Korg Polysix w/ Kiwisix resistant to tuning

Hello experts,

I’ve been shoveling money into getting a beloved Korg Polysix into full working order for many months now. I’ve paid a tech (whom I believe to be competent) to transplant the synth into a new custom housing, install the Kiwitechnics Kiwisix mod and the Kiwi power supply upgrade, and then attempt calibration and tuning. Unfortunately, the VCO tuning has not been successful.

Here’s what we’ve attempted:

  1. After centering VR1, VR2, VR3, VR14 and VR15 per the tuning procedure and confirming that TRANSPOSE feature of the KiwiSix is OFF/at 0, and ensuring that the “Stretch Tuning” Switch is set towards the rear of the instrument, VCO0 couldn’t be tuned to pitch according to the front panel tune knob. Even with VR11 of VCO0 fully CW and VR2 set fully CW, the highest pitch reached was about 2041Hz (keyboard C5, goal pitch C7, 2093Hz).
  2. Attempted to calibrate VCO0 to be in tune “ratiometrically” so the synth is in tune with itself, but it fell sharp compared to standard tuning: I centered VR2, set the front panel tuning knob almost fully CW, and calibrated VCO0 to be 2093Hz using VR11. Then I attempted to calibrate keyboard note C2 of VCO0 using VR1. Not successful. Even with VR1 fully CCW, the highest pitch reached was 238 Hz (goal 261 Hz). Going back at this point and re-adjusting VR2 to ensure 2093 Hz at keyboard note C5, then returning to try again tuning keyboard note C2 showed similar results. VCO0 can not be scaled properly. VCO1 tuning/scaling was attempted in a similar manner. The results were almost identical to VCO0.
  3. I implemented the following suggestions from Murray at Kiwitechnics, but the VCO tuning results are virtually unchanged and still unsuccessful:
  • Confirmed that the front panel tune pot is sending the correct signal (-5v to +5v) to the 4051 P4, and that zero volts is output near the center of this tuning knob.
  • Confirmed that the optocoupler labeled PC-1 is correctly between -2v and -6v (according to Murray, the photocell functionality is fine based on the fact that the measured voltage is in range and that the VCOs generally follow the V/Oct control voltage from the KiwiSix PCBA).
  • Checked the ENV RELEASE and it was at 5.07V, when the REL front panel knob was set fully CW, so I adjusted the Kiwisix trimpot until it was 5.00 V, and attempted to tune VCO0—no significant improvement.
  • Replaced IC31 and IC39, and attempted to tune the VCOs—no significant improvement.
  • Finally, a 2nd voice board was temporarily installed in the unit, and the VCO tuning issues were nearly identical to the first board.

Any suggestions, wisdom or guidance for next steps would be so appreciated!!