How To Test The Magnetrons Used In Microwave Ovens

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 magnetron uses permanent magnets and a half-wave voltage doubler circuit (the HV capacitor and HV diode) to oscillate and produce the 2450 Mhz cooking frequency, thus converting the 60 Hz supply voltage into microwave energy. 

See Magnetron Structure and Operation for a detailed description of the magnetron tube 

Important Safety Information

The following resistance tests will conclusively reveal a magnetron that is shorted or one that has an open filament winding.

Test 1

1. Unplug the oven (make sure the oven is completely disconnected from any power source)
2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS. (Procedure)
3. Carefully remove all leads from the magnetron terminals. Proper wiring is important so make notes of the wire locations.
4. Set the ohmmeter to its lowest resistance scale.
5. Measure the resistance from one magnetron terminal to the other in either direction.
6. The magnetron filament resistance should be less than one ohm.

Test 2

1. With the meter set to the highest resistance scale, check from the magnetron terminals to the metal magnetron housing. (Be careful not to touch the meter leads as this will result in a false reading).
2. The meter should read infinity (open circuit) regardless of meter polarity.
3. Even a slight reading would indicate a defective magnetron.

Replacement Considerations

1. Be careful not to strike or touch the antenna dome area
2. Be sure to transfer any add-on parts, such as an air duct or thermal fuse
3. Insure that the wire mesh RF gasket is intact and in place
4. Examine the rim of the opening where the magnetron dome is to be inserted into the waveguide. Smooth out any irregularities, such as dents, pits, and burns. The rim surface should be bare metal, smooth to the touch. Use light-grade sandpaper - do not use steel wool.
5. If there is evidence of poor terminal connections (i.e., discolored, burned, pitted connectors), repair or replace the slip-on connectors on the filament leads

Comments and suggestions: j.gallawa@cox.net

The COMPLETE MICROWAVE OVEN SERVICE HANDBOOK Operation, Maintenance, Troubleshooting and Repair Copyright © 2008 by John C. Gallawa

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