Need help finding the cause for a feedback loop

[workbench]

Hello Everybody,

this is my first post.

I ported the hihat section of a Roland TR606 for Eurorack synthesizer to a PCB.

At the first test, the Hipass Filter began to oscillate on the last quarter of the potentiometer (CW). It lies in the area B6 in the attached PDF.

I have already tried to add 220pF capacitor in parallel with the potentiometer, unfortunately unsuccessful.
Maybe you can give me a hint why it could be.

Thanks

Markus

[dmills]

Check the value of R48, 52-55.

You have an opamp configured for a gain of -2, and some positive feedback. To a first approximation if the gain between the opamp output and its non inverting input exceeds a magnitude of 0.5 you are asking for the thing to take off.

Looking at it I am having a slightly hard time seeing how it could be stable, the analysis is not straightforward, but I am seeing a LOT of positive feedback when only R53 is in play. Check your values, I suspect a dropped decimal point somewhere or other.

Incidentally, if that little cap was going to help stability it should have gone across R54 IMHO.

[workbench]

Thanks a lot.
I swapped r53 for 12K. I have no oscillations anymore