Hi,
What happens if you ground the signal CN06-2?
I am not sure what you mean by "it is visible at R9". Does this mean
that you can see the LFO wave at R9 (outside end) with the mod wheel
turned down completely?
What happens when you short the mod wheel wiper to GND?
What happens when you remove KLM-369 / R16?
Try cleaning the PCB.
Check again that the cuts you made to the PCB traces completely
separate.
Good luck!
Johannes
On 2013-02-16 07:30, klosmon wrote:
> 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
>
>
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