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I'm sorry to have to give you some information that you won't want to hear.
You stated that you everything else works. So, if you can hear the fan running when you try to heat something up then, we would (without taking the machine apart) state that the most likely problem is the magnetron tube. This is an expensive part and not worth doing a repair (even if you are trained to do it).
Please let me know if you would like more information by posting back to this question.