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Apple ][ Plus, Keyboard works (now) except Shift?


Well-known member
Recently, I got an Apple ][ Plus in externally decent condition, but internally not so great (lots of dirt and debris, some corrosion on the motherboard). I've replaced the motherboard (tested to work, and boots). The old motherboard has no video signal whatsoever, no beep (did beep for a bit, but then stopped). Unfortunately, the Keyboard had been plugged into it backward (something that I didn't realize until I tried using it as a guide on the new motherboard). The new board still boots. But most of the keys didn't work (mostly the letters).

Long story short, I ended up replacing both 74LS00s and the 74LS04 on the encoder board, as well as both 74LS257s at B6 and B7, along with the 74LS74 at B10. Tested the switches (all good), and replaced the ribbon cable (broken pin). Things were better after that, but there were still some keys that were nonresponsive. Then I noticed that the power light was cutting out when the ribbon cable was jostled, so I added some sockets in between the cable and the boards to give it a tighter fit. Now everything works except the Shift keys?

I should note that the Shift keys did work for a moment when I swapped back the old 74LS74, but others didn't. I don't think the IC had anything to do with it, as several keys were nonresponsive, but all the functioning keys were working fine (including Shift). I did try swapping them back, but it didn't bring back the Shift key, while the other keys that were nonresponsive remained nonresponsive.

I'm guessing that it's still a cable issue, but in case I'm wrong, maybe someone knows any other IC failures that could cause this issue? Alternatively, if it is the cable (made it myself, so entirely possible), maybe someone could tell me which pin is responsible for the shift keys? At the moment, the only source I have for a new cable would have the same loose pin issues, so if I can figure out which pin, maybe I can address the issue until I can make a new one with better parts.