Microwave Ovens

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This page covers domestic microwave ovens.

Summary

On safety grounds, we recommend leaving the repair of microwave ovens to a professional, since internally, they use very high voltages at high power. We don't normally work on microwave ovens at a Restart Party on account of the risks (see below) and the difficulty of controlling casual bystanders.

There are, however, a few simple things that can be tried.

Safety

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It could be EXTREMELY DANGEROUS to operate a microwave oven with the cover off, much more dangerous even than working on raw mains electricity. Even after switching off, the capacitor may hold sufficient charge for an extended period to give you a DANGEROUS, possibly LETHAL electric shock. You should NEVER remove the main cover unless you have a complete understanding of the risks.
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An improper repair could lead to potentially dangerous leakage of microwave energy.
Never work on a microwave oven when you are alone. All other people present must be warned of the risks.

Principles of Operation

At the heart of a microwave oven is a cavity magnetron, a device invented during WWII for the generation of high power microwaves for use in radar. This generates radio energy which is piped into the oven by a wave guide in the same way that light can be piped through an optical fibre. The magnetron is powered by a very high voltage DC, provided by a transformer, rectifier diode and capacitor. A fan provides both cooling for the magnetron and air circulation for the oven. A motor turns the turntable to ensure the food is evenly cooked, as the microwaves don't spread evenly through the oven. Finally, a simple mechanical timer or an electronic programmer controls everything.

Simple Faults and Repairs

Always switch off and unplug the oven before doing even the simplest investigation or repair.

If there is no sign of life at all, check the fuse in the plug. However, fuses don't normally blow for no reason and this may indicate another more serious fault.

There are just a few simple faults that can be fixed, listed here.

Failed bulb

Look for a small cover plate on the back of the oven or a window inside secured by one or more screws which you can remove in order to access the bulb. Replace it with one of the same type, voltage and wattage.

Sparking

If you can see sparking inside the oven, the first thing to do is clean the inside thoroughly. Remove all traces of any burnt food or burn marks from the sparking itself.

Pay particular attention to the wave guide cover where the microwaves enter the oven. In a bad case, this can get burnt through. There are instructions online to clean or replace the wave guide cover, but these depend on it being being removable by undoing screws accessible from within the cooking compartment, which may not be the case. Be aware that cutting a new wave guide cover to size may create harmful dust.

Plate not turning

Assuming everything else seems to work, turn the microwave upside down (not forgetting first to remove the plate and anything else inside it). Check to see whether there is a separate cover that can be removed to inspect and possibly repair or replace just the turntable motor and mechanism.

Door problems

There is normally a mechanical linkage on the door latch which can cause problems. This activates a switch to switch off the power when the door is opened. Access can normally only be gained by removing the main cover, which is not advised. Read the Safety section before doing this. A broken plastic lever or cam might be repairable but glue on its own is unlikely to hold without some form of extra reinforcement.

For extra safety there may be two separate switches attached to the door. If they disagree about whether the door is open or shut they may deliberately blow a fuse in order to disable the oven for safety. It would be extremely unwise to replace, reset or bypass this fuse without performing a complete repair, which should be left to a professional.

Timer and Controls

As with the door mechanism, access to the timer and controls may only be possible by removing the main cover. Read the Safety section before doing this. There is some possibility that faulty buttons or switches could be cleaned or replaced, but beyond this electronic diagnosis and repair skills will be required. A mechanical timer might be improved by cleaning but failure may simply be due to wear.

Other Components

You may find websites which describe the replacement of other components, in particular the transformer, diode, capacitor, magnetron and fan. On Safety grounds, you are strongly advised to leave these to a professional repairer. If not mated properly with the wave guide, the magnetron may cause sparking, overheating, and interference with WiFi, Bluetooth and other wireless technologies.