Microwave Comes On But Doesn't Heat

[jimmy12345678]

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

[johntech]

To start I would reccommend testing the diode with a 9v battery. They can fail under a load. Google "how to test a microwave diode with a 9v battery. A voltage sniffing pen when held a foot away from a microwave will detect the high voltage if the transformer is working, so carefully turn on microwave with compartment open and move beeper pen towards unit, if the pen beeps your transformer is working to some degree. When you run into a transformer that has a failure to the secondary windings that usually means that you have a short down stream that if not corrected can fry the new transformer. Magnatrons can and almost always  test good in ohms but fail under a load. If the diode checks out and the pen reacts without getting close to any wires, added to the fact you said someone replaced the capacitor, then the magnatron would be the only thing left in the high voltage section to fail. 

[jimmy12345678]

2 hours ago, johntech said:

To start I would reccommend testing the diode with a 9v battery. They can fail under a load. Google "how to test a microwave diode with a 9v battery. A voltage sniffing pen when held a foot away from a microwave will detect the high voltage if the transformer is working, so carefully turn on microwave with compartment open and move beeper pen towards unit, if the pen beeps your transformer is working to some degree. When you run into a transformer that has a failure to the secondary windings that usually means that you have a short down stream that if not corrected can fry the new transformer. Magnatrons can and almost always  test good in ohms but fail under a load. If the diode checks out and the pen reacts without getting close to any wires, added to the fact you said someone replaced the capacitor, then the magnatron would be the only thing left in the high voltage section to fail. 

 

I have a parts microwave that I took the transformer from, so I have a space capacitor, diode, and various other parts.  The only thing I replaced on the microwave I'm trying to fix is the high voltage transformer.

 

I tested both diodes using the 9 volt battery method and both checked out (went from 9 volts to approx 5.9).  I also checked that the transformer was getting power to the primary leads with a homemade test light (old lightbulb socket, wire, alligator clips) and it lit up.  I also checked the various power settings and the bulb would kick on and off at various times depending on the setting. 

On another forum a user told me to unhook both wires going to the magnetron, run the microwave for a few seconds, and discharge the capacitor.  If it makes a big spark and crack it means the high voltage system is working up to the magnetron, which it did.  I've ordered a replacement magnetron on ebay and am waiting for it to be delivered. 

I also ran the microwave a few times with the new transformer installed and it didn't overheat like the last one, there was just no heat, so I don't think there's a short before the transformer. 

 

Thanks for your help!!!  I'll let you know how it turns out. 

[johntech]

Sounds like your going to have a working microwave again. Good job.

[jimmy12345678]

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!!!

[johntech]

Good job.. Were glad to help.