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Common Types of Magnetron Failures That Are Easily Detected With Simple Observation

© 1989-2013 J. Carlton Gallawa All Rights Reserved Worldwide

Important Safety Information

The microwave oven is a very dangerous appliance to work on. For your personal safety, we respectfully ask that you read, fully understand and be prepared to follow carefully the list of very important safety precautions as well as the disclaimer at the bottom of this page BEFORE proceeding with any tests or troubleshooting.


When some of the following problems occur, it can result in an unusually POWERFUL ELECTRIC CHARGE stored in the high-voltage capacitor, even after the oven is unplugged, making it particularly DANGEROUS if the HV capacitor is not properly and completely discharged.

The magnetron can fail in a number of ways, many of which produce obvious visual symptoms that can be clearly seen and require no testing with a meter.

When the magnetron must be replaced, here are some 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
  6. If possible, perform an RF leakage check around the magnetron

Following is a pictorial list of common magnetron failures that are visible, along with their respective symptoms and solutions.

Insulator breakdown begins with a tiny burned spot on the magnetron insulator, then with each subsequent cook cycle, progressively produces stronger arcing and burning, eventually leaving clear visual evidence of the failure as shown in the picture to the right

Symptoms: Loud hum, no heat, arcing sound, electrical burning smell

Solution: Replace the magnetron

Cracked Magnet(s)

Symptoms: Weak or no heat, magnetron gets extremely hot (overheats), intermittent arcing or "snapping" sound

Solution: Replace the magnetron

Burned Dome (or Antenna) caused by arcing due to backfeeding microwave energy. When this occurs, check for a stalled or arcing stirrer blade or non-rotating antenna assembly. In many commercial models with multiple magnetrons, the cooking tray must be raised in order inspect the condition of the lower antenna assembly.

Symptoms: Weak or no heat, arcing sound during cook cycle

Solution: Replace the magnetron and, if necessary, the respective antenna or stirrer assembly.

Loose Magnetron Filament Connectors / Discoloration of the connector(s) or plastic insulator(s)
If the connectors that slip onto the magnetron filament terminals become loose or are improperly crimped, it causes a buildup of resistive heat. As this occurs the connection further deteriorates causing the following visual symptoms
  • Small blackened pits in the magnetron terminal(s)
  • Melted, and decayed appearance
  • Also, as noted above, an unusually fearsome spark is produced when discharging the capacitor.

Symptoms: Intermittent and/or low heat initially, then eventually no heat

Solution: Repair defective terminals as follows:
Either (1) Clean the burned/pitted magnetron terminals and replace the slip-on connectors, making sure they fit tightly on the terminals; or (2) Cut away burned wire and connector(s). (Make sure there is enough remaining wire to reach with some slack) Clean terminals to prepare for soldering. Solder the filament leads directly to the magnetron. Be careful not to apply soldering heat any longer than necessary.

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Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted © 1989-2013 by J. Carlton Gallawa. The material available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

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