I have a LG microwave that powers up but does not cook. Before it stop heating, for the past couple of weeks there was a sporadic zapping sound, similar to what happens if aluminum foil touches the sides of the oven wall. This did not occur every time something was heated. From reading other posts it seems to be either a diode, capacitor, or magnetron. How do I eliminate the possibilities? Just a methodical process of elimination?

Thanks,
Garry