Microwave Oven Troubleshooting With Failing Diode - Page 1
Forum section: electronics / repair

author: Eccleston
author_rank: Newbie
timestamp: May 20, 2025, 09:32:51 pm
content: Hello,I'm trying to troubleshoot a microwave oven and would appreciate some insight.The symptoms are as follows: all LCD segments scroll continuously, there's a high-frequency noise around 10 kHz when the unit is powered, and after opening the door, the relays click and the internal lamp flashes.For troubleshooting, I measured the resistance on the low-voltage side of the transformer, which came out to 2.2 ohms and seems normal. The magnetron shows a short between its terminals, which is expected. Using an analog multimeter, I tested the HV capacitor. In ohm mode, measuring terminal to terminal, the resistance rises and settles in the high megohms range. Measuring from terminal to the body shows infinite resistance, so the capacitor appears to be functioning as expected.Now for the strange part. I tested the HV diode using an analog multimeter. Compared to a known good diode, the forward current looked normal, but in reverse bias, the diode showed about 50 percent of the forward current value, much higher than it should. That suggested excessive reverse leakage and i decided to replace it with another diode of the same specification, known to be good.After replacing the diode, I powered on the microwave without the back cover in place. It worked fine: it heated food normally and the high-frequency noise was gone. I then unplugged the unit and waited about 30 minutes to let the capacitor discharge. Just to be safe, I discharged it using a resistor and confirmed the voltage was at 0 volts with a multimeter. After reassembling the back cover, I powered it on again, and immediately heard the same high-frequency noise as before. Upon inspection, I found that the new diode had failed as well.Any ideas what might be killing the diode so quickly? Is it worth continuing with the repair?Thanks in advance.Edit: Got a suggestion IRL to disconnect the LV terminal that runs from the control board to the transformer and test it. For a while, the microwave behaved normally, then eventually returned to the behavior described earlier. After tapping the board lightly with the blunt side of a screwdriver, the issue stopped at least for now. I’ll probe and test the control board on Monday. I still believe something on the HV side is borked, but the only way to know for sure is to fix the board first.

author: ondraN
author_rank: Regular Contributor
timestamp: May 21, 2025, 09:30:55 am
content: The capacitor is probably also faulty. It will have a leakage current, which will only become apparent at higher voltages.

author: Eccleston
author_rank: Newbie
timestamp: May 21, 2025, 10:58:57 am
content: Thanks for the reply.Is there any way to be sure it's the HV capacitor that's killing the diode and not something else, like the magnetron or the HV transformer? I don't want to buy a new capacitor just to end up ruining it because another component is faulty.

author: ondraN
author_rank: Regular Contributor
timestamp: May 21, 2025, 11:47:23 am
content: If it worked for a while, then the capacitor is most likely defective. It is possible to measure it, but not with a regular multimeter. If the magnetron was defective, then after replacing the diode the oven would not heat at all, or only very little. But it's not entirely possible that there's another problem. You have to consider for yourself whether it's worth trying again or buying a new oven.

author: Seekonk
author_rank: Super Contributor
timestamp: May 21, 2025, 12:59:47 pm
content: I have a microwave that was just a week ago shooting out flames into the cooking area. It even burned a hole thru the mica cover, surprised it wasn't plastic. I opened the case to see what microwave tube it used as I had an old microwave sitting around. Removed the tube and it had arcing on the end of the emitter. I also noticed that it had been dropped sometime before purchase as the transformer mounting was severely bent. Microwave chamber had a diagonal sheet metal in front of the exit with arcing on it. I just bent that out slightly away from the tube. This had worked for four years after purchase. Don't know what eventually made it go crazy.

author: wilfred
author_rank: Super Contributor
timestamp: May 21, 2025, 02:00:31 pm
content: As far as I understand it the HV diode is a series of diodes and has a higher forward voltage drop than a single diode that a multimeter is designed to measure. When I was diagnosing my microwave I found a reference online to a simple circuit using a 9v battery to test the HV diode.I've pulled apart half a dozen generic and relatively modern microwaves and they were so similar inside. Oddly, in the HV section the strangest difference that presented difficulties in swapping components was the size of the terminal spade lugs on the diode, HV fuse holder and capacitor. I needed a fuse and a microswitch.MAybe you can source a discarded microwave to see if parts are suitable. Even if only for testing.