There are two keywords that I noted while reading through this entire thread today: keyboard & capacitors.
While it may not be the root problem in this case, I feel it important to mention that the electrolytic capacitors used in some keyboards leak and need replacing as badly as the caps on many logic boards (such as the SE/30 logic board).
I myself had many troubles with a IIgs keyboard attached to my SE/30. Sometimes I would tap the keys but I wouldn't get a response on screen. But after powering off (for safety) and then disconnecting and then reconnecting the IIgs keyboard to my SE/30 and then powering on, the keyboard worked fine. In the past, I had opened up the keyboard and fixed some cracked traces, cracked because the ADB ports themselves were merely soldered to the PCB without any strain relief, hence anyone pressing in the cable too hard could apply undue pressure on the powers, which in turn ultimately cracked the traces on my IIgs keyboard. But even after I had fixed those traces, I would still get the same problem from time to time, where I would tap on the keyboard but nothing would show on screen.
Last week I opened the keyboard again and examined the two caps with a magnifying glass. Sure enough, they had leaked. I swapped them out with tantalum caps and found that all is well now. (I also used some hot glue on the back side of each ADB port to eliminate any future trace breaking.)
So I don't know if capacitors and/or leaked fluid is playing a role here. But in my experience, it likely is.
While it may not be the root problem in this case, I feel it important to mention that the electrolytic capacitors used in some keyboards leak and need replacing as badly as the caps on many logic boards (such as the SE/30 logic board).
I myself had many troubles with a IIgs keyboard attached to my SE/30. Sometimes I would tap the keys but I wouldn't get a response on screen. But after powering off (for safety) and then disconnecting and then reconnecting the IIgs keyboard to my SE/30 and then powering on, the keyboard worked fine. In the past, I had opened up the keyboard and fixed some cracked traces, cracked because the ADB ports themselves were merely soldered to the PCB without any strain relief, hence anyone pressing in the cable too hard could apply undue pressure on the powers, which in turn ultimately cracked the traces on my IIgs keyboard. But even after I had fixed those traces, I would still get the same problem from time to time, where I would tap on the keyboard but nothing would show on screen.
Last week I opened the keyboard again and examined the two caps with a magnifying glass. Sure enough, they had leaked. I swapped them out with tantalum caps and found that all is well now. (I also used some hot glue on the back side of each ADB port to eliminate any future trace breaking.)
So I don't know if capacitors and/or leaked fluid is playing a role here. But in my experience, it likely is.