Magnetron failures and why the fail.

Magnetron Failure.

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.

Components in a magnetron.

Inside a Magnetron

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 or cutouts
  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., discoloured, 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

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

Terminals showing signs of burning. 

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 and change the terminals making sure they fit properly.

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Cracked Magnet(s)

This is caused by the magnetron over heating, some instances this is because of reflected microwave energy.

Magnetron with cracked magnet

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

Solution: Replace the magnetron and check why the magnetron has overheated.

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Burnt or melted antenna cap.
Burned Dome (or Antenna) caused by arcing due to reflected microwave energy {back feeding}. 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.

Burnt Antenna  or you may see this inside the cavity Burnt cavity

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

Solution: Replace the magnetron and, if necessary, the respective antenna or stirrer assembly. Clean the cavity wear the arcing has happen. To much carbon build up will cause the arcing to return within a very short period of time..
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Loose Magnetron terminals

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 build up 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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