Polymoog Keyboard Modulation

[Jacob Hunt]

Ok, so over the past few weeks I've been wrapping my head around this broke down Polymoog keyboard of mine best I can. Just trying to understand the signal flow and getting a general idea of what's happening has been a chore all on it's own given the complexity it is famous for. Anyway, now I've got it most of the way playable after studying and repairing the individual subsections. There is one thing I don't quite get:

After repairing the osc 1 frequency modulation, it just won't stop! It sounds great on patches where it's supposed to be, but it keeps going even on the others, even piano, just slowed way down. I checked the mod amount input from the preset board, and it changes to the right amount so that's not it, and besides, it modulates with it unplugged anyway. The trimmer even works, but won't bring it down to zero. I noticed when I put my meter to the osc 2's 3080 modulation amp output, it stopped happening, but it still worked properly on patches where it was supposed to work.

I guess what I am asking is how I could search for the cause of a problem like this, because I really don't know what this might signify. The only solution I could come up with is replacing all related components (again- didn't replace the tantalums on time, and boom, short circuit), but I would like to know how to properly search for the problem. Thanks!

[Jacob Hunt]

Here is the related schematic- A14 is the amp I referred to

[Jacob Hunt]

Turns out not long after replacing q6, a12 which I had already tried swapping started to smoke! Replaced it and now it's good! Just a few more little problems here and there..

[madtheory]

I can't help you with any ideas on this, as you're clearly more advanced than I am with electronics! But I'd just like to say kudos yo you for restoring this beast, and it is a beast, probably the most complex keyboard ever devised. A really tough one to diagnose and repair! Looking forward to the sound demos

[Jacob Hunt]

Thanks! The actual original problem ended up being on an op-amp socket I had soldered in- seems the input and output of the amp had bridged together with solder from my sloppy work.. So a good reminder to myself and anybody reading to always check the simple things as well, and well, be less sloppy

After that I've popped in some of the replacement 5823 dividers from Flatkeys to bring back the lost notes- they work perfectly and come highly recommended, couldn't have done it without them!
Now if I can source some Polycoms without them costing double what the synth costed me it will be mission complete- unfortunately I have 6 bad polycoms.. other than that one happy Polymoog finally!