I've had a couple where I had to remove the connector header from the board,
clean the bottom of the header and the board, then solder it back on. That
was a real pain but it fixed the problem when doing the jumper didn't help.
Apparently there was some leakage from pin 1 to pin 2 of CN06 underneath the
header.
Don Backshall
_____
From:
PolySix@yahoogroups.com [mailto:
PolySix@yahoogroups.com] On Behalf Of
klosmon
Sent: Saturday, February 16, 2013 1:31 AM
To:
PolySix@yahoogroups.comSubject: [PolySix] LFO problems again
I'm currently repairing a battery-damaged CPU board, and I've just about
got everything working again -- except for the LFO bleedthrough into the
VCO modulation circuit.
I've repaired over three dozen of these things the past few years, and
come across this problem repeatedly.
I was able to solve it several times in the past by cutting the circuit
board trace at CNO6-2 and at the outside end of R9, and joining them
with a jumper (taking care to move C43 back into the circuit). This
bypassed the parts of the circuit board that caused the LFO signal to
bleed into the modulation circuit even when the mod wheel was fully down.
In the case of this board, however, that procedure isn't helping --
there's still LFO modulation audible on the VCO (and visible on the
scope at R9). I though it might be something from the other LFO mod
path from the front panel mod switch (through ICs 1 and 2, finally
through R11), but grounding that signal doesn't stop the mod effect.
And, it's only appearing at the VCO mod circuit; no sign of it at the
VCF or VCA.
Thinking it might be something on the front panel, I swapped in a
working CPU board, and the problem went away; obviously the fault is in
the first CPU board.
My eyes are starting to blur going over these schematics; has anyone
here dealt with this problem successfully?
If so, I'd love to hear about it.
Thanks.
~GMM
[Non-text portions of this message have been removed]