A Buddy of mine had a similar thing happen to him. This was his response to your question..
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It is probably one of two things, the diode, or the magnatron. I have included a couple of websites which show how to test each. Replacing either of these is very easy. Just make sure that whoever is doing this: DISCHARGE THE CAPACITOR!!! by shorting it out across the two poles with a screwdriver (without touching the metal on the screwdriver). The magnatron should be about $40 and the diode is about $3. I have seen websites selling these diodes for $19, but if you pay over $3, you are getting bent over. They cost pennies in bulk.
It is usually the magnetron about 90% of the time
http://www.partselect.com/microwave+...ode+repair.htm
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