Hello everyone!
I posted on here possibly over a year ago, but unfortunately I can't find my old post to resurrect it. Basically I was attempting to fix an 'S' key on the Apple II Plus which was giving no output at all and in the process of doing that I plugged the keyboard back into the motherboard with the ribbon cable upside down most likely frying something. I was told by some that it was the daughter encoder board that needed to be fixed or replaced. Some keys give their correct output, and then many other keys either don't give an output or the wrong output. For instance the letter 'I' would work, and it would also come out from a different key. I had swapped the encoder board twice to no success. I believe it's an issue on the motherboard now. Does anyone have any idea how I can tackle this?
Thanks!
If you ruled out other causes (keyboard/encoder/broken traces) and are sure it's the motherboard, and you can read some keys, but not others - then it's most certainly related to the two 74LS257 (B6 and B7). They are responsible for connecting the data bits from the keyboard to the CPU's data bus. If one of them has some "stuck bits", then some keys will be incorrectly read.
The 74LS257 are cheap to replace. If it's not them, then it's not on the motherboard and you need to go back to investigate the keyboard.
Thanks for replying!
Mine say 74LS257AN 8122, and I only seem to find them on ebay. I tried the microcenter website but nothing comes up. The only thing is that the ones on ebay aren't 8122, they are 8117 like this one:
https://www.ebay.com/itm/193482021503?hash=item2d0c6d5a7f:g:-asAAOSw1s9ey~qa
Does it matter if the last number is different?
Thanks!
The "74LS257" matters - that's the part number. "8122" is the date code - meaning "produced in (19)81, week #22". You can ignore the production daten when ordering a replacement - unless you are a very picky collector. :-)
You can also try to swap the two 74LS257s in your machine (B6/B7). They are normally identical. If the behaviour changes when swapping them, you can be 100% sure that at least one of them is bad. If nothing changes, then you still don't know more: they could both be bad in the same way (or they are good after all and problem is still be with the keyboard itself).
OH MAN! I swapped them and got a big difference. Still some keys don't work but a majority came back to normal. You are awesome! The only interesting thing also is that when I swapped them, the screen flickers after each time I press a key. I'll await for the new chips I just ordered. You rock! :)
Edit: It looks like most of the keyboard came back, except for the space bar and maybe the number 1 and another here and there. I wish I could fix the 'S' key though, it seems completely dead since I got it, but the traces look fine from what I saw. I didn't see any noticeable cracks from it. I even desoldered it and it examined the key itself and it looked fine. It must be somewhere on the keyboard in the traces, but I don't know how to find it. I have a multimeter, but I'm clueless as to what to check. Continuity maybe?
Ok, so that confirms one of the 257s is bad. Indeed those two chips also connect to the latched video data bus. Normally they only provide read access, but when one of them was bad, it might also interfere with the video data. Well, you'll have to replace them anyway - that should improve things a lot.... :)
If 'S' was the only key which didn't work at all (doesn't trigger a keypress) then it's likely the key switch itself. Did you check with the multimeter the resistance/continuity between the two switch contacts when you press the key? Obviously you would measure the resistance while the machine is not powered...
I recall swapping the 'A' key with the 'S' and noticed that when the 'S' was in the 'A' position it worked fine, but the 'A' key switch didn't trigger anything when it was on the 'S' position. That confirmed to me that the key switch itself isn't the issue. Something on the board was the issue.
I did just now take my Fluke multimeter and touched the contacts on a few key switches. I first did it by doing continuinty tests and when I pressed down on the A key, I got a sound, and the other keys gave sounds also, but no sound came from the 'S' key. I then switched it to the Ohm's for resistance and I was getting some readings from the other keys switches and nothing from the 'S'. It's dead, but what do I do now? Now that I know there's nothing flowing between the two pins?
disregard this post.