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6 5
with
David Herres
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.