64 / 116
6 4
E L E CT R I C AL CONNE CT I ON
W I NT E R 2 01 5
APPLIANCE REPAIR
Don’t always wave goodbye
T
he first microwave ovens were
intended for laboratory use. When
they became available to the
public, they were extremely expensive.
But, a low-end unit can now be had for not
much more than $100.
Because of this, it may seem like
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.
So, if there is no sign of life in your
microwave, 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.
A WARNING
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 2.4GHz. 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.
THE FIX IS IN
Microwave radiation is generated by a
device known as a cavity magnetron, first
The microwave oven is a
simple appliance and repair
is not difficult, as long as the
hazards are recognised.
David Herres
explains.
When replacing a cord, never go to
a lower ampere capacity; this will
set the stage for an electrical fire.