[Norm K]

Likely the diode to the magnetron or the magnetron itself. Take great care around the capacitor (or whatever they call it in a microwave oven) if you decide to dig into it yourself because, although not certain, the possibility of death awaits within, in the form of a significant stored charge. If it was just a decent unit it probably makes more sense to just buy another decent one given how low prices are. This is particularly true if you get one that uses the same mounting apparatus, as it'll take you less time to hang a new oven than it will just to remove the screws necessary to begin an attempt at diagnosing the problem.