Original post by: John ,
Over the years I have repaired several microwave ovens. All of them had the same symptom - more noise than usual and no heat. The high voltage circuit of these units is extremely simple comprising a large transformer, capacitor, diode and magnetron. Personally I have never seen a capacitor or a transformer fail so the diagnosis usually comes down to the diode or magnetron - about $10.00 or $50-$80 respectively. You do need to be extremely wary of the killer voltages when checking these units. As already mentioned disconnect the power cord and using a well-insulated screwdriver short out the terminals of the capacitor just in case. A magnetron can be checked with a standard test meter on the ohms range. First check the resistance between the two magnetron terminals (remove the wires first). The resistance should be very low - in the range of a few ohms. Next check the resistance between each terminal and the metal fins of the magnetron. The resistance here should be too high to measure (in essence an open circuit). If the magnetron passes both these tests then it is quite likely fine. A microwave diode can be open or short circuited. A short circuited diode creates noise and no heat and an open circuited diode creates no additional noise but also no heat. Testing the diode using a regular test meter is not usually successful because the testing voltage has to be quite high. I always have a spare good diode to substitute as they are quite cheap. I also concur with other readers that if a magnetron is suspected as the failure cause then it may be more cost-effective and safer to just buy a new one. However I repaired my own over the stove model by replacing a faulty magnetron for a quarter the cost of replacing the unit. Above all stay safe.