Blown fuse in HV transformer primary circuit or HV fuse (if used).
Bad connections (particularly to magnetron filament).
Open thermal protector or thermal fuse.
Open HV capacitor, HV diode, HV transformer, or magnetron filament.
Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
Defective HV relay (not commonly used).
ecogeorge
July 2007