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Microwave stopped heating after being cleaned (could be co-incidence). Besides not heating appeared to run properly. Let sit for a day hoping something was wet and would dry, tried heating glass of water and heard a pop microwave and microwave completely off. Opened up unit and found 20A fuse blown. I am hoping by describing the events leading to fuse being blown that part that needs to be replaced is more readily identifiable (fuse & HV cap or HV rectifier or transformer or magnetron) Thanks MikeG

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  • mikeg8 May 22, 2008

    replaced fuse and diode (diode was open)



    Microwave runs again but still not heat.



    Capacitor did not spark when discharged. However, it appearred ok when tested with ohm meter.



    none of transformer windings were shorted to ground.



    When trying to heat something, noticed that audible noise of microwave was different than when working. I'd say frequency of noise generated by microwave when attempting to heat something is lower.

  • mikeg8 Jun 05, 2008

    I ran microwave for 5 minutes plus and fuse did not blow again. Since fuse did not blow I ruled out transformer.



    I then replaced the magnatron and microwave still does not heat.



    Microwave still groans/hums loader when trying to cook.



    Next suggestion?? Replace both capacitor and high V transfromer??



    Thanks MikeG

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  • Microwave Ovens Master
  • 8,820 Answers

If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.

If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.
If it goes dead or blows the breaker when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.

But you might replace the fuse and do another quick heating test to see if it blows again. If so, you'll need to do some checks.

Here are some links you or someone you know can use for test help, but read their safety warnings first:

http://www.gallawa.com/microtech/cap_test.html
http://www.gallawa.com/microtech/diode.html
http://www.gallawa.com/microtech/xformer.html
We're happy to help and we appreciate your thoughtful rating of our answer.

William E. Miller, AS-EET
prototech@usa.net
http://www.microwavedisplay.com

Posted on May 12, 2008

  • William Miller
    William Miller May 22, 2008

    The diode was probably not open because normally when the diodes fail, they short.

    The oven will groan or hum loader when you try to cook, but a shorted high voltage diode alone will not blow the fuse.

    If the diode is tested with a common meter it will look open, since a much higher voltage is needed to forward bias (turn on) a high voltage diode. It will look open.

    I would put a cup of water in it and set it for about 5 minutes.

    Stay with it, then after it stops, see if you smell any electrical odors coming from the transformer or excessive heat from the magnetron.

    Also look for a cracked magnet on the magnetron. Excess heat can cause that, and it won't heat.

    It's best to do this test with the control panel pulled away or the outer cover off so you can check it faster after the cook cycle..

    I'd say one of either the mag or transformer is bad.

    If the fuse blows again, I'd suspect the transformer. Sometimes bared internal windings will not actually short until it is heated up. Expansion & contraction may cause them to touch and short it out, blowing the fuse.

    In some cases a bad mag will cause excessive current flow and a blown fuse, but further testing is needed.

    Let me know how it goes.

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http://www.espares.co.uk/advice/microwave/a/5/1443/how-a-microwave-oven-works.html

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http://www.espares.co.uk/advice/microwave/a/5/1443/how-a-microwave-oven-works.html

So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off. Check the wave guide covers, if food had deposited
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Now keeping clear of the microwave, switch on; use a cup of water inside to test.
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http://www.espares.co.uk/advice/microwave/a/5/1443/how-a-microwave-oven-works.html

So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off. Check the wave guide covers, if food had deposited
then it must be checked or replaced. See link:
http://www.espares.co.uk/advice/microwave/a/5/1146/how-to-replace-a-microwave-guide-cover.html

Now keeping clear of the microwave, switch on; use a cup of water inside to test.
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