A Rare Defect In A Microwave Oven
Appliance: Electrolux modelo MEP41
Defect: Heating active even without command
The customer complaint in relation to this appliance was that the heating cavity becomes hot even without issuing any command from the frontal keypad. The customer, with a good knowledge in technical affairs, was aware of the dangers concerning microwave frequency radiation.
I opened the set and did a visual inspection, in which I could realize the excellent manufacturer constructive technique. Although the set was not new, the internal conditions were pretty good, with no signs of previous maintenance. With the aid of Google, the service manual was easily found:
https://drive.google.com/file/d/18FFPhuB1uvPF1AtwmV059E-es9b6ztE-/view
It is in portuguese language, but as Electrolux is worldwide active, finding it in other languages must be easy.
The quality of the service manual caused me a good impression: well-written, didactic, with good and elucidative drawings and good quality color pictures, providing the repair technician a plenty of usefull information. The wiring diagram give good details, including the wire colours in each point, greatly facilitating in the case of disconnected wiring. However, there is a negative aspect: the electronic command board is not shown, and this hindered the present maintenance, as will be seen below.
I opened the set and first of all disconnected the two wires that lead the line voltage (127 V in this case) to the big transformer that supplies the Magnetron tube. I connected the multimeter probes to the two already free wires and plugged the set to the wall outlet. Immediately the multimeter showed the presence of line voltage. An abnormal situation in this case since that, for this to happen, a command would be issued in the panel keypad, which had not occurred. If the two wires were still connected to the big transformer, the Magnetron tube would be on, in its full power. And worse: without the assistance of the fan, that remained off. This condition leads to an overheating of the Magnetron tube, substantially shortening its lifespan.
I opened the cavity door, and with this action the multimeter showed that the line voltage was not being applied anymore, demonstrating the proper functioning of the three microswitches associated to the door. In order to be shure of the good status of these microswitches, I carried out a detailed check in them; the testing corroborated the correct operation of the three microswitches.
The big question: why the big transformer receives line voltage without any command? The classical arrangement of practily all the microwave ovens involves the switching logics of the three door microswitches and the intrinsic operation of the command board. The output of the command board is in the form of a sturdy relay, with contacts adequate to bear the high current consumed by the Magnetron. As the correct status of the microswitches assembly was already verified, my attention turned to the control board and the power relay.
The board of this set has three relays, and the one responsible to supply the Magnetron through the big transformer is the bigger of all. This relay is identified by the red arrow in the left picture below:
The contact connection of this relay is by the upper part of the component via white connector showed in the right picture above. From this connector is output the line voltage sent to the big transformer. Inside the relay this circuit reaches the printed board, apparently without use there. When there is continuity between these two contact blades, the line voltage reaches the big transformer, which supplies the Magnetron and causes the heating in the cavity. As the Magnetron was being supplied even without any command in the panel keyboard, it turned evident that the problems could be in the contact blades. In order to check, the white connector (right picture above) was taken out and a quick check with the multimeter proved the spurious continuity. This can happen due to a mechanical locking inside the relay (very rarely to happen) or due to melted contact blades. This melting is a result of arc formation between the two contact blades when the switching occurs, remembering that the higher the power switched, the higher the temperature in the point of the arc formation. In the present case, the entirey of the power supplied to the Magnetron is conducted by these contacts, thus aggravating the thermal effect of the arc formation. Whit the continuous use, the contact blades become more and more degraded and, with the high temperature resulting from the arcings, it turns out to melting of the two blades each other. I thought that this could be the case.
I removed the circuit board and dessoldered the relay, which can be seen in the picture below:
The relay was then checked isolated from the circuit board, with the multimeter showing that the short circuit still remained, even with the relay de-energized. This check also removed the possibility that this spurious continuity came from any point of the printed circuit board. The point for access to the contacts are the two studs in the upper part of the figure above, in which point the white connector is pluggued.
In order to verify mechanically the problem I tried to open the relay, but I discovered that this component was built in a way to not allow this. A chinese piece, for a change. In many conventional relays it is possible to take out the cap, leaving its inside exposed, thus allowing the mechanical working of the relay to be checked. I had to crush the relay cover in small parts until the inside could be seen:
The entire relay is shown at left in the picture above. To the right the two contact blades are shown (arrow), in which could be seen that the blades are flush with each other. I have come to discover that they were not melted, but rather there was a mechanical clamping between them (the rare defect I formely referred), pushing the blades against each other and avoiding them to disconnect. In other words, the entire relay was clamped. Chinese quality…
I easily got a replacement relay from the local market. At least it was cheap – which explains the questionable quality.
The replacement relay was mounted and soldered to the printed board, that was replaced in its position and bolted. Without still connecting the connectors to the board, a check with the multimeter was carried out again, which showed that the spurious continuity was not occurring anymore. The connectors were then plugged to the board and the two wires were connected to the big transformer. A new visual inspection was then carried out, which showed that all was correct.
I plugged the set to the wall outlet and the front panel display lit up, suggesting a return to normality. A check with the multimeter probes connected to the big transformer primary side showed the the line voltage was not there anymore.
