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Simple service tips
for your
•
nncrowave oven
Most people take their microwave
oven for granted and never do
anything to ensure that it is always
working at top efficiency. In this
article, we detail some routine jobs
which will help you to get the most
from your microwave oven.
By LEO SIMPSON
Why should you have to do anything to a microwave
oven? After all, modern electronic appliances are very
reliable and so you may feel well justified in not
touching anything on your microwave oven, apart
from giving it the occasional wipe over.
The problem is that simple cleaning is not good
enough.
Essentially a microwave oven is just a big unfiltered
DC power supply feeding a magnetron. And a
magnetron can be regarded as a big thermionic valve
which pumps microwave energy into the oven.
Whenever the microwave is actually cooking
THIS PHOTO CLEARLY SHOWS the difference between
Pozidrive (left) and Philips head screwdrivers. The
Pozidrive has extra flutes between the blades.
6
SILICON CHIP
something, the magnetron is dissipating quite a lot of
heat and that is removed by a ventilation fan. You can
hear that fan running when you are using the oven.
For that fan to work properly, all the vents in the
rear and base of the oven cabinet need to be kept
clear. If your oven has been in use for several years,
you will probably find that these vents are clogged
with grease and dust. It's easy to understand how the
dust gets there - it is pulled though the oven by the
fan - but how does the grease get there? It comes
from the food being cooked in the oven.
The ventilation fan which cools the magnetron also
removes steam and fumes from food cooking in the
oven. All these fumes are pulled right through the oven
and vented out the back. So if you cook a lot of fatty
foods and the food is not covered with some sort of
plastic film then not only will grease be deposited inside the oven itself, it will also be deposited inside the
enclosure at the rear and sides of the oven.
Mix this grease with dust and you will find a pretty
daggy mess in the vents of your oven. The problem is
likely to be worse with ovens that are permanently installed in kitchen cabinetry, although ovens which sit
on bench tops can be pretty messy too.
So every few years or so, it is a good idea to clean all
this mess out of the vents. This is best done with a
vacuum cleaner, using the brush nozzle fitting. You
can do most of this basic cleaning job without having
to gain access to the cabinet. To do the job properly,
though, you need to remove the outside casing of the
oven.
WHILE YOU HAVE THE OVEN casing off, it is a good
idea to remove and clean the interior lights and the glass
window. Dry them thoroughly before re-installing them.
Now it might seem obvious, but the power plug must
be disconnected from the power point before you even
think of removing the screws. Microwave ovens contain a large high voltage transformer and capacitor inside. If you accidentally came in contact with live
wires to these units, it could very easily be fatal. So
disconnent the power plug from the power point.
screwdriver which doesn't fit properly. In an extreme
case, if the screws are very tight, you might damage
the heads so badly that it won't be possible to remove
them. We don't want that, do we?
How do you tell a Pozidriv screw from a Phillips
head screw? Our photographs show the difference.
Unfortunately though, they don't tell the whole story.
Crosshead screws from some Asian sources may look
like Phillips head types but a Pozidriv screwdriver will
be a better fit.
Most hardware stores these days stock Pozidriv
screwdrivers, along with their Phillips head types.
Those made by Stanley have blue handles while their
Phillips head drivers have green handles. The size to
fit the cabinet screws of most ovens.are Phillips point
1 or 2 or Pozidriv point 2 or 3.
For a comprehensive article on the many diverse
screws and screwdrivers in use today, see "Screws &
Screwdrivers: What You Need to Know" in the
November 1988 issue of SILICON CHIP.
Having removed the casing from the oven, you are
likely to find more dust and grease inside. This should
all be cleaned off as well as possible. But while you
are doing that, make sure you don't disturb any of the
wiring.
Cleaning the lights
To remove the screws from the casing of the oven,
you need the correct screwdriver. Most likely, the
screws will be a crosshead design but a Phillips head
driver will not necessarily be a proper fit. They might
need a Pozidriv (pronounced "pozzy drive") screwdriver instead.
Because the screws are likely to be very tight, you
are likely to damage the heads if you use a
Many microwave ovens have interior lights
although these are not in the oven itself. They shine
through perforations in the oven wall and are further
isolated by a layer of glass. As time goes on, you will
find that these lights become more and more ineffective, particularly in microwave/convection ovens.
Ultimately, you won't be able to see anything inside
the oven, even though the lights are on.
What happens is that grease circulating around the
oven is progressively baked onto the lamps and onto
both sides of the glass. This happens even in the most
fastidious households so that even if the oven itself is
kept spotlessly clean, the globes and glass will still
have a coating of baked on gunk.
COCKROACHES CAN CAUSE big problems in microwave
ovens. The dark patch indicates where a cockroach has
urinated on the PC edge connector, leading to erratic
keyboard operation.
