So I'd start with the Cutoff Pot contacts at the back of the front panel circuit and test with a Voltmeter if the pot itself is wiping correctly. I believe it wipes -5V to 5V when testing the centre pad.
If that's fine, then perhaps testing the INH A B C Data lines for pulses with a Logic Probe could be interesting.
You have to make sure your main KLM-367 board isn't corroded too. Is there a new battery on it, any signs of blackened traces, dull greenish/grey solder pads?
Another easy thing to try would be to just remove connector by connector and clean them up thoroughly (both ends, both contact points), and re-seat them all firmly but carefully.
Reflowing connector pads can also be worthwhile, and it's not difficult at all.