I recently started up on an old MemoryMoog+ project that I put aside a while ago. The MM came in with virtually nothing working on it, but after a ton of scope probing and staring at the huge schematics I got it completely operational… with one tiny irritating problem. It seems that Voice 2/OSC 1 reads around A6 A6 A6 when in tune, instead of 7F 7F 7F. Obviously this means the autotune does not work for that oscillator. The oscillator can be perfectly tuned and stable but reads incorrectly in the hex chart, or I can bring it up to the autotune area and it WILL “successfully” tune but then is audibly out of tune. Here are things I have tried thus far:
-Check incoming CV: Matches all others
-Check incoming autotune voltage: Matches others when autotune cleared, but still reads hex incorrectly.
-Replaced autotune 4051 mux/dmux. No change.
-Replaced TL072 post 4051. No change.
-Tried swapping the mux connectors and the problem moved to different voice. It is definitely on the mux/dmux board and not the voice cards.
-Checked resistors (in-circuit) around the 4051/TL072 and they match up with other voice channels.
I can’t really find any where else the problem could come from. It shouldn’t be before the 4051 otherwise it should affect all 8 outputs, and it isn’t after the output connector. That leaves the 72 op-amp, resistors and capacitors. I feel like if it is any of those components then I would be back where I started… pitch change, but incorrect hex.
If this was my synth I would just leave it as-is since everything can be tuned, but this one is for sale and I feel like someone will get a bit irritated that it would say “5 tuned” even though it can stay in tune anyway. Anyone have an idea where I should look?