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HOW TO SAFELY TEST THE HIGH VOLTAGE CAPACITORS USED IN COMMERCIAL & RESIDENTIAL MICROWAVE OVENS

The Complete Microwave Oven Service Handbook: Operation, Maintenance, Troubleshooting and Repair
Excerpt from the distinguished e-Book, The Complete Microwave Oven Service Handbook: Operation, Maintenance, Troubleshooting and Repair on CD-ROM © 1996-2013 J. Carlton Gallawa . All Rights Reserved Worldwide

Typical high-voltage capacitor used in microwave ovens
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The high-voltage capacitor works along with the high-voltage diode to effectively double the already-high voltage from the secondary (output) winding of the power transformer. This high DC voltage provides the boost necessary to fire the magnetron into oscillation.  The capacitor can hold a fearsome electrical charge long after the oven has been unplugged. So before making this or any other test:
ALWAYS MAKE SURE THE OVEN IS UNPLUGGED AND THE HIGH VOLTAGE CAPACITOR IS FULLY DISCHARGED

How To Discharge The High Voltage Capacitor

First Method The capacitor is discharged by creating a short circuit (direct connection) between the two capacitor terminals and from each terminal to chassis ground (bare metal surface).

  1. Do this by touching the blade of an insulated-handled screw driver to one terminal, then slide it toward the other terminal until it makes contact and hold it there for a few seconds. (This can result in a rather startling "pop!" Note:If there is a spark, the capacitor is evidently holding a charge, thus it is most likely not defective)
  2. Repeat the procedure to create a short between each capacitor terminal and chassis ground.
  3. If the capacitor has three terminals, use the same procedure to create a short circuit between each terminal and then from each terminal to ground.
  4. Older Amana-made models (generally those manufactured before 1977) have red, round filter capacitors mounted in the base of the magnetron tube which can also hold a charge. Ground each magnetron terminal by creating a short circuit to chassis ground using the blade of a screwdriver as explained above.

Although most manufacturers specify the above procedure in their service manuals, some prefer the following alternate method involving the use of a resistor to slow the discharge rate.

Alternate Method: As shown in the illustration to the right, attach one end of a 100K - 150K ohm, 25 watt resistor to the bare metal chassis with an alligator clip. Attach the other end of the resistor to the blade of a well insulated screwdriver.

  1. Touch the blade of the screwdriver to one of the capacitor terminals and hold it there for a few seconds. Then repeat the procedure for the other terminal.
  2. To ensure that the capacitor is fully discharged, follow the first method (shown above).

Capacitor Test Procedure

  1. Unplug the oven.
  2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS.
  3. Note the wiring and carefully remove all leads from the capacitor terminals. (If there is a bleeder resistor, it need not be removed. But, bear in mind that some measurements will reflect the meg-ohm resistance of the resistor)
  4. Set the ohmmeter to its highest resistance scale.
  5. Measure from one terminal to the other for a normal reading of infinity (or the value of the bleeder resistor).
  6. Now reverse the leads. The meter should momentarily deflect toward the zero mark, then slowly drift back to infinity.
  7. Reverse the leads once again. This should produce the same meter deflection.
  8. Next measure from each terminal to the capacitor's metal case for a normal reading of infinity. (If there is an internal diode, the meter readings will reflect the diode's forward bias resistance. (See HV diode test procedure)
  9. A visual inspection will also reveal certain defects, such as:
    • Evidence of arcing or burning at the insulators
    • The presence of an oily film or smell suggests a dielectric (non-conductive medium) leak
    • A bulging case indicates dielectric breakdown

Any such defects or abnormal readings would require replacement of the capacitor

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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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