Hello fellow atarians, I am having same problem with my 800xl, been fine for 30 years and suddenly it doesnt goto basic on boot up. followed your posts and its 40 ram blocks normal and 48 if hold down option. ive reseated the mmu and basic chips already and they were clean and probably didnt need it! Please let me know where to get a MMU chip replacement. Dont need to solder, my 800xl is REV A no. 034 so its a real oldie! the chips are socketed, so heres hoping to get a new mmu. Its funny, very very rare do I play cartridge games, but prob has been since I have done recently. Thinking of checking connections for dry joints ect. will liet you know! thanks again!