A shorted microwave oven repaired. Model: Universal Uni-2040
A guy brought this microwave oven for repair. It has a strange symptom; sometimes it heats up, and many times it causes the earth leakage switch to trip.
I powered up the microwave oven and nothing happens. The moment I took the screws off this is what I saw.
There was no fuse on that board. When I was moving the microwave oven around, I could hear something loose inside. I looked for that loose part and guess what I found? A fuse; 8 amp fuse and when I checked it, it was a blown fuse.
What caused this fuse to blow? Was it over load or shorted component in the oven?
I checked the power board and all checked fine. I added a light bulb to the fuse connection and look what I got.
Here, the microwave oven was not turned on, just plugged in power and as you can see, it is shorted. After checking all the parts inside this oven, my mind was set to two problems. Either the magnetron is shorted out or one of the safety locks is shorted.
This is one of the main safety lock inside this oven. It is the only one that has three wires connected to it. When I took it out, it was jammed. It did not click. I pressed the button harder and it started clicking again. A very good idea to replace it and I did. Replacing that safety lock did not help me in my repair but I was one step closer to fixing this problem.
I moved on to the second option, the magnetron head. The magnetron is in charge of heating in this oven.
I unplugged the two pins coil connecter on the top of the magnetron and tested for shorts on the head; there was none. The testing is done by using the ohm setting on the multi meter by placing one probe to one of the pins and the other to the body of the head. If any reading fond, the head is shorted.
I decided to take it off and replace it with well know working head. Guess what? The short was gone and the microwave oven was working again. It seems like when there is a load on that dead; it trips the earth leakage switch. I tried to take a photo of it while it was on but as you can see; it was too dark to see inside it.
Mission Accomplished
This article was prepared for you by Waleed Rishmawi, one of our ‘Master Authors’ and currently working in the Bethlehem area of Palestine repairing electrical and electronic equipment.
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Anthony Paterson
August 18, 2017 at 4:44 pm
Any microwave repair should always begin by highlight the fatal voltage that can exist in an unplugged microwave and the imperitive to safely discharge that voltage before doing anything.
See: http://www.microtechfactoryservice.com/safety.html#dischg
Furthermore your partial tests of the Magnetron didn't find anything wrong?
1. On 'high ohms' check between each terminal and chassis and it should be infinity.
2. On 'low ohms' check continiuty between terminals and expect a reading.
Overall, a poor article with glaring ommisions regarding safety.
Waleed Rishmawi
August 21, 2017 at 4:48 pm
Anthony: thanks for your comments about this article. yes, you are right I should have mentioned something regarding safety first but I have written about this safety procedures very often and i am sure many people who read my articles before know I have spoken about it before. thanks.
Amir Mukhtar Ashrafi
August 23, 2017 at 12:37 pm
thanks you Sir Waleed
Keep Sharing yours Expertise with us .
A Student of Electronics
Amir Mukhtar AShrafi
Justice
August 18, 2017 at 8:38 pm
Thanks Waleed, I gained a lot of technical information on this article.
Parasuraman S
August 18, 2017 at 11:00 pm
Very good and informative article! Thanks!
Albert
August 19, 2017 at 3:55 pm
Finding the exact same microwave head is always a problem but luckily you had one at hand that matched with the HV capacitor circuit. I myself never try turning on any microwave oven without putting the metal cover back first. It after all could be a bit unsafe in case the oven leaks radiation which is invisible to the human eye. (an old colleague had looked into a IR laser head of a DVD or CD player thinking it was not working and now has to live with some eye damage eversince). And the lightbulb test always easily can be done on the outside AC mains plug too anyway. Thanks for the good article Waleed.
Waleed Rishmawi
August 21, 2017 at 4:51 pm
Albert: most heads are almost the same. the only thing is the size. they come in different sizes but the operation is almost the same.
about testing the microwave one with metal part on is a very good advice. even with metal off though, it will not harm you in any way unless the body of the microwave oven is old and torn then I would not do it that way. in this case it was in good shape and based on this I did what I did. thanks for your comments.
Bibhas
August 21, 2017 at 5:59 am
hallo waleed sir, why you change the magnetron? as you tell your artical that "I unplugged the two pins coil connecter on the top of the magnetron and tested for shorts on the head; there was none".So plz discuss what is the reason for change a new one magnetron. there was nothing to short. ------------ fake artical
Waleed Rishmawi
August 21, 2017 at 4:43 pm
during testing it was fine but when it has a load on it, it causes the earth leakage to trip. very normal in microwave oven repairs.
Rick Smith
January 10, 2018 at 1:54 pm
Nice and informative article. Thanks for sharing the information with us. Although I am not sure if the process can be performed by non-professionals. But it was worth reading.