Any microwave techs here? Magnetron question.

[i6pwr]

Our OTR LG microwave (LMVM20333BM) last time we used it, made a laboring hum for about 5 seconds after startup, then seemed normal for the next 3 min. Fan didn't stop, platter kept rotating, but didn't heat the food.

After some poking around, I found the high voltage fuse was blown, also I disconnected the magnetron leads to the filament to check for a short and I have .35 ohms to ground on both filament leads, but I also forgot to unplug the sensor to the left of the filament connector and when I did I now have 80 ohms to ground from each of the filament leads.

I assume even 80 ohms is too low?

 

[mk378]

In operation the filament carries a few thousand volts relative to the anode (ground). With filament cold there should be infinite resistance. There is a short inside the magnetron.

The round sensor is just a thermal switch to cut power in case of overheating it has nothing to do with the high voltage side. The short is usually a thin "whisker" of metal that can be displaced by vibration.

 

[i6pwr]

I removed the magnetron and found this little guy. Might be why this was a floor model years ago...lol.

Is the magnet isolated or can it cause a short to ground. I would assume it hasn't moved since we owned it but someone has been inside it and it has worked for a number of years...but maybe at a reduced capacity...who knows.

 

[i6pwr]

With filament cold there should be infinite resistance. There is a short inside the magnetron

You are referring to these terminals in relation to ground correct?

 

[Geekhead]

You are referring to these terminals in relation to ground correct?

View attachment 288368

 

[mk378]

Yes. That is the magnetron filament. There should be low resistance between the two terminals and open circuit from either terminal to ground.

The entire anode structure (frame, heatsink fins, and magnets) is DC ground. That screw was not going to cause a short circuit. The high voltage is all inside the box at the base where the wires from the transformer plug in.

 

[rstcso]

Please be extremely careful working on HV circuits of MWOs. A typical voltage is -2,000 volts with current of several hundred milliamps. These can, and do, kill people!

I concur, it sounds like the magnetron is shorted. The loose screw, although a bit alarming to find, most likely has nothing to do with the failure.

 

[i6pwr]

There should be low resistance between the two terminals and open circuit from either terminal to ground.

Yes, about .35 ohms between terminals, and 80 ohms to ground so definitely not infinite.

Please be extremely careful working on HV circuits of MWOs. A typical voltage is -2,000 volts with current of several hundred milliamps. These can, and do, kill people!

Oh absolutely. I discharged the cap as well.

 

[JimPghPA]

The inside of a magnetron may have beryllium in it as a high temperature insulator. Barillium is extremely toxic, and very small flakes of it can be inhaled and you won't even know that you've done that. For that reason alone it is best to not mess with the inside of a magnetron.

I never open up a magnetron. When a neighbor's MWO had enough electric leakage to trip her ground fault I had her get a different MWO. Again, I do not open any magnetron.

 

[benjamming]

Do they have thoriated parts too?