Troubleshooting a microwave oven using the Divide and Conquer Method
Detailed Countertop Microwave Oven Diagram
Detailed Built-in Microwave Oven Diagram
Lots of times, when you're troubleshooting a microwave problem, it's helpful to figure out early on whether the problem is in the low voltage section or the high voltage section because this narrows the problem down considerably. Professional Appliantologists call this the Divide and Conquer Method of Troubleshooting and it is a very powerful technique indeed. At this point, I'll turn it over to Academy Fellow Budget Appliance Repair (Willie), Professor Emeritus of Appliantology, to explain this sublime troubleshooting kata:
One way to test this is to remove the ?black (L1) power wire from the high voltage transformer, or both wires going to the the primary 110 volt side of the high voltage transformer if you can't figure out which is the line and which is neutral, (be sure to tape them safely away from touching anything).
Then turn the microwave on and it should startup and count down and cooling fan should run, just won't do any heating, (by disconnecting the high voltage transformer you have completely eliminated the high voltage system from the picture). If this works OK then you know the switches and complete control circuit is OK.
If the test with the high voltage system disabled above works, next reconnect the transformer and disconnect the two wires going to the mag tube, (make sure to mark the terminals they come off of), and make sure they are safely away from everything and give it another try. If again it works without blowing the fuse you can be pretty sure you have a bad mag tube, even if it doesn't show as a directly shorted mag tube when checking continuity from the terminals to the mag tube case.
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Source: GE Spacesaver JVM1441BH04 Microwave continually blows fuse after slamming door