Ibanez AF201 repair

[Mathias]

I'm having an Ibanez AF201 on the bench. Great pedal and valuable. Everything works except on the BP (bandpass) mode.
There is just a very loud synth like frequency coming out. Anyone had his run in with a pedal like this or has a schematic laying around?

 

[Mathias]

Found schematic attached that is somewhat resembling this pedal. The switch is according to the schematic with the BP on connector pin 7 and 15.

If anyone has a suggestion for what to look for I'd be glad to hear.