[sdiy] Moog Opus 3 TDA1008/ sub octave issue

Nathan Trites nathan at idmclassics.net
Tue Feb 2 23:47:43 CET 2021


Hi all,

I don't have much experience with organs using top octave generators or TDA
divider/keyer chips, so this issue I was working on had been new for me and
I thought I'd document it here.

A customer brought me an Opus 3 that needed a bunch of TLC on the front
panel controls, envelope triggering issues, and octaves 1 and 3 had a sub
octave mixed in. Octaves 2 and 4 were fine. I didn't find anything online
about the same problem, but when reading up on the TDA1008 I saw Florian
had a similar issue in 2008 on a Crumar Performer:

http://synth-diy.org/pipermail/synth-diy/2018-August/169555.html

This was the same issue I ended up having. The keyboard drive/trigger
circuit had failed and was being held too high, except for C keys that
would trigger a pulse correctly. The kind of fluke from the keyer chip for
the C keys w/ an oxidized socket - it went away when reseated.

Q1 3904 and U13 LM358 were both bad, so replacing those fixed the suboctave
issue and the triggering problems. Not entirely true - the envelopes were a
bit flaky until replacing the sockets someone put in there in the 90s.
After new sockets and CMOS they work well.

Just thought I'd share! Interesting synth/organ with a nice mix of ideas
(and now rare parts) under the hood.

Nathan
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