Getting closer (see below)! Unsoldered and socketed IC5 & IC6 and checked continuity thoroughly.
Noticed a few things along the way: I thought all Electrolytics caps on the right side were okay, but hiding beneath the negative leg of C19 were some greenish residue and dust... Didn't have any spare 0.33uF/50V in my box so used the next closest one: 0.47 uF/50V Nichicon. Noticed that on clone boards, a tantalum is used. I may have this, need to check values with those cap codes.
Each of C1 and C2 (new caps I had to put in) had a pin unsoldered today, so had to redo those as well.
I tested the resistance of R17, the resistor array and two pins were giving me infinity. Reflowed and re-tested. Fine.
Now, on powering on the Polysix with IC6 and IC5 swapped, I get all LEDs ON, and this won't change no matter how long I wait.
Swapping back the previous IC6 in its IC6 socket reverts the Polysix to its previous functionality. It powers on with one voice activated, but I guess that's just the Reset/VR1 calibration.
Therefore, IC5 is most probably bad. Maybe IC6 also has some issues. Too bad I don't have any spare LM393! I saw a few ones on the voice boards, but I'm loather to use those. I don't want to touch the voice board yet if not absolutely necessary.
Next I will test each of these LM393 comparators on my small breadboard to pinpoint where the issue is.
Onward!
Yash