I think it sounds alright. I can see it being something you hook up to a load of effects pedals and muck around with.

^ I was reading about that a few days ago. Strange looking things when opened up.

Which components?

Check the trace and connector that link the switch to the CPU board for connectivity. A trace may be broken or the old molex connector is loose.

Save up for a mid-range desoldering station, seriously. Everything else will just be pain.

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I changed my mind. I'd hit him and then steal what I wanted.

The trilogy I'm fixing at the moment (uses lots of the same circuits) had a few dead op-amps that caused similar problems.

...

Now that is pretty damn cool.

I suspect that one of the demux chips (4051) that feeds control voltages to the affected chip, or one of the associated voltage buffers, has failed. If it's the VCF on voice 5 that's playing up then I would focus on IC23 (the demux) and IC18a (quad op-amp) on the module board. With a multimeter try ...

Forgive me if I don't immediately s**t myself with excitement.

Yeah, the thing with power switch is that one of its poles actually controls the reference voltage for the D/A. If the contacts get poor (as can often happen) that mucks up the rest of the circuit. This post on analogue-heaven describes a way of calibrating the VCO with just a guitar tuner. Even if ...

Ok cool - come to think of it the fact that D/A is sending out the correct control voltages ought to rule out the power switch problem. Do you have access to an oscilloscope for VCO calibration procedures?

Have you tried any of the calibration procedures detailed in the service manual? You can make sure the digital/analogue converter is accurately configured just with a basic multimeter.