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Kenmore Microwave 721.66339 burning electrical smell coming from back. Six month old oven

New oven, using properly but unable to use due to safely reasons. Fan in back isn't working, when in use smells like electronics burning.

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I have this same problem with a similar Kenmore elite, it tested good when I bought last week, heated a cup of water to boiling in 70 seconds or so. I used to help rive around my next door neighbor who was a microwave repairman, I learned what went wrong with them and typical fixes. Back then there was only 1 brand being sold the Amana Radar range which sold for well over $1000 for the cheapest models. They had no sensors like they do now. Only a few things could go wrong with them making them simple to make money off repairs. So I do know my way around a microwave, I sell 4 or more a month and repair maybe 1 or 2 a month. But this one has me stumped. I took a look inside and cleaned the grease off the cover right away as I always do. I also had to repair a few tabs that hold the panel and top cover piece. .after the cleaning I reassembled it and tested it again. It worked fine. Then 2 days later I found the missing platter for it and went ahead and cooked a frozen macaroni and cheese family sized dinner to give it an official test to see if I should just keep it and sell my current oven. After 5 minutes it seemed like it was cooking kind of slow. It was in my garage during this so I didn't notice any smells till I checked on it. It should have been completely thawed but not hot yet (if it was in my 17 year old GE) so I turned the dial to cook a frozen dinner and not just times cook. I watched it and that's when I smelled the burnt plastic smell, strong on the right side. But that's also where the light bulb is, that bulb only comes on when the oven is cooking?
I put may face closer to see inside and only saw the bulb, I took it apart and tested everything and the only issue I found was the humidity senor is reading low resistance by about ? of what it should be. This would not account for any burning smells since it's on the left and only tells the oven how the food is cooking. It would either keep cooking longer or say it's done before it really is depending on how the resistance is read by the cpu connected to that sensor.
the diode checked fine, the hf cap is fine, the transformer was good, the magnitron was not shorted and tested within the speced ohms range. Ok controls work and I can set it to run at different heat settings without any problems. I can hear it kick in a run for 50% when I set it to that setting, but it still isn't even getting a cup of water hot now. All wires are connected good, the entire board works properly, the readout and button select and work. I haven't tried the fan yet. I'm going to check it out but I did doin it by hand when the forget was off and it spins freely. I don't have an extra humidity sensor to test it but I'll get one very soon and report back here. I was hoping that this was a problem that was specific to my model and hoped to find the quick answer but this is a common problem in general which seems to have only one typical response provided by every single person but a select few. The answer given 95% of the time is buy a new oven! That's not the answer to fixing the oven. I'm pretty sure that anyone who comes here looking for a solution isn't hoping that 9 out of 10 answers will be buy a new one or they would not be here in the first place, they'd be out buying another oven that will likely break even quicker than the last one that they are dealing with fir the first time after 15 years. New microwaves are junk, I fix more ovens that are 5 years or less old. I sell ovens from the 1980s that still work perfectly! Every Samsung is junk and every sharp and magichef are still going strong 30 years later. It's because the magnetron is built using pure elements and not made by Deawoo or Toshiba, that's why Panasonic's last so long too, they aren't using substandard parts to make their expensive ovens. This really has become a race to the bottom when it comes to making new appliances these days. All are built with end of life factored in, at least until they figure out that they can no longer cheaply mine those elements needed and find a new way to make things using even poorer substitutes.

ill post my findings as soon as I get this figured out it can't be that hard, there's only a handful of components that can burn or give off any smell at all I suspect it's the light bulb and there's something else unrelated that's dying. My guess is the magnetron or the transformer is faulty even though they read within ohms specs. Magnetron made by Toshiba, I've replaced many toshiba on deawoo magnetrons.

Posted on Oct 05, 2019


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Posted on Jan 02, 2017



  • 579 Answers

SOURCE: Burning smell , Microwave not turning off

the control circuit shorted out from the cleaning session and kept the microwave on so long with nothing to cook that it has cooked itself I'm sorry to tell you. Probably not worth repairing

Posted on Jan 01, 2008


Jim Goldsmith

  • 2005 Answers

SOURCE: Kenmore over the range microwave 721.63684300 - nothing works

you must remove the MW take off the cover then look for a large metal can (capacitor) and a white (ceramic) fuse...look and, smell and, you should find the cause

Posted on Apr 22, 2009


  • 2 Answers

SOURCE: The fan on our Kenmore microwave oven is not working.

My husband took the unit off the wall looked down into the fan and found a dead mouse keeping the fan from spinning. We removed the dead mouse cleaned up the area. Turned on the oven. The fan worked perfectly. Then he rehung the microwave.

Posted on Aug 21, 2009


  • 1 Answer

SOURCE: Generic problem for all Kenmore Microwave

Loud hum is usually due to faulty hv diod, resonance capacitor, or the magnatrone feed thru capacitor , chk it with a ohm meter , one by one n replace it .. zaheer Islamabad

Posted on Dec 31, 2009


D. Floyd Kolb

  • 43501 Answers

SOURCE: microwave smokes and smells like burning plastic

Here is a tip that will help you to figure out what is wrong with your Microwave Oven....

Microwave Oven Basic Troubleshooting Tips


Posted on Aug 07, 2010

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