[sdiy] VCO oscillation problem

[Peng]

I'm having a problem with my VCO 4E from EFM. Assembled and looks nice but the wave outputs all produce the same high-pitched screalling. It responds to keytracking and LFOs as cv mod. Tried the trimmers: couldn't bring the pitch down enough. Quadruple checked pcb, wiring, solder joints, etc. Started poking my finger around the board ( as a beginner this is my scientic troublshooting method ) , Got to the op amp that sums the cv inputs and touched the capacitor in its feedback loop and the pitch changed slightly. Tried attaching a bigger cap but this had only a very small effect. My best guess is a problem with the feedback loop. Any pointers, suggestions, or links would be of great help.
 
Thanks,
Peng


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