CEM3330 VCA oscillating

Ingo Debus debus at cityweb.de
Tue Mar 7 16:51:21 CET 2000


it seems another CEM chip has died in my synth. After having it not used
for several months, I turned the synth on and - squeeeeee... short
positive pulses, about 400Hz, 13 volts amplitude. It's the chip, not
anything else on the board; I swapped the 3330 with another one from
another board. Anyone else experienced this yet?

The circuit is much like the one in the datasheet, but there's a 1 nF
cap from pin 12 to pin 15 and from pin 2 to pin 7 (this was on the board
- an old Doepfer kit from 1982); and a 10nF cap in series with a 1k
resistor from pin 13 rsp. 4 to ground (this was a mod I had done myself
back then - can't remember why). Only the "linear" control inputs are
used.  The output buffer is a TL072. The other four boards/chips I have
(same circuit) are still working fine.

Both VCAs in the 3330 are used: one as audio VCA and the other one to
control VCF envelope amount.

As the 3330 is no longer available, perhaps I can make a piggy-back
board with a 13700 on it? But there's little space left, and I don't
like to make five of these boards. Are there any chances a 13700 behaves
like a 3330? This is a polyphonic synth, the VCAs should be similar at least.


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