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[TI99/4a] Keyboard issue

Kehvarl

Member
Joined
May 15, 2022
Messages
30
Location
Phoenix, AZ
I've given my 4a a good cleanup, built a cable and got it running. Now I have a strange and intermittent keyboard issue.

At some point during typing, the entire 2,W,S,X,.,L,O,9 column of keys will stop responding. If I press 9 a few times, that will usually restore functionality for a random amount of time. Also, the 9 key when used to type open-paren "(", will often produce 2 parentheses instead of 1.

I disassembled the machine again to check the keyboard connector, but there don't appear to be any bad solder joints on the keyboard's ribbon cable nor the pins on the motherboard side. I sprayed some deoxit into the connector, but the pins all looked to be in great condition. The key switches all seem to be soldered in properly too.

Any suggestions on the next thing to check?
 
Thank you both for the advice.

I disassembled the machine again last night, and started testing traces and pins with my continuity meter. Everything checked out both on continuity and to a visual inspection.

When re-assembling the computer, I noticed that the plastic shield that insulates the solder pads on the keyboard from the RF shielding had a couple of tiny holes in it. I added some kapton tape, and re-positioned the shield slightly, and now the keyboard is functioning properly.

Out of curiosity, how many keyboard variants are there for the TI99/4a? My two machines both seem to have key switches that look similar, but feel very different, and the keyboard PCBs themselves look different:
  • The working machine has a multicolored ribbon cable, and the PCB has a green solder mask which has some crinkling due to age.
  • The unrestored machine has a PCB with no solder mask, and the ribbon cable is grey.
  • I've also seen restoration videos with a membrane keyboard which is different from both of mine.
 
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