Thanks very much!
I've stripped out most RAM modules, still no joy, I will now try swapping a few of them around.
Does the machine need a cache chip in order to boot?
Can I use the CPU board from a 7500/100 (630-1219-A) ?
All the directions I find for "reset the PRAM" are not precise on what gets held down when.
Should I be holding command-option-p-r while I press the triangle key that starts the machine, release the triangle key but keep holding the other 4?
Or is it press power key and after releasing it...