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I've been cleaning up an Atari 400 that had a bad RF cable, and cleaned the machine thoroughly so it's looking great. It's working well apart from the keyboard, where most of the keys aren't working: some columns on the far left work (Q, A, Z and W, S, X) but almost everything else doesn't, and backspace even acts like reset.

Is this likely to be an issue with the membrane or is it more likely that there's some cracked solder joints / faulty components on the main board? Is there a good way to test and diagnose the keyboard outside of the machine?

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  • With what did you clean it? I really hope it wasn't water or some liquid. – Bálint Jun 5 '16 at 11:33
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    You did not mention whether the Atari's keyboard worked before cleaning - this is to determine if your action actually caused the problem, made it worse or not. Also, I think you should expand the subject a little, "faulty keyboard" is a bit generic. – Kuba Tyszko Jun 5 '16 at 11:47
  • I should clarify I didn't clean the keyboard apart from wiping the front of it with a damp cloth. I disassembled the machine so I could clean the plastics. I actually don't know the state of the keyboard prior to cleaning (a mistake) I'd opened it up to replace the RF lead and from there decided to clean it while it was already open. – Matt Lacey Jun 5 '16 at 12:26
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    One advice I can suggest - assuming Atari has simple matrix keyboard interface, you could try booting it up without the keyboard and simulate keystrokes with a jumper wire on the keyboard connector (according to the matrix and connector pinout). This would let you rule whether it's the keyboard at fault or something in the actual computer. – Kuba Tyszko Jun 6 '16 at 11:13
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    Well I did that and generated plenty of the 'broken' key strokes, so I guess it is the keyboard. Will leave this un-answered until I've played with the keyboard itself and found a way to test it. – Matt Lacey Jun 14 '16 at 2:22
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Chip checking time! Pull, inspect, and re-seat the 4051s (two of em, the small chips near the center of the board) and the POKEY chip on the motherboard. Hopefully they are socketed and not soldered, I'm not sure how consistent Atari was with those.

If that doesn't resolve anything, and since some of the keys do work, try swapping the two 4051s and see if a whole new group of keys is not working (each controls half the board). If yes, bad ICs. Lucky thing is, they are cheap and CD4051 ICs can be found on ebay and the usual electronics shops. Socket the new chips if they aren't already.

If the keyboard itself appears to be bad, Best Electronics and B & C ComputerVisions sell replacements.

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    Any hints/tips on testing/repairing the membrane itself? I should have edited the question by now but I have narrowed it down to the actual keyboard. Still up-voted as I had heard the 4501s can be problematic! – Matt Lacey Jun 16 '16 at 7:27
  • I haven't tried a repair of the membrane kb itself, but I'd look for broken traces that happen to be in common with the keys that arent working, especially as it's happening in columns and not scattered specific keys. Let us know if you resolve it, curious to hear what the fix is! – driph Jun 20 '16 at 0:03
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    "Pull, inspect, and re-seat the 4051s" Worked perfectly for me and my new to me MINT atari 400. Thanks as i was starting to assume the ribbon cable was the culprit. Now to get a belt set for the 410, an SIO cable for the 810 and a sio2pc/usb. 8 bit heaven. – CanTom Apr 21 '18 at 16:52

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