Discussion about the Korg PolySix synthesizer

I am in the middle of restoring a p6 I aquired recently and have been unable to troubleshoot the cutoff not working. Nothing effects the cutoff (EG, KBD trk, etc) I have traced the +-14v signal from the pot on the face of the synth to pin 2, ic13 on klm366. Up to that point everything works as I would expect (the voltage varies from -14v to +14 as I turn the knob from low to high). After the first opamp stage of ic13 (pin 1) the voltage jumps dramatically when I turn the knob from -13 volts on the lower half of the turn, then jumps to +14 on the upper half of the knob turn and next to nothing in between. I have replaced ic13 and that didn't fix it. Could it be the thermistor th4 after ic13? Any help would be appreciated! Thanks!