[sdiy] Oberheim 4-voice issues (SEM, keyboard)

[Eric Frampton]

> Electrolytics fail very rarely and ripping them all out and replacing them pretty much always does more harm than good.  Tantalum beads fail but tend to go shorted which makes a lot of very messy smelly greasy smoke, and disc ceramics go leaky.

I did the electrolytics on all the modules and the PSU, since this unit's 35 years old and was gigged out extensively, and upon its retirement lived in a non-climate-controlled barn. So far everything else has come up fine, save the keyboard assigner module which has some bad logic happening (and is a topic for another day) and most likely the programmer (which I haven't dug into yet).

> I'd have left the electrolytics alone, and shot the tantalums in the head as a warning to all the disc ceramics ;-)

Heehee! I like that! I've always read that one should leave tantalums in non-power parts of a circuit alone, so that's why I did that.

> I've replaced about half a dozen genuinely faulty electrolytics in 25 years.  I replace a dozen leaky disc (or more usually surface mount block) ceramic caps a week…

But how does one accurately measure them without pulling them completely off the board?

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