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How to repair a microwave, common problems.

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Note: Before touching any internal parts, be sure to discharge the capacitor. The capacitor stores additional voltage and can hurt you even if the unit is unplugged. To discharge a capacitor safely, you will need the following: a screwdriver, a wire-wound resistor with a 2 watt-20,000ohm rating, and a pair of jumper wires with alligator clips on the ends. Clip a wire to each end of the resistor. Clip one wire to the metal shaft of the screwdriver. Clip the other wire to one of the capacitor’s terminals. Now, touch the other terminal with the tip of the screwdriver. There may be a small spark. If the capacitor has three terminals, do the same process with the middle terminal and each outside terminal.



Problem: The microwave oven won’t run at all.
Solution: Unplug the power cord and check for voltage at the outlet. Inspect the cord for any damage or burn marks. Because of all the safety devices in a microwave oven, any one of them could be the cause. You will need to remove the outside shell of the microwave. Unscrew the screws underneath and on the back that hold the shell in place and slide it off. Here’s a list of things to check that may be causing this problem.

  • The fuse could be blown — Remove the fuse with a set of fusepullers. Place it on a paper towel so it doesn’t roll away. With your VOM set to RX1, place a probe on each end of the fuse. The reading should be zero. If not, replace the fuse with an identical one.
  • Door switch could be bad — Locate the door switches and remove the leads. With the VOM on RX1, probe the terminals. The reading should be infinity with the door open and zero with it closed. If not, replace. Check both door switches.
  • Fan motor could be bad — Locate the fan and remove the leads. With the VOM set to RX1, probe the terminals. If the reading is infinity, then it is bad.

Problem: The microwave oven keeps blowing fuses.
Solution: Check the door switch as described above. The capacitor or diode may be bad. Discharge the capacitor as described earlier in the article. Test the capacitor by removing the leads and setting the VOM to RX100. Probe the terminals. The reading should start in the low ohms and increase toward infinity. Reverse the probes and re-test. The reading should do the same thing. If not, it’s bad. To test the diode, disconnect the diode from both the appliance and the capacitor. With the VOM set to RX100, probe the wires. Then reverse the probes and read again. You should get infinity for one reading and low ohms for the other reading. If not, replace. Another cause could be a faulty magnetron, but due to the sensitivity of that piece, it’s best left to a professional.



Problem: The microwave oven cooks slowly or unevenly.

Solution: Check the voltage at the outlet supplying power. If it is lower than 115 volts, there is a problem with your electrical service or breaker. A bad turntable motor may also be the cause. To check it, turn the microwave over onto its top. Remove the bottom grill. Set the VOM to RX1 and remove one lead from the motor terminals. Probe the terminals. If the reading is infinity, then replace the motor. The magnetron and the wave guide may also be the culprits here. They need to be serviced by a professional.



Problem: The microwave runs but it’s not cooking anything.
Solution: Check the thermal cutoffs for both the oven and the magnetron. The thermal cutoffs are little disc shaped devices with a wire connecting the two of them. Remove a lead and set the VOM to RX1. Probe the terminals and look for a reading of zero. If not, replace. Check both thermal cutoffs. If these are OK, check the capacitor and diode as described above. The magnetron or transformer could also be bad, but they need to be serviced by a professional.



I hope we were able to help you determine the cause of your microwave’s problem. These are the easiest and least expensive repair situations. Any problems not covered here will require a professional in most cases. As always, have the make and model numbers handy when heading to the parts shop. If your microwave isn’t the only appliance giving you headaches, this website has repair guide for many of them. Pick your next project, heat up a cup of coffee in your now-working microwave, and read on. For more info you can also watch the video below:


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