Do you know anybody in the UK with a working 800XL? Maybe spend an evening swapping parts as you appear to have a fully socketed machine. I was able to fix my 600XL by swapping chips with an 800XL to determine that the 600XL must have been subjected to a bad power supply that killed multiple chips (CPU, Pokey, PIA, & RAM). If not, perhaps FJC can fit you into his repair queue.