If I have this correct, the 1200XL has a +9v power supply and through some magic is supposed to step it down to +5 volts?
Except everywhere I expect to pick up +5, I get 10. Like on R63, which you should replace with a jumper so the SIO get's the full 5 volts. R63 for me reads 10.
Another place I read was the top of C11 should get me +5, but I show 10.
Oddly enough, everything works fine on the 1200xl. So I'm not sure what's going on.
What does your 1200xl show?
Frank