I recently purchased an old Atari 800XL off eBay. The machine was untested/as-is and not guaranteed to work, but it was cosmetically in mint condition with original box, manuals, and even the original sales receipt! The only thing missing was the original power supply.
After receiving the unit and doing a little googling for 800XL pinouts, I was able to scrounge up a 5 pin DIN plug in my junk pile and wire up the +5v. Now the original connector is 7 pin, with 3 pins tied to ground, 3 pins tied to +5v and 1 pin tied to shield. I am only providing the +5 on 1 of the 3, and likewise the ground is only connected to 1 of the 3, but I can't see how this could be remotely related to my actual problem. The machine appears to work just fine, but I wanted to mention this in unlikely case it has any relevance on my actual problem.
After writing the obligatory "hello world" in basic, I noticed the letter A on the keyboard does not work. I can print the letter A to the screen from basic with a
chr$(65), so I know the computer/character generator/etc. are all seemingly groovy. But pressing the A key by itself, or with the Shift or Control, does not create any character input to the computer. Further, every key on the keyboard makes that distinct ATARI 'click' on the audio except for the A key.
I found this thread Atari 400 Faulty Keyboard - Some columns work, most do not and a few random newsgroup conversations which all suggest swapping U24 and U25, which are the 4051's used (I assume) in decoding the keyboard matrix. But the symptoms described generally involve more than 1 key acting weird.
After taking the 800XL apart (wow, crazy shielding Atari!) I discovered the machine has a Rev D motherboard and the 4051's are soldered, so I cant do any swaptronics troubleshooting to see how swapping the chips effects the problem.
I have also read anecdotally that the flat keyboard membrane sometimes simply needs a good cleaning, but looking at the 18 screws in the keyboard, I'm a little gun shy about just ripping it open.
Can anyone advise me on how to proceed? My soldering skills aren't horrible. I could probably get the 4051's off the board and replace them with sockets, order some new ones (?) and go down that road... Or I could dive into disassembling the keyboard and trying to clean the 'A' keys membrane.
I welcome any experience or input. My gut says (because it's a single key) that it's mechanical in the keyboard. But I'd like to hear someone else come to the same conclusion...