While continuing to investigate my malfunctioning Polysix, I inspected the leftmost control board, the one which has a blue connector soldered to the power supply.
If you follow that trace, you may find a break in the line somewhere as I did. A break was also found in the same board of the spare Polysix (now being parted), and also from the Polysix of a member on French forum anafrog.
I repaired both boards.
Yash