12V is coming in to the CPU board at 1J17 I think, near the battery and the sound diagnostic switch. I think the two pins closet to the batteries are 12V, furthest out is +, next one in is -.
I've got a Space Shuttle with similar problems. But on mine the 7 never goes away, and I've ruled out the PS as a problem. Verified the ROMs (which is the other thing the repair guide says could be wrong) and I've found a bad chip at U3. Have to get a replacement and see what happens from there. If I'm reading the schematics right (still new to this), U3 tells U23 to pass the ROM data to the CPU, which would account for a ROM error if it didn't work.