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So there are actually 2 more fuses in addition to wthat 20 amp inline fuse. One is mounted on the side of the magnetron which is identified as part 10 in the diagram below.
The fuse is a dime sized round device that is silver and black. Unplug the oven and test continuity through this fuse and if it's open, you'll need to replace both the magnetron as well as the magnetron fuse.
The other fuse is also a small found dime sized device and it's mounted on the top of the microwave above the microwave oven cavity. This is your cavity thermal fuse and it tends to fail if the microwave was ran without anything inside or something caught on fire so it overheated and shut the microwave down. If this fuse is open, replace it and you should be good.
Now if both of those fuses as well as the 20 amp in-line fuse all test good, you'd need a new display circuit board I'm afraid to say--this is assuming you're getting power to the microwave from your house.
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