800XL not responding to keyboard input

[+hueyjones70]

I have an 800XL that does not respond to keyboard inputs for ESC, and the 1-6 keys.  I thought I had a bad mylar but I tried two different mylars and a mechanical keyboard.  When I used the mylars and the keyboard on a different computer, everything worked as it should.  Where do I start my search for the problem?

[+DrVenkman]

Swap the keyboard mux chips (the two little 4000-series ICs) and see if the non-functional keys change.

[Colleton]

Look at the two keyboard mux/demux chips just to the left of the keyboard ribbon connector.  Try swapping them and see if the fault travels.  I had a keyboard issue with my 1st 800XL and it ended up with both chips being bad.

 

https://www.digikey.com/product-detail/en/texas-instruments/CD4051BE/296-2057-5-ND/67305

 

At least they're cheap.

[erichenneke]

Check the connector that the ribbon cable plugs into.  Maybe some debris, dirt,  gunk or oxidation or something only on that contact point for that trace preventing a connection there?   

 

Just trying to think of the simple stuff first to rule out.

[+Nezgar]

Does the red LED turn on when you turn on the computer?

 

Is it one of the mechanical keyswitch type keyboards? If so the keys may need considerable excercise (hundreds of repeated pressing of each key) to start registering again after years of disuse.

 

I agree swapping/replacing the 4051's is the first target though.

 

XL keyboard types for reference if you can match up which yours is:

 

[+hueyjones70]

1 hour ago, Nezgar said:

Does the red LED turn on when you turn on the computer?

 

Is it one of the mechanical keyswitch type keyboards? If so the keys may need considerable excercise (hundreds of repeated pressing of each key) to start registering again after years of disuse.

 

I agree swapping/replacing the 4051's is the first target though.

 

XL keyboard types for reference if you can match up which yours is:

 

I tried one mechanical and two mylar and got the same result. I will try cleaning the contacts and check the ICs. I forgot to mention that before the unresponsiveness started, when I turned it acted as if a key had been pressed and had gotten stuck. That led to the disassembly of the computer and checking other keyboards. The stuck key effect didn't happen again.

[wildstar87]

Those are all on the same row in the matrix, I'd suspect bad contact with that particular pin/slot in the connector.

http://members.casema.nl/hhaydn/howel/logic/burched/b5_800xl_kbd.htm