Discussion about the Korg PolySix synthesizer

If you have an oscilloscope, monitor pins 1, 2 and 3 of IC6 when calibrating the DAC
Show us the waveforms.  This is the comparator
that compares the DAC voltage to the voltage of the selected pot for ADC conversion.

If that chip is socketed, you might try replacing it.  Also, maybe replace the one you 
said was hot to the touch.