I’ve now got one of the four keyboards fixed. Actually I probably have mixed parts of one keyboard with another. I’ll have to make or buy more foam pads for the other three.
Two of the keyboards had foam pads with mylar disks on both sides, as I expected, and those foam pads remove easily. Unfortunately two of them had mylar disks on only the bottom side. As a result, when the foam deteriorates, it winds up stuck to the plunger, so those two keyboards will require a lot more effort to clean up.
I was somewhat surprised that even the keyboards with mylar on both sides of the foam still had some sticky keys after the pads were removed. I suspect this is due to dirt getting between the plunger and shaft. I probably need to take all four completely apart and clean all of the mechanical pieces thoroughly. I’ll have to test whether 97% isopropyl alcohol will damage the keycaps, stems, or frames.
One Sol-20 has now been reassembled with the repaired keyboard, and seems to be working fine, aside from the previously noted instability during the first few seconds after powerup. I need to look at the power supply output on a scope to see if it has a problem.