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LG Microwave Oven MS3840SR Repair
One of the earliest electronic repairs I did was on a microwave. It seemed fairly basic in construction and with a little help from my friends YouTube & Google, I was able to carry out a successful repair. I now had a chance to expand my skills with another microwave (I have done several since).
The customer complained about the microwave working as normal, but the food would not heat. From experience, I had already decided where the possible fault may lay, but I have been caught out before by just repairing by experience, instead of diagnosis, so I was determined to carry out tests and let the results lead me to the correct conclusion.
Sure enough, after trying to heat a glass of water for 30 seconds, the fault was confirmed – no heating occurred even though the power supply, all the display, turntable motor and other features were working as normal.
Firstly you MUST drain the main capacitor, whose job is to effectively double the already high voltage of the secondary circuit of the transformer. You should not do this by shorting the terminals with a screwdriver or something similar. You should use a load of some description, such as a high wattage globe or high Ohm resistors.
I have made a capacitor drain using a plastic pair of tweezers with 2/ 550 Ohm resistors in series, covered in shrink wrap. This works well when draining the residual voltage of capacitors. After this step has been done, always check for remaining voltage with a multimeter.
After removing the casing, a good visual inspection was carried out. The power supply fuse was intact as I suspected and 240 Volts was being provided at the transformer primary circuit. It appeared that the supply voltage and system was operating correctly. Next, the high voltage circuit needed to be tested.
NOTE: This circuit uses up to 5kV and is very dangerous. Use extreme caution for both the safety of yourself and care of your equipment. Most multimeters, for example are only rated up to 1kV. (This is how I blew up one of my multimeters on my first microwave repair)
I have done several microwave ovens & have found that the magnetron is often the source of the fault, however, as I said, I wanted to test the complete system.
Firstly I tested the high voltage fuse and this was found to be blown. These are special fuses 850mA with a 5kV rating. The correct one must be used. Any variation can cause other problems.
What had caused the fuse to blow? These were expensive and I couldn’t just fit another one in the hope that the fault was a random problem. I needed to search further.
The high voltage diode needed to be checked. Due to their high resistance, they can not be tested with a normal diode tester. They need another form of testing. This can be accomplished by using a 9 Volt battery (in this case measured 9.69 Volts), together with your multimeter in series, on the voltage range. In this case, the reading should drop to about 6 Volts for a healthy diode. The voltage drop can be seen by using a multimeter across the diode itself. This showed a reading of 3.28 Volts.
Next the high voltage capacitor needed to be tested. Bridge the 2 output terminals together. This allows the capacitor to discharge. Connect a capacitance meter on the 2 outer terminals (there are actually only 2 terminals – these are just joined). When removing the bridge wire, the meter should slowly climb up to the correct reading. This is noted on the side of the can. The example shown below (right) is 1.00uf, however the one used in the microwave read 1.10uf and so the reading below was within specifications.
As already stated, due to the extremely high voltages produced by the secondary circuit in the transformer, it is difficult to test for maximum output under load. However the resistance readings could be noted:
Primary windings (0.9Ω)
Primary to the body of the transformer (no continuity)
Filament winding (0.6 Ω)
Filament to the body of the transformer (no continuity)
Secondary winding to body of the transformer (95.5 Ω)
All readings were found to be within specifications.
The next step was to check the condition of the magnetron itself. As the shielding was removed the fault became obvious. The magnetron contains 2 circular ferrous magnets that help distort the path of the electrons which create the oscillating microwaves that cooks your food. One of these was cracked. This would not create the field distortion in the center vacuum tube.
At least the reason for the blown fuse could now be determined.
However after pricing a new magnetron and comparing the price of the microwave (which by the way was only out of warranty by about 6 months), it was determined that the repair using a new magnetron would outway the price of a new microwave.
Don’t panic! I haven’t finished yet!
After checking my collection of salvaged microwave parts, I found another LG magnetron. However, this suited a 900W and not a 1100W oven that I was working on. After consultation with the customer, it was agreed that it was worth the effort to try and repair the oven.
After all the components had been refitted to the oven, including the 900W Magnetron, I decided to use the old globe trick in place of the fuse to make sure the circuit was performing correctly. But I did not realize that I was now testing a high voltage circuit, up to 5kV!
The globe glowed very brightly and then produced a blue flash before the filament broke! At least I knew there was high voltage now operating through the circuit….
I felt I could confidently install the high voltage fuse and finally test the microwave.
A glass of water in for 30 seconds and now hot water was produced. With the customers’ approval, the microwave was used to cook our evening meal and proved to be in good working order and no noticeable difference was noted of its power output difference.
Sometimes we need to think outside of the box to save these appliances from recycling or worse yet – landfill.
Sorry guys, I didn’t get a chance to do a video on the repair this time. Next time I’ll do one for those interested.
This article was prepared for you by Mark Rabone from Australia.
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Note: You can check out his previous repair article below: