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Author Topic: Microwave Comes On And Hums But No Heat  (Read 335 times)

Offline jimmy12345678

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Microwave Comes On And Hums But No Heat
« on: June 27, 2017, 04:19:39 PM »




The other day I went to heat up food in my Kenmore microwave and after about 15-20 seconds there was smoke rolling out of it. So I unplugged it and took the cover over to see what was wrong. I ran it with the cover off and noticed that the transformer was getting very hot and that's what was smoking, so I found another one that would fit and installed it. Now the microwave turns on and doesn't smoke, but it also doesn't make things hot. I put a cup of water in there for a minute and it was still ice cold.

I've searched online and found Microtech Factory Service Microtech Factory Service. Expert microwave oven repair for the Florida-Alabama Gulf Coast and they had diagnostic procedures to test the various components inside a microwave. I'll post the results of those tests below:

Transformer

Between Primary Windings - 0.2 ohms (Old and New)
Between Primary Leads And Ground - Open
Between Filiment Wires - 0.2 ohms (Old and New)
Between Filiment Leads And Ground - Open (Old and New)
Between Voltage Tap and ground - Old One - 848 ohms (spec says 55-70 Ohms) New One - 102 Ohms I don't know if this is too far out of spec or not

Magnetron:

Between Both Terminals - 0.2 Ohms
Between Terminals And Ground - Open

Capacitor:

Between both terminals - Slowly climbs to OL, when switched goes back to zero and starts climbing back to OL
Between terminals and ground - Open

Diode:
0.2M Ohms one way
OL other way


Thermal Protectors (2)

Both 0.2 ohms across terminals


It just doesn't make any sense why it doesn't work now. It worked on Saturday, then yesterday I used it and that's when the transformer overheated. And I didn't touch anything else inside, just wired up the new transformer just as the old one was. Same specs on both of them (120v, 60 Hz). Unless this new transformer is also defective (as I said, the 102 Ohms on the high side is above the spec I found).

If anyone can help me or they need me to conduct more tests I'm more than happy to help. Thanks!!!

Kenmore Model # 566.8832980 
Manufactured September 1987

Offline olyteddy

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Re: Microwave Comes On And Hums But No Heat
« Reply #1 on: June 29, 2017, 11:05:01 PM »
FWIW you can't test the diode with an ohmeter or with the diode test setting on a conventional meter either. This is because it's actually a stack of diodes in series (to handle the voltage). I use a 9 Volt battery and measure the Voltage passed - around 6 Volts conducting, just a few Millivolts reverse. Also the TCOs are in the primary circuit and will usually cause a total shut down if they are open. Examine the Magnetron for cracked or displaced magnets, I've seen bad magnets cause no output.

Offline mz3own

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Re: Microwave Comes On And Hums But No Heat
« Reply #2 on: June 30, 2017, 06:08:03 AM »
did I read that right? you are trying to fix a 30yr old microwave? :o
Keep us posted what you find to fix it.

Offline jimmy12345678

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Re: Microwave Comes On And Hums But No Heat
« Reply #3 on: June 30, 2017, 03:07:59 PM »
FWIW you can't test the diode with an ohmeter or with the diode test setting on a conventional meter either. This is because it's actually a stack of diodes in series (to handle the voltage). I use a 9 Volt battery and measure the Voltage passed - around 6 Volts conducting, just a few Millivolts reverse. Also the TCOs are in the primary circuit and will usually cause a total shut down if they are open. Examine the Magnetron for cracked or displaced magnets, I've seen bad magnets cause no output.

Thanks guys.  Sorry for my ignorance about anything, I'm still green when it comes to working on microwaves.  I did test the diode using a 9 volt and it dropped to around 5.9 volts.  I have a parts microwave that I garbage picked, which is where I found the new transformer. 

A user on another forum told me to hook up a test light (lightbulb socket, wire, alligator clips) to the primary input on the transformer and check for power.  I stayed solid on high power, and would click on and off depending on which power setting it was on.  He then said to remove the wires going to the magnetron and run the microwave for a few seconds (2-3) and then discharge the capacitor.  If it makes a big crack and a spark that means it's making high voltage but not using it.  Therefore the magnetron is bad, and I've ordered a replacement one off of ebay.

I'll let you guys know how it turns out once the new magnetron arrives. 
 

Offline jimmy12345678

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Re: Microwave Comes On And Hums But No Heat
« Reply #4 on: June 30, 2017, 03:12:49 PM »
did I read that right? you are trying to fix a 30yr old microwave? :o
Keep us posted what you find to fix it.

Yes you read that right.  My mom got this microwave on Christmas in 1987 and it has been going strong up until this past Saturday (never even had to replace the bulb).  The reason I'm fixing it instead of just replacing it is because everything now is cheap, Chinese garbage, including microwaves, and I guarantee if I got a new one it wouldn't last 5 years.  They don't make things to last anymore.  That microwave I garbaged picked I can pick up with one hand with the cover off, there's no way I could do that with this old Kenmore.  Everything is thinner, cheaper, and designed to be disposable, and disposable is not a word in my vocabulary.   
« Last Edit: June 30, 2017, 03:13:36 PM by jimmy12345678 »

Offline jimmy12345678

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Re: Microwave Comes On And Hums But No Heat
« Reply #5 on: July 03, 2017, 02:15:02 PM »
My new magnetron arrived in the mail today, so I tore the old one off and transferred the mounting bracket and air ducts mounted on it to the new one.  Then I installed the new one and tested everything and it's working as it should again.  I tested it with a cup of water and after 1 minute the water was pretty warm, not ice cold as before.  This repair has been a total success, and I've learned how to repair microwaves in the process.  Thanks so much to everyone who helped me along the way!!!

 

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