Well, finally I'm home. And I've disconnected the wire harness coming from the VCO board so no signal can come from there at the filter input. And it still exhibits the same behavior.

I've also checked the +15v dc for possible ripples on the oscilloscope and it's perfectly flat, pure direct current at 15.01 volts (at the source on the power supply board). Since VR2 is biased with that same 15v dc, I thought it might have been a source of noise there, but I guess not.

I've also trimmed down the maximum resonance level a little bit (with VR3 trimpot) since I've always found that it was twice as loud as the loudest sound coming from the VCOs or noise source. That has diminished the harmonics just a little, but they're still audible. Especially around 800hz and higher.

Since this MP4 is now about 28 years old, I thought it might be components starting to degrade. I wasn't rich enough back in 1982 (I was 17) to buy a brand new MP4 then, so I've never heard its filter feedback when it was new. But I did hear an SSM2044 feedback in other circuits than the MP4, and it sounded perfectly sinusoidal with no artifacts whatsoever.

Alain.

--- In korg_mono-poly@yahoogroups.com , Florian Anwander <fanwander@...> wrote:
>
> Hello Alain
>
> I could imagine, that the output potentiometer of one (or all) VCOs does
> not "shut" the VCO ouput completely. Though the VCO itself might be not
> audible for us, the Filter on high resonance may react on it.
>
> You may try to disconnect the VCO pcb from the VCF/VCA pcb and check the
> VCF then.
>
> Florian
>