I have a fully socketed 800XL mb. It doesn't boot up, meaning get any display or the pbllllt sound when turned on.
It flashes a white line when turned on and has a three very thin lines, red, green blue at the top of the screen, on a grey, not quite black screen.
I know U2 is good. I think I've swapped chips before, just getting back to trying to fix it again.
At one time, I tried to get a keyboard socket off of it and messed it up. Today I would know where to buy that.