A CLEAN-UP WITH A TOOTHBRUSH and some
methylated spirits was all that was needed to cure the
erratic keyboard operation. A squirt with silicone water
repellant completed the repair.
Use the right screwdriver
A PRIL 1990
7
SUSPECT THE THERMISTOR IF the cooling fan runs
continuously. It's easily removed for replacement by
unplugging the leads and undoing two self-tapping
screws.
THE VENTILATION SLOTS in the back of the cabinet
should be cleaned regularly so that the oven's cooling fan
can work effectively. This is best done using a vacuum
cleaner with a brush fitting.
While it is usually possible to change the lamps from
a compartment on the side of the oven, it is not possible to remove and clean the glass, unless the oven casing is removed as we have just described. Once you
have the casing off it usually is an easy matter to
remove the whole bracket which supports the lamps
and glass. Be careful when removing the screws that
you don't let the glass fall out and break. Give the
glass and bulbs a thorough cleaning and dry them off
before reinstalling them.
fortunately, the place which is often the cosiest for a
cockroach is just behind the vacuum fluorescent
digital display or behind the keyboard. Sometimes they
die there and that can spell a problem for the
electronics.
If the clock display on your microwave oven starts
to dimly light segments which are not supposed to be
lit or the keyboard starts behaving erratically, the
most likely suspect is a dead c9ckroach. Inevitably,
the cockroach will have urinated on the printed circuit
board and the resultant leakage will cause the erratic
behaviour of the keyboard or clock display.
If left too long, this problem can also lead to severe
corrosion on the printed circuit board tracks. The
oven in our photos had this problem and the cure was
to thoroughly clean the affected area of the board
with a toothbrush and methylated spirits. If you have a
can of CRC 226 or similar silicone water repellant,
it is a good idea to give the affected area a squirt or
two to seal it. This will help avoid a re-occurence of
any "ghosting" of the clock display.
One point to be watched in disassembling the oven's
control electronics is to make sure that you note the
position of every screw and cable connector. You will
find that a variety of different screw threads and
screw lengths will be used and it is almost impossible
to keep track of where each screw goes. So make notes
as you go. Better still, get a piece of white styrofoam
and stick each screw into it as you go, and note where
it came from.
Removing the cockies
Another prob!P.m which even the most fastidious
households can suffer is cockroaches. And these little
beasties love microwave ovens. Why? Because they
have nice litte niches which are warm and cosy. Un-
Thermistor replacement
BE SURE TO NOTE the position of every screw and cable
connector when disassembling your oven. Check that the
power plug is disconnected from the wall before you
begin work.
8
SILICON CHIP
The oven in our photos, a combination convection/microwave type, has also had a recurring fault
with the temperature thermistor which controls the
fan. It monitors the temperature of the cooling air and
causes the fan to run until the oven cools down. When
the thermistor fails, the fan runs continuously. The
cure is simply to replace the thermistor, as shown in
one of the photos.
Lightning protection
Before we conclude, we should say something about
lightning protection. Because microwave ovens are
usually permanently powered and they contain sensitive electronic components, they are . particularly
prone to damage during thunderstorms. This is due to
high transient voltages caused by nearby lightning
strikes.
Now most microwave ovens these days incorporate
surge protection components and these will be
sacrificed if a voltage surge occurs. The problem is, if
such damage occurs the microwave will not work
afterwards. It will have to be repaired at a cost of
somewhere between $50 and $100 and perhaps a lot
more if the damage has gone further than the surge
protection components.
Many areas of Australia have had very severe
thunderstorms in the last few months and if the present trend continues, we are likely to have many more.
Therefore, if a thunderstorm is coming your way, it is
a good idea to disconnect your microwave oven (and
any other electronic equipment such as VCRs, computers and TV sets). Sure, the surge protection components will work but why put them at risk?
For the same reason, if you are going away for the
weekend or a longer period, disconnect your
microwave oven and any other electronic equipment.
ONCE THE CASING HAS been removed, it is a good idea
to vacuum out the interior of the oven to remove dust
and foreign bodies. Take care that you don't damage any
parts or wiring during this process.
ANTRIM
That way, you don't have to worry about thunderstorms while you are away.
For more reading on the subject of lightning damage
to electronic equipment, see this month's article in the
Serviceman and an article entitled "Lightning! How
To Protect Your Electronic Equipment" in the
February 1989 issue of SILICON CHIP.
~
TOROIDAL TRANSFORMERS
General
Construction
OUTER
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MANUFACTURED IN U.K. NOW AVA ILABLE
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INSUL AT ION -
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15VA
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100.00
112.00
Enquiries from resellers and OEMs welcome. Quantity prices and data sheets available on request.
Distributed in Australia by Harbuch Electronics Pty Ltd , 90 George St., HORNSBY, NSW, 2077
Phone (02)476-5854 Fa x (02)476-3231
APRIL 1990
9
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