First let me say that yes, I've replaced R63 with a jumper. Everything seems fine (video, sound), but it won't recognize my SIO2SD. Actually tried multiple SIO2SDs on it without success. Tested those SIO2SDs on other 1200XLs and they work. I swapped the PIA (6520) and not change. I swapped the POKEY and no change. I swapped the 6502C and no change. The LCD on the SIO2SD lights up and I can see the file in D1: on the SIO2SD LCD display, just no load. The fact that the SIO2SD displays both lines on the lighted LCD display tells me its getting power. It powers up pretty reliably, but always as if nothing is plugged into the SIO port. Once in 20 tries it did boot to black screen.
I would be greatly appreciative if someone has any guidance on what might be causing this. I've modified and fixed scores of Atari's over the years, but never encountered this particular issue.