[sdiy] Sequential Conundrum (no, that's not a type of drum machine)
greg.montalbano at ucop.edu
Thu Aug 1 19:07:09 CEST 2002
There is a type of Zen koan which, upon repeated contemplation, will
drive one insane; I'm beginning to think this Prophet 600 I've been
wrestling with would be a contender...
The voice board has six individual voices, each containing two CEM
oscillator chips and one CEM 3372 VCF/VCA chip. There were two dead 3372s,
which I pulled and replaced. Now, the two voices with the new chips are
responding to VCA control voltages, but NOT to the VCF controls.
OK, maybe it's a bad socket or bad filter circuitry, right?
--Wrong; if I switch one of the old working 3372s with the new ones, then
the voice will work just fine; and now the voice with the NEW 3372 is not
responding to filter control.
Ah, you say: it must be that the new chips are faulty.
-- Nope; I built a test circuit (from the 3372 data sheet), and found that
the two new chips perform just as well as the four old ones ... in the test
circuit; as soon as I put them back in the Prophet, it's back to filter
The only other factor that could be involved is the tuning routine -- upon
power-up, before the voices become active, the Prophet runs a tune
procedure that, if it encounters any oscillators or filters that it
considers to be un-tunable, scales them such that they are disabled.
But WHY would it be having trouble tuning the new chips, if the new chips
I have heard un-documented rumors over the years about the unreliability of
later CEM chips; does anybody here have any real info regarding the 3372s?
Can anyone suggest anything else I might be overlooking here?
At this point, I'll try just about anything....
"Remember, class: there are no stupid questions -- just stupid people."
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