Troubleshooting and reparing an Atari 800XL with a single key not working

Question

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...

Answer 1

In the end, I decided that it had to be easier to take the keyboard apart than to de-solder the 4051's and so I carefully removed the 18 tiny screws from the back of the keyboard. It wasn't nearly as bad as I expected (I was concerned that I was going to have springs everywhere). After disassembly I fired up the Atari to try and see if I could get the A key to work by pushing directly on the membrane, which did indeed work as expected. So I turned off the machine, swabbed the membrane contacts with alcohol, let everything dry thoroughly and reassembled the keyboard. The A key is now working as expected. The problem was obviously purely mechanical.

Answer 2

I've found that the old Atari keyboards can lose conductivity and that pressing they key repeatedly, not necessarily with a lot of pressure, can revive them. I've had Apple II keys start working again after pressing them a hundred or two times.

Assuming the above isn't the issue, if the rest of the keys, particularly in the same row and column work (not based on the geography of the keyboard, but of the circuit underneath) then the problem is most likely the mechanism of that one key. In theory you could try swapping the mechanism from another key to prove that fixes it but I'd probably just order a new mechanism.

Answer 3

After finding this community just in the last couple days, some of these questions bring back great memories from my younger days. I used to repair the ATARI computers and did modifications to bump up the RAM in the ATARI computers. I remember opening the keyboards to do cleaning on the membrane. Just don't rub too hard or use any heavy cleaners on them as you will remove the silver off. If you get some silver corrosion use a pencil eraser and gently rub the contact point. Don't bother with the traces connecting them as they are thin and can be easily broken. use a multi meter to test continuity along the traces as they did seem to break causing key outages.

Answer 4

I have an 800XL that does this once in a while though the keyboard in my rig seems 100% mechanical. Others seem to be less so with a sheet of mylar in there.

Anywho - in my case I'll simply pop the keycap and use compressed air to clean out the switch. I do this a few times and magically the key starts working again.