Noob 800XL Questions...

[RobFromOak]

On 8/12/2019 at 10:12 PM, shoestring said:

Without me thinking about this too much, if your machine is socketed then try reseating or swapping the two 4051 analogue keyboard multiplexers ( C4051BE ) and see if the problem moves to other keys. That's usually the most common fault after the mylar and the keyboard followed by the POKEY ( much less likely ).

 

 

 

 

Thanks! I had a few issues - at first, none of the keys except function keys worked. Took apart the keyboard and noticed some of the traces on the mylar membrane looked suspicious. Replaced the membrane and it helped restoring half of the keys. I searched for the Atari service manual (http://www.atarimania.com/documents/Atari 800XL Service manual.pdf) which identified 3 potential suspects - one of them exactly what you suggested. I did the swap of the two multiplexers and boom! Just ordered replacement chips.

[RobFromOak]

On 6/13/2020 at 12:52 AM, RobFromOak said:

Thanks! I had a few issues - at first, none of the keys except function keys worked. Took apart the keyboard and noticed some of the traces on the mylar membrane looked suspicious. Replaced the membrane and it helped restoring half of the keys. I searched for the Atari service manual (http://www.atarimania.com/documents/Atari 800XL Service manual.pdf) which identified 3 potential suspects - one of them exactly what you suggested. I did the swap of the two multiplexers and boom! Just ordered replacement chips.

I have an update - I got new chips made by Texas Instruments (CD4051BE) yesterday - not recycled parts. Replaced both old RCA ones and everything is back to normal now.