My GE model mc0165uw above range oven blows its 20amp ceramic fuse when i close the microwave oven door. I have checked the continuity of all 4 door interlock switches, and as far as I can tell they check out ok. What would be the next troubleshooting step here before heading to the pawnshop for a new one.
Years ago I had experience with helping an electronics shop owner next door to my office fix a similar problem. The problem turned out to be alignment of the micro switches which were all on plates with adjustment slider holes. On his unit the door interlock micro switches had an interesting contact arrangement with wiring such that when the switches sequenced incorrectly a direct short would be imposed across the power line through the fuse. This would cause the fuse to blow and render the unit non-operational. Best we could determine it was designed as a safety interlock system. It turns out that we guessed correctly.
It is a requirement that microwaves have an interlock monitoring system to pop the fuse if the switches are not operating correctly. There is a good description here: