of the sound generating sub-circuits on it, hence the longest list of parts to be swapped. These are R51 – R63, R66 – R75, R77 – R89, R92 – R98, R102 – R104, R108, C6 – C9 and C12 – C19.
The broken parts were Osc 1 square output (bad Z1), LFO output (bad A1), capacitor C4 (measured 2nF instead of 10nF) and LFO modulation for the S/H circuit (bad A3). I also swapped A2a and Z2 while I was at it.. Although these were ok, given the high amount of failed ICs on this synth, this seemed like a reasonable preventive maintenance action. I didn’t have a proper replacement for A3 (CA3080 OTA, canned version) so I glued a DIP-socket to the solder side of the board with a small dab of silicone and connected the required pins with a bit of flat cable cut to exact length.
A few oddities from some previous repairs were present on this board. R37 had been removed and replaced with a trimmer AND the two 10µF 35V electrolythic power supply decoupling capacitors (C10 and C11) had been replaced with two 10nF ceramic caps. Classy repairs..
Whilst calibrating Board B, the -15V feed suddenly dropped to -7V causing the synth to go totally dead. Serious double-triple-WTF. Powering down for a minute or so didn’t seem to cure the problem so I just gave up for the day. No idea what the fuck happened. Maybe I poked somewhere with the multimeter probe I shouldn’t have, maybe this was a first sign of PSU failure.. Who knows. Nonetheless, the next time I carried on with the repairs the -15V feed was back to normal again. As the levels haven’t glitched again after this single instance, I think I’ll leave it as-is for now.
After this power failure (and giving the repairs some days off) I was briefly planning a replacement PSU. The idea was to simply strip the parts for both +15V and -15V regulation stages and drop in 78 / 79 series linear regulators (7815 and 7915 respectively). At a glance, the board layout wouldn’t pose any problems and the implementation would be a no-brainer. But like said, saving that for later then..