Discussion about the Korg PolySix synthesizer

The multiplexer/demultiplexer thing:

Correction voltage from VR1 enters IC39 X7. This voltage must come back
from IC31 X7 through IC18 at the bottom left of your schematic.

The two controlling signals for this seem to be MUXSELA for the Offset
correction (X7) and INH for the other correction (X6 to optocoupler).