The Atari 800XL has two main custom chips, the GTIA and the ANTIC, both of which are getting harder to get hold of – unless you want to pay premium eBay prices. After more investigation, I decided that it was probably one of these that was the cause of my problems. Searching online, it looked like the best way forward was to buy another complete unit with known working chips; and, after a few weeks of eBay lurking I won one that worked but was missing the keyboard membrane.
Now the moment of truth, I swapped the ANTIC chip first, and the original Atari booted into life. For good measure, I also tried switching the GTIA to find the one with the best video output.
Both motherboards are the same Rev D version, but the latest one has a capacitor mod on the back which appears to be something to do with the RF output. The original motherboard now has almost all chips in sockets so I decided to keep this as the main board.
With all the inbuilt tests passing, I did the normal full clean – the keyboard was disgusting as usual.
Interestingly, the two units have a completely different keyboards, one has standard switches the other a membrane keyboard? Anyone know why?
The unit turned out very nice, and I have a mostly complete spare.