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Spent some more time with the control boards: socketed the 4501 so that I can swap more easily. Tested with the new 4501 and it does look like I get control over the pots more easily than before, but it is still flaky.
I checked all the D and P lines with the Logic Probe, ensuring that I get a pulse everywhere, so that's good.
CPU-P1 does not provide a pulse but a varying level now.
Did calibration again, this time making sure that EG intensity was at 0 and not at Min (an easy mistake to make!)... Of course, not having an oscilloscope blocks the total calibration. I tried using my Arduino for that but the oscilloscope which did work does not give measurements precise enough. On the other hand, I could see the waveform when adjusting MG.
Based on the schematics, I gather that all the pot values are multiplexed by the 4501s on the control boards, then sent through CN11 as INH | A| B | C | DATA to KLM-367 so that the CPU uses them as well as the 4501s at IC18 and IC19 to Demux them, and there's a comparator involved too.
I don't have enough new 4501s to swap them all, which might have been cool, so I may have to test them individually in a circuit.
It would be interesting to know which end points I can test by either the Logic Probe for a Pulse Train or else with the Digital Voltmeter.
Yash