I have the same issue with my GE Advantium. I have no shorts in any of the magnetron, diode, capacitor, door switches (3 of them and one on the magnatron), tempswitches (3 or 4 of them), high voltage transformer. I have looked and used my multimeter to see if the control boards have continuity through out the boards and don't see any solder that looks burned/shorted out. Some discoloring but I think that is just because of its use throughout the years (no true burn marks).

Any idea's.

I am going to try to replace all the door switches anyway because it seems that is a tell tell sign of fuses blowing when trying to start up the microwave part of it.

Just FYI. I can use the convectional oven and heat lamp all day long and it doesn't seem to want to short out the 20A fuse. Just when I hit the microwave quick start or timed cooking using the microwave function to heat the food.