In went the cup of water for tea and pop went the fuse. So I replaced it. But then it popped again. A pattern was emerging. It seemed to me, and my wife, that it only happened when opening or closing the microwave door. Since this was a built-in with a rather expensive trim kit I figured I would take a crack at fixing it. The first trick to replacing the fuse was finding the bits for the security torx screws that attached the case on the back of the microwave. At the hardware store, if you don’t see it on the shelf, ask. In this case they keep the security torx in a cabinet. They are not on the sales floor with the other driver bits, at least not in this particular store. So, with that challenge out of the way it was on to figuring out what was causing the fuse to blow.
Since it seemed that it was related to the door, I started looking around the interlock. The interlock on a microwave does several things.
- It turns on/off the light inside of the microwave.
- It stops the turntable if you have one when you open the door.
- It prevents you from running the microwave with the door open.
It is this last thing that is the most important. You are not going to glow in the dark if exposed to microwave energy. It is non-ionizing radiation. It is more like a powerful radio transmitter. But if the interlock failed and the oven was still running when you stuck your hand in there, it would not be a good thing. 1000 watts of microwave can heat a popcorn kernel to over 400 degrees in a few moments.
So, where was I… the interlock. On my microwave there are a series of microswitches along the front and back of the interlock mechanism (4 total in my particular model) The mechanism is held in with only 2 screws, so it is pretty easy to remove. Somewhere on the inside your microwave (once you get the screws off and remove the cover) should be an envelope with a wiring diagram of how it is all connected. But just to be sure, grab your camera and use the macro(close up) settings on your lens to get some close up shots of how the wires are connected to each switch before you start taking anything apart.
The microswitches are fairly easy to remove. They are simply held in with some plastic tabs. Be gentle, you don’t want to snap any of the tabs off. In my case, it was pretty obvious which switch was bad. It was a simple game of “One of these things is not like the other ones”. On one of the switches the little plunger on the top of the switch was stuck down inside the body of the switch. The plastic shield around the lug that attached the wire to the switch was blackened on the inside, and the lug on the switch was discolored like it had been overheated at some point. When I removed this switch from the interlock, it fell apart. Inside the contacts were clearly charred where some severe electrical arcing had occurred.
My guess is when the plunger became stuck in the switch body it caused a dead short when the door was either opened or closed, thus blowing the fuse. There is a set of upper and lower microswitches that work in tandem to detect that the door is properly closed. Activating one, must have caused the dead short in the other since it was stuck.
A trip to a local electronics supply house provided me with a plethora of replacement options. I chose a nice Cherry microswitch with the same dimensions and ratings. It was simply a matter of paying $1.25, sliding the new switch in place, reconnecting the wires, and installing one last replacement fuse.
I am happy to report that it has been several days, and several cups of tea later and it is still working fine.
Never attempt this type of repair if you are not comfortable working around electricity. And Never Never plug in the microwave with the cover removed. You do not want to energize the magnatron (that big metal thing with the metal fins that heats the popcorn to 400 degrees in seconds) with the cover off. The inside of the microwave also has a couple of fairly large capcitors that would curl your hair, in a bad way, if you poked at the wrong thing. This is also clearly going to void your warranty if you have one.
I already have curly hair (What’s left of it) and have always been perfectly at ease taking things apart. This repair saved me about $500 – The cost of a replacement microwave + a new trim kit.