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Vintage Synth Repair

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Siel DK 600, strange problem

2008-11-27 by Scott Nordlund

I repaired the NiCd battery corrosion problem in my Siel DK 600 (and also the Expander module), then attempted to address a CV demux/sample and hold problem (envelope release time was too short because a defective component was draining the charge from the capacitor). I replaced the associated parts (4051 and TL084), and it fixed the problem, but also introduced some new ones. The CVs associated with these chips (filter cutoff, oscillator detune, etc.) are unstable when the synth is first switched on, and will actually change (in an interesting way) when there's some kind of mechanical vibration. It mostly resolves after the synth has had a few minutes to warm up. I do appreciate the added "vintage driftiness" and "innovative performance controls", but I'd really rather have it do what I want it to do. I've tried reseating the chips.

It won't be a big deal to just rework the whole thing by replacing all the parts (sockets included) and then thoroughly cleaning off the flux, but the crappy DIP connectors seem to have a very limited lifespan and I don't really want to go plugging and unplugging any more than absolutely necessary. ; I'd like to fix it and be done with it. I'll redily admit that using salvaged components (sockets even) probably isn't the best idea, but I'm too cheap and lazy to order parts, and it's probably not any more likely to cause problems than the original flaky construction.

So based on this, is the problem more likely to be a bad solder joint or bad socket, or is the solder flux on the board interfering with the high impedance stuff? I'd guess the former, but again I don't want to risk breaking a pin on a DIP connector if I have to rework it again.

There's also some sort of interference that seems to affect the pulsewidth and cutoff CVs (?) of voices 5 and 6. I probably won't be able to address this without an oscilloscope, but it's not extremely intrusive, so it can wait (though I'm also open to suggestions here!).

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