The magnetron is the heart of a microwave oven. It is much like a radio transmitter in that it produces RF energy and radiates the energy into the cooking cavity where it is absorbed by the food. The microwave oven is a very dangerous appliance to work on. The following resistance tests will conclusively reveal a magnetron that is shorted or one that has an open filament winding. Test 1 Unplug the oven.
Proper wiring is important so make notes of the wire locations.
Set the ohmmeter to its lowest resistance scale. Measure the resistance from one magnetron terminal to the other in either direction. The magnetron filament resistance should be less than one ohm. With the meter set to the highest resistance scale, check from the magnetron terminals to the metal magnetron housing. Even a slight reading would indicate a defective magnetron.