Model #PLMVZ169HCA, this over the range microwave and hood combination would appear to turn on when the start button was pressed, but only the timer would could down. The normal operating sounds were not heard and when the door was opened, the cavity lamp was not working and neither was the surface lighting or vent fan. It was acting like a dead unit except the display and touch pad were still functioning.
Microwave ovens that stop working are not to difficult to diagnose, but they can be very difficult to service. I must say that if you are unfamiliar with the servicing of any microwave oven that you do not attempt your own repair. Microwave ovens use very high voltages during their operation that may still be present even when the unit is unplugged from the outlet. Touching the wrong part can result in serious injury or death so do not be casual about working on one of these units. Nuff said.
Given the behavior of the unit, and the fact the display was illuminated and counting down, I at least knew the control board was operational, and the secondary interlock switch was working. But with everything else on the broken list, it was time to get out the meter and take some electrical measurements.
With the panel removed, I was able to get to the connections on the back side of the board and started verifying voltages to the board. The one I was most concerned with is the voltage that comes out of the control relay and goes to the high voltage transformer. This is also the input source for all the other components so I figured it was a good place to start. In making my check at the connection, the cavity lamp turned on a for a brief instant which was surprise enough that I quickly pulled my test leads away. But my meter did register 120 voltage AC going into the relay, so I knew power was up to that point. But it was the flicker of the lamp that hinted at the reason for the failure.
Control boards contain electrical components that are soldered to wire traces on these boards which provide paths for current to flow. Think of them as wires that lead from component to components making a complete circuit. And just like wires, when a connection becomes broken, current cannot flow and the circuit will no longer function.
I removed all the connections from the panel so I could take it down from the microwave. A few screws removed had the control board separated from the panel so I could take a closer look at it. After some close inspection, I found the reason for the light flickering, and why nothing else was working. One of the solder connections for the main relay had become broken and was now loosely connected to the trace on the back of the board. This happens to be the point current flowes into the board before it goes out to the other components that were all not working. By pushing against the relay, I was able to visually see the broken connection and damaged wire trace.
The solution was as simple as installing a new control board given this microwave was still under a manufacturers warranty. Had this been a COD call, an acceptable solution would be to repair the electrical connection with a soldering iron and wire. Most techs I would with don’t have much soldering practice let alone a soldering iron, but if you do, fix the board and save the customer some money.
With the new board installed, everything started working like it was new.
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