In order to do an operational test I put a glass of water in the heating cavity and programmed to 1 minute. Upon pressing the Initiate button on the panel, the multimeter finally showed that the line voltage was normally applied to the big transformer. The fan was spinning. After one minute, the water in the glass became heated up, showing that the problem was finally resolved. I closed the set, ending this maintenance work.
A doubt remained concernig the status of the Magnetron tube, considering that it perhaps worked without the cooling from the fan by a time that it is not possible to estimate by sure. Mainly because a close check in the tube revealed that one of the cooling fins in the body of the tube was loose, a thing I never had saw. According to the test, the tube is working, although its lifespan from now on is an open question. When the set was returned to the customer, I notified him about this issue.
This article was prepared for you by Henrique Jorge Guimarães Ulbrich from Curitiba, Brazil. Retired telecommunication and electronics technician. Loves electronics, telecommunications, cars, wines and grand children.
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Note: You can read his previous article on A Failed Bidirectional Diode In A Microwave Oven
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17 Comments
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Waleed Rishmawi
March 1, 2025 at 6:23 pm
yes I have encourtered such issue in a microwave oven before...thanks for sharing and have a blessed day.
Henrique Jorge G. Ulbrich
March 1, 2025 at 9:25 pm
Thanks Waleed. Have a nice day.
albert van bemmelen
March 1, 2025 at 7:14 pm
Excellent written article that was easy read and understood Henrique! Thanks for sharing!
And the link to the service manual is always good to keep as download. Indeed odd that the controller board was not mentioned in it.
I always keep defect relays for the thin wire coils that are very useful for pcb layer repair or even in new projects.
Henrique Jorge G. Ulbrich
March 1, 2025 at 9:31 pm
Thanks for your comments, Albert. To keep defective relays for reusing the thin wire is a good idea. Remembering that the copper thin wire is enameled, needing a pretreatment for enabling soldering.
Albert van Bemmelen
March 1, 2025 at 11:46 pm
That is even better Henrique because the enameled thin wires prevents short circuiting on small surfaces also when they cross each other. And the fact that the wire is so thin may also protect against way too high currents to protect circuits from getting more damage!
Parasuraman S
March 1, 2025 at 9:05 pm
Very informative and detailed article! Many thanks for sharing, spending your valuable time!
Henrique Jorge G. Ulbrich
March 1, 2025 at 9:52 pm
Many thanks for your words of encouragement. I happily write my bench works following the same principle of all of us, i.e. contentment in sharing experiences. Indeed, time is valuable for all, but by sure it worthwhile. Makes me happy.
My articles are not frequent because I don´t work directly with maintenance. I have many and diverse activities and only fix some gadget in the case of attending myself or a relative/friend. Sometimes using components saved from other devices, obviously without charging the owner, the same with my service. And last but not least: always respecting the environment.
Francisco Maciel
March 1, 2025 at 10:35 pm
Great investigation deep enough to find root cause. Congratulations and thanks for sharing.
Excelente investigación.
Cuando encuentras la causa raíz... Encuentras el porqué del problema.
Y cuando compartes el aprendizaje, encuentras sabiduría.
Gracias !
Henrique Jorge G. Ulbrich
March 2, 2025 at 4:11 pm
Gracias, Francisco. Sempre é bom encontrar defeitos fora do convencional.
YOGESH PANCHAL
March 1, 2025 at 10:38 pm
Good Job! Sir
Being active to fulfill our job satisfaction is energy for our mind.
Keep sharing your experience.
Henrique Jorge G. Ulbrich
March 2, 2025 at 4:14 pm
Thanks, Yogesh. No doubt, when an opportunity arise, the experiences will be shared gladly.
Mark J
March 2, 2025 at 4:26 am
Henrique great article. It was informative.
Henrique Jorge G. Ulbrich
March 2, 2025 at 4:15 pm
Thanks, Mark. Have a nice day.
Tito Kanshulu
March 2, 2025 at 8:18 pm
Excellent repair article Sir, well written, easy to understand. A rare problem indeed
Henrique Jorge G. Ulbrich
March 3, 2025 at 10:53 pm
Thanks Tito. I did really appreciate your flattering comments. With this rare problem I had the opportunity to spend more time and replace only the relay, and not the entire board. Nature will be thankful and this made me happy.
Liviu Filimon
March 3, 2025 at 3:33 am
Well done!
Liviu
MUYKIT
March 13, 2025 at 11:18 pm
Thanks senior electronics repairer, Ulbrich. Though not paticularly efficient electro-mechanical switches called relays still find their way in modern electronic circuits. A relay that fails while in a state of relaying voltage may cause so much damage to an equipment. I learned this from car electrical and electronic operation systems, where mostly blame is shifted to a bad relay when a cars "computer" gets "roasted" .In your oven repair case, you had no choise but to replace a fake faulty relay with another one. But you are not very sure of a long time service of the oven. One day I lacked electricity due to a failed relay in my supply meter, but couldnt replace it due to restrictions by electricity supplier that, only them should open the sealed meter. You look younger than 80 years in that photograph making me to doubt if you are indeed older than one of the regular article contributers on this website. I hope you are passing your immense knowledge of electronics repair to your loving grandchildren