Some of this seems to point at Board A (octave switch and and possibly the unstable behaviour). I tend to exchange all opamps from Board A (with the same type). In the past I've witnessed improvements several times; probably the manufacturing quality of even generic opamps is higher today than 40 years ago. Also clean Board A very carefully. Board A is notorius for instability when contaminated.
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Re: [vintagesynthrepair] Need help with Unstable Oscillators in Arp Odyssey 2800
2012-08-06 by Malte Rogacki