The microwave oven is a simple appliance, and repair is not difficult as long as the hazards are recognised. David Herres explains.

The first microwave ovens were intended for laboratory use.

When they became available to the public, they were extremely expensive. However, a low-end unit can now be had for not much more than $100.

It seems microwave ovens are not worth repairing, yet repairs may take only minutes. Some built-in models are still quite pricey and well worth restoring.

Also, there are always good reasons for learning about something new. You can find out about waveguides and cavity resonators, both of which are used in satellite dish receivers and many other types of equipment.

If there is no sign of life in the appliance, of course begin by checking the power source. Next, inspect the power cord for damage, carefully moving it from side to side, especially where it enters the cabinet. If there is some sign that the unit wants to power up, you’ve found the problem.

When replacing a cord, never go to a lower ampere capacity; this will set the stage for an electrical fire. Also, the section of cord inside the cabinet may be exposed to heat, so check the conductor rating.

The connector may be moulded into the cord, so you may have to hunt down a replacement. Be sure everything is like new so that a shock or fire hazard is not created.

Before opening the cabinet, check the door interlock switch. This ensures the oven can’t operate when the door is open – something that would be extremely hazardous. When a microwave oven won’t power up, the fault is often here.

The switch typically consists of one or two pins that go into holes when the door is closed. These pins, or another mechanical linkage, actuate an electrical switch in series with one or both legs of the incoming power.

Sometimes you can press firmly on the door, nudging it up and down or from side to side, causing the unit to spring to life. If this does not happen, the interlock switch may still be at fault, the problem being electrical.

The term ‘microwave’ refers to the wavelength, which can range from 1mm to 1m. Frequency varies inversely with respect to the wavelength and, by definition, microwave falls between 300MHz and 300GHz. The latter is a mind-boggling 300 billion vibrations per second!

The domestic microwave oven generates about 2.45GHz. This electromagnetic radiation heats the water content of food. Microwave cooking is very efficient, because the material is heated from within, wherever there are water molecules. (Foods such as potatoes may explode, so they should be pierced with a fork. And don’t try to cook eggs in their shells!)

You can’t be harmed by microwaves from a properly functioning unit. The energy is non-ionising – it does not knock electrons out of their orbits around atomic nuclei.

However, microwave radiation could cause burns – just like any heat source – particularly in the eye’s cornea where it can cause cataracts. There is no blood circulation to carry away heat, so the cornea loses its transparency like heated egg white.

To prevent this hazard, the oven is lined with metal shielding, which confines the microwave radiation. The glass door is fitted with a grid composed of evenly-spaced holes whose geometry is such that microwaves cannot pass through, but shorter wavelength light is conveyed, permitting the user to view the contents of the oven.

There is a health hazard if the door interlock switch has been defeated and the oven can operate with the door open. Also, a user may sustain radiation burns if the glass is broken, the door is warped or poorly fitting, or if any part of the protective metal shielding has been removed,.

Do not pull apart a microwave enclosure unless you are fully aware of the dangers and take appropriate safety measures. The ovens contain hazardous substances that can be carcinogenic. It is not wise to remove any internal parts of a microwave oven for use in educational projects.

Microwave radiation is generated by a device known as a cavity magnetron, first built in connection with radar research. This device operates on high-voltage DC, which is produced by an internal power supply.

As in a TV or computer monitor, the high voltage is stored in electrolytic capacitors, which retain the charge for a long time after the unit has been powered down. Correct procedures must be followed to discharge these voltages because they are high enough to be lethal.

If the power supply has no output, check diodes and capacitors using a multimeter. Replacement is straightforward and parts are readily available.

The magnetron attracts electrons from a small heated wire. Magnets then rotate the electrons in a vacuum. Due to the cavity dimensions in magnetrons, the electrons create microwave radiation, which is emitted by a very small antenna into a waveguide that channels the radiation through a small transparent window into the oven.

The waveguide is an air-filled tubular structure fitted tightly to the magnetron body with a gasket. The radiation travels without loss through this pipe. If a cable were used, there would be too much loss due to capacitive shunting at such a high frequency.

The magnetron creates a certain amount of waste heat, and in such a small volume the temperature would be high if it were not for the air cooling. The magnetron’s cooling fan makes the sound you hear when a microwave oven is running. If this sound is absent, the fan has quit.

If you catch it in time, before there is component damage, the fan can be replaced or perhaps its bearings can be lubricated with a few drops of light machine oil.

Besides components that produce and convey the microwave radiation, there are controls that turn on the magnetron, fan and oven light, and limit the duration. Circuit board repair for these controls involves careful soldering with heats sinks so that components are not fried. It is feasible to replace an entire board or keypad if needed.

Most modern microwave ovens have a carousel to ensure that the food (especially frozen) is evenly heated. It is best to place food to the outside of the turntable so there is no a dead spot in the centre. Of course, the carousel motor can quit and may need lubrication or replacement.