Sublime Master Willie in the Samurai School of Appliantology dispenses these indispensable pearls of appliantological wisdom for troubleshooting a microwave oven that keeps blowing the internal fuse:

Quick n’ easy way to see if the high voltage system is blowing the fuse, disconnect the wires to the primary side of the high voltage transformer and make sure they are safely away from any part of the microwave.

Turn the unit on and if all operates OK, (except no heat of course), then the high voltage system is blowing the fuse and you would need to diagnose that section. If fuse still blows with transformer disconnected then it time to replace all the door switches.

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