Hi - I have a Jenn Air JMV9169BAP Microwave - approximately 6 years old - last week it stopped heating food - I replaced the Megatron - turning the microwave on to boil a glass of water, the unit went on for about 10 seconds then cut out - front panel LED screen going completely dead . I replaced the 20AMP fuse and the microwave powered up again. Turning the microwave on again, this time it powered off again this time within 3 seconds. I took the capacitor and diode to my local parts supply store to have those things tested - all ok according to the knowledgeable store folk - I bought a couple more 20 AMP fuses - to check it was not the new Megatron causing the problem - I unplugged that completely. Inserting a new fuse again and turning it on again , it again powered off again within 2-3 seconds blowing the fuse again . So I can rule out the Megatron, diode, capacitor here - what else could be causing this fuse to blow ? or should I give up and call a technician ?

Aviator,
Here's a relatively quick test- unplug the power to the transformer that feeds the magnetron. Since they will be electrically live during the test, wrap them with electrical tape.
Then, invest another 20amp fuse as a test.
If it doesn't blow, the transformer is shorting out.