So here's a head-scratcher ... I have two 1200XL's, one in fairly good physical condition, one "ugly duckling" with a horribly yellowed case, broken plasic over the LEDs, one joystick port with a broken pin. Both were working when stored away. A couple years ago, I fitted a U1MB inside the prettier one. Worked great at first but within a day or so started showing instability - settings would reset themselves in the U1MB, occasional garbled display in the U1MB menu, etc. I removed the U1MB, installed that board inside one of my 800XL's and it's worked great since.
Today at long last I decided to return that 1200XL to stock configuration - I reinstalled the original MMU, OS ROMS and replaced the jumpers back to their original locations (W7, W8 and W9). I hadn't been using my 9 VAC bricks for awhile so I plugged one in, checked it to be sure it was good (it shows 10.2 VAC without any load) then plugged it the machine and powered up. Red screen. If there's any audio pops I didn't hear them.
I then dug out the Ugly Duckling, powered it up and it worked fine - rainbow logo and all. So I started trying to isolate any bad chips. Since it couldn't seem to get running, I decided to see if the CPU was bad first. I removed the CPU from the bad board, plugged it into the Ugly Duckling and gave it a try - now the Ugly Duckling had a red screen too. So to check, I tried the known-good CPU in the first board. Still had a red screen. Huh. So I put the good CPU back into the Ugly Duckling where it came from and now it boots to a red screen too. Hmm. Both machines now boot to a red screen.
Anyone have any suggestions?