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Microwave stopped heating and I fixed it. Answered

Our microwave is about two years old and it stopped heating. The fan ran and the carousel went around, but food did not warm.

I took it out of the upper cabinet mount and opened the case. I know about the big capacitor and discharged it with a screwdriver after disconnecting the power. For extra safety I clipped a jumper wire on both terminals.

I watched numerous videos at YouTube on troubleshooting a microwave and on testing the various major components at home, but no component showed any signs of failure. Then it began to work again as suddenly as it had stopped working. About ten days later, it stopped heating again. After more testing, still no components showed signs of failure.

But, I did squeeze the female spade crimp connectors a little with a pair of pliers before connecting them again so they fit more tightly when reconnected. The microwave has continued to work as it should. My wife remembers a time a few months ago when she ran it and ran it to get something to cook as she wanted it and the microwave stopped because it was overheated. I suspect a connection was weak and heated the metal in the connector enough that it became weak. Later it failed intermittently.

We could have called warranty service, but we did not want to wait. Knowing what I know now, I doubt a serviceman would have patiently checked for weak connections, but would probably have simply replaced some parts. When those did not really solve the problem, the microwave would have been condemned and replaced. A replacement oven may not have matched our other appliances as well as our present microwave does.

Test procedures for components in a microwave are different than they are for similar components used in (for example) a sound amplifier. This applies especially to the high voltsge diode. Do not connect a voltmeter to the high voltage side of the transformer. When running the microwave, be certain to have a dummy load in the cavity, like a cup of water. A very useful test that checks for power to the transormer primary is to disconnect the primary side transformer connectors and attach aligator clips from a voltmeter to the transformer leads. Then run the microwave for a few seconds and check for a full line voltage reading each time.

A clamping ammeter to measure line current helps because the sound with the magnetron working is not too much differnt from the fan and carousel running without the magnetron working, but, it is easy to see the extra needle deflection when the magnetron is also drawing current.

What seemed to be an expensive problem had an easy inexpensive solution.


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2 years ago

Start with the pure basics first.
A lot of microwaves have temperature sensors as well as safety switches by the double.
A failing microswitch for the door can turn off all functions off, some or just the power to the transformer.
The temp switches open with too much heat, so you only need a basic multimeter across the switch to check it has continuity.
If you can't hear the usual humming when the microwaves heats then start with the transformer too.
With no power to the transformer you eleiminate all need to check the high voltage side or magnetron ;)
I suspect a faulty microswitch in the door or some not so perfect connection.
Check all connections that are not soldered.
Take the time to get the safety switch assembly out if you can't get the wires off.
You will need to check each switch without the wires on it, so take a pic or mark them !!
Some will be normally closed, some normally open.
Either way the reading on your multimeter or tester should change clearly if you push the switch.
Anything not perfect here means you need to replace the switch in question.
Having a scrap microwave at hand comes in handy here ;)

For further work: Just unplug the microwave and wait an hour.
The capacitor has a bleeding resistor and should be empty by then.
And unless you mess directly on the high voltage side of thing you are safe anyway.

Phil B
Phil B

Reply 2 years ago

I took some steps not mentioned above because I wanted to stress the point the problem can be something so simple as a weak or intermittent connection, and I did not want to clutter what I wrote with lots of not quite necessary details. I did remove connections from the door microswitches and test them. One of them may have been unreliable to some degree. (In trying to free it from the snap in catches the top came off and parts fell out.) I did replace the switches as a precaution of relatively low cost.
I found only one thermal cutout, and my Ohmmeter indicated it is good. I did test it out of the circuit to avoid spurious readings due to feedback through a back channel.
The normal sound of the fan and the carousel made it very difficult to distinguish the presence or absence of any sound from the transformer or the msgnetron. If there had been an abnormal growl, as with a bad capacitor, I could have noticed that. It was so much easier and more certain to use a line current ammeter normally clamped around one conductor and watch for an 13 amp. draw compared to a 5 amp. draw. I did check the bleeder resistor in the capacitor, and it was good. My digital multimeter has a capacitance function and I did confirm the value of the capacitor. I also checked it for shorts and opens. I have no scrap microwaves available. Thank you for your detailed discussion.