Microwave was cooking potatoes and started making a rumbling noise.
I could not tell if it was below the cooking surface (thinking the Antenna Motor; there is no turn table) or behind and towards the upper end of the Control Panel on the right (thinking Fan Motor). Not sure if either of these parts have failed.
What is the best way to determine what part has failed?
Additional info: The Exhaust Fan (Blower-Fan) is operating normally. Microwave was heating the potatoes, but I cannot say whether or not the heating was slower than normal.
Answer Hello Dennis. If the unit does not heat now, You most likely have a "high voltage" side malfunction. Transformer, diode, magnetron, Capacitor. You will need to be extremely careful if you decide to test the components with a multi meter, Very high residual voltage is present, so make sure you remove power to the microwave and short across the terminals on the capacitor. The magnetron should have a reading of less than 1 ohm resistance across the two terminals, and an "open" circuit between each terminal and chassis ground. The diode/rectifier should have a resistance reading in one direction only, no resistance, or resistance in both directions indicate a faulty diode. The primary winding on the trans former should read approximately 450 Ohms resistance, the secondary winding should read 80 ohms resistance, and the filament transformer should read 100 ohms resistance. If all the readings are proper and in range, replace the magnetron.