[sdiy] SCI Pro-One problem (audio leaks to CV or something)

[Matti Kari]

Hello,

my first post to the first & quick introduction first: I'm 25 years old 
rocker from Helsinki, Finland. I've been doing some electronics DIY 
before but mostly with tubes (practically some guitar / organ / electric 
piano amps). I am also planning to build my own modular synth someday, 
mostly with tubes on the signal path, etc.

Anyway, to the subject itself:

I have an SCI Pro-One which has the following problem: when the filter 
cutoff CV is between some specific value (say: cutoff pot between 11 
o'clock and 3 o'clock with no other CV's applied), signal from 
oscillator 1 and / or noise source seems to be leaking so the filter CV 
or something.

Please listen to this mp3: http://maza.fi/tmp/pro-one-noisyfilter.mp3
The filter is swept using the EG on this example.

Does anyone have ideas what could cause this? Feeding the 3320 filter 
straight from cutoff pot works fine and I've even swapped that chip, so 
it can't be that one. Also I've replaced all the resistors and the quad 
opamp that has the filter CV summing amp but the problem stays.

Any ideas?

Just a note: I've also replaced the PSU electrolytics to fresh ones and 
those on signal path were changed to plastic ones to improve reliability.

Thanks in advance & best regards,

Matti

-- 
Matti Kari   ||   Vesalantie 38a, FI-00940 Helsinki
maza at maza.fi || http://maza.fi/ || +358 40 838 6607
hammondleslierhodestubeanalogsynthlinuxinternetetc.