Question about Panasonic NN-T990SA Microwave Oven

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Panasonic MW- Arcing

If heating fatty substances (melting butter for example), as the product on the turntable gets near the area where I assume the MW waves come out (a square in the center of right side vertical panel inside, covered with a removable panel of some strange material) a brilliant flashing and the sound of heavy electrical arcing (1 to 2 sec., off and on) takes place until the product rotates out of proximity to the panel. I removed the panel and cleaned it pretty good and for a while things were fine. Now it is doing it again and freaking out my wife. She wants a new MW, I want to easily fix it. By replacing that panel with a new one will the problem go away? I had read that food built up on the surface can cause this and cleaning it seemed to help the first time. Ever heard of this?

John Ewers

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You're on the right track. That panel is the waveguide cover.

This is usually due to grease or food particles stuck to the inside of the oven or stuck on or behind the waveguide cover in the waveguide just out of sight.

It can be fragile, and the metal edges behind it can be sharp, so be careful!

The cover and the metal behind it should be cleaned with a mild, non-ammonia cleaner.

If the cavity metal of the oven is chipped or charred, it will have to be sanded smmoth and clean.

Then you can apply some high-temperature appliance paint, available from places like Wal-Mart or Lowe's.

If your waveguide cover is burned, it will need to be replaced.

To order Panasonic parts see

Posted on Nov 18, 2007


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MCD1611W New, popping popcorn in glass MW corn popper. 3-4 minutes to pop. After popped I open the door and after a few seconds the MW shuts down completely, no lights, no response to pad pushing.

Popcorn is a low-moisture food which can cause a microwave to overheat, as can a failure of the cooling fan or blocked vents.

You should always turn a paper plate upside-down on the turntable (slightly off-center for better heating) and place the popcorn container on top of that.

The paper plate elevates the popcorn for better exposure to the energy and protects the turntable from excessive heat.

Placing food off-center instead of in the center increases exposure to the energy and prevents the same spot on the turntable from being overheated repeatedly, which leads to material fatigue and cracking.

It's also a good idea to put about 1/2 cup of water in a 1-cup microwave-safe container in the corner of the oven when cooking low-moisture foods like popcorn, bacon, and potatoes.

The microwave's magnetron has a thermal protector (thermostat, TCO, or thermal fuse) attached to it.

If the magnetron gets too hot, it can be damaged or destroyed, so the thermal protector opens and disconnects the power to the whole oven.

If it is a resettable device, the power will be restored when the device cools. If it is not resettable, the device must be replaced.

FixYa does not directly notify us experts of responses, so if you have other questions, please contact me directly.

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Microwave Oven Control Panel Repair Nationwide
"Recycling by repairing since 1982"

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Shuts off after approx 5 - 7 sec. some smoke

If it stops after 7 seconds, it has a problem with the inverter.

If the digital programmer circuit / controller does not get a feedback signal from the inverter for a period of 3 seconds, the controller will stop the cook cycle.

It may be a loose connection in the wiring between the inverter, magnetron, and/or control board, or maybe one of those components has failed.

Due to the lethal voltages used and produced in inverters, I do not recommend that an inexperienced person work on these.

You can find an authorized Panasonic servicer here.

In Canada, you can find an authorized Panasonic servicer here.
If you prefer to replace the inverter yourself, you can find links to helpful exploded view diagrams and part ordering help here.

I recommend that you bookmark that link to your favorites.We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

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You should have no arcing within the unit. Assuming you have adequate food/water in the cavity, it should not arc. You NEVER run the unit without something to be heated in it!!!

You might check the stirrer fan which is often in the roof of the unit to see if contamination might be in the area causing arcing. The electric fields in th area are very high and arcs can jump.

If the insulating window is charred or has charred grease, it should bbe replaced.

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Considering the melting pooint of glass ...

Normally, microwave travels around round surfaces (Look at the B-1 for example) If you put flat metal in a microwave, you will create sparks ... lots of em. Is it possible you have something going on with one of the wheels ... like a piece of tin foil?

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If it is burning and arcing at the antenna, it is probably arcing in the wave guide area. Probably will have to replace unit. I have had some success by removing the antenna, sanding and repainting the burnt areas of the cavity and sanding off any burnt marks on the antenna. Need to look at top of mag to see if also arcing there too.

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(Sorry, but your subject line got cut off.)

Quite often this is due to grease or food particles stuck somewhere in the cavity of the oven or stuck on or behind the waveguide cover.

It may also be in the area of the turntable support, coupler, or motor.

The waveguide cover (also called a spatter shield) is usually about the size of a playing card and often held in place by a plastic rivet.

It can be fragile, and the metal edges of the oven behind it can be sharp, so be careful!

The cover and the metal behind it should be cleaned with a mild, non-ammonia cleaner.

If the cavity metal of the oven is chipped or charred, it will have to be sanded smooth and clean and dried thoroughly.

Then you can apply some high-temperature appliance paint, available from places like Wal-Mart, Lowe's or Builder's Square.

If your waveguide cover is burned, it will need to be replaced. It provides important protection for the magnetron.

If you need further help, please reply back with your brand and model number so we can help you.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

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