, this over the range microwave stopped heating for the customer, but the vent and light functions continued to operate. When the door was closed and a cycle started, the unit made all the normal noises, but it just didn't heat. The customer was limited to what they could do to resolve the issue, so they called for service to see if it could get working again.
Microwave ovens are a bit unusual in there operation because of the way they heat food. This process essentially involves vibrating the atoms within the food itself generating friction which then cooks the food. The magnetron is the component responsible for generating this microwave energy, as it is essentially a radio transmitter mounted to the side of the oven. These microwaves reflect off of the metal sides of the oven and are absorbed by the food during the cooking process. With the door sealed tightly, all the energy is contained within the cavity allowing us to stare in as we watch our popcorn being popped.
Because many microwave ovens have become all but disposable due to low manufacturing costs, not all microwave parts are available for every model, but many of them can still be easily repaired if you are willing to do a little work. I do want to mention that microwaves use very high voltages to produce the needed energy to cook your food. This voltage can be present even if the unit is unplugged from the wall. Care must be taken when working on microwave ovens to avoid getting to close and personal with this voltage. So if you decide to open up your own microwave to do a repair, pay attention to what you are doing and never touch any part of the high voltage section without discharging the capacitor first. Please see my microwave disclaimer for details.
The microwave in this post was doing everything except heating, but it was still counting down the timer and beeping when done. This is a good sign that most of the microwave parts are working properly, but in order to test everything else, I am going to need to pull this unit off the wall and remove the cabinet. Don't try this by yourself as many of these units are heavy. I (along with a helper) removed the unit from the wall and proceed to take all the screws out that hold the cabinet to the chassis. With a clear view of the interior components, I didn't need to look to far to find one of the thermostats had literally broken in two. This one is mounted directly to the magnetron as a safety device in the even of overheating. If the magnetron gets to warm, the thermostat will open the circuit to the high voltage transformer cutting power to the unit. Once the magnetron cools back down, the thermostat contacts will close and again completes the circuit.
Magnetron's generate quite a bit of heat when in operation, so it is essential they are cooled in some way. Most microwaves use a cooling fan to do this, while others (like this one) use the ventilation fan to help keep everything cool. The fan was working just fine on this microwave, so I felt comfortable just replacing the thermostat. With the new thermostat in place, I plugged the unit back in to do a quick heating test with a cup of water. Making sure everything was clear of the electrical components, I turned the unit on and a minute later I had hot water. Now its just a matter of putting everything back the way I found it, and reinstalling the unit back up on the wall. This microwave is once again heating, and the customer is glad to have it back working.