Is the signal actually entering IC23? Have you checked with a scope if the CV for all six voices is there? What enters pin3 of IC23 is not just the keyboard track - it's the whole sum of all Cutoff-affecting parameters. I find it not unlikely that other parameters are to blame here. A likely candidate: Envelopes.
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[vintagesynthrepair] Re: Korg Polysix dead voice
2013-06-13 by Malte Rogacki