[sdiy] Difference between early and late revisions of the CEM3340?

[Tom Wiltshire]

Hi All,

Does anyone know what the differences are between early and late revisions of the CEM3340?

I have a customer who is trying to fix a dead oscillator in an OBXa. He’s replaced the faulty CEM3340 rev.C chip with a modern Rev.G which fixes the dead voice, but the voice won’t tune. Swapping in a Rev.C from another voice cures the problem, so we’re pretty sure it’s down to differences between the chips.

The only thing I’ve been able to find is a Analogue Heaven thread where someone mentions a note in the Sequential Circuits documentation about reducing the value of the resistor on pin 1, but they don’t reference where this is found:

> "Modification To VCO Circuits
> For correct scaling, all Sequential synthesizers with Rev G CEM3340 VCO chips need the 26.7K 1% resistors (part #R-146) in the VCO circuits to be replaced with 25.5K 1% resistors (part #R-516).  This change MAY affect tuning of Prophet-5, -10, -600, and -T8s.”


(From http://analogue-heaven.1065350.n5.nabble.com/CEM3340-revG-reissue-td135312.html)

This isn’t a massive change (-5% or so) but it was obviously important enough to warrant the effort. Oberheim had wads of Engineering Change Orders for a lot of their stuff, and I’ve checked the ones I have but haven’t found anything relevant to VCO chip revisions.

Does anyone have any clues?

Thanks,
Tom

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       Electric Druid
Synth & Stompbox DIY
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