LG MH-1355M Service Manual page 19

MAGNETRON TUBE
• The magnetron tube is the heart of the microwave
oven. It is basically a diode with the addition of a
magnetic field. The magnetron tube assembly con-
sists of cylindrical cathode (negative) terminal,
enclosed within a cylindrical anode (positive) termi-
nal, evacuated of all air, and surrounded by a perma-
nent magnet.
• In order to create an electron flow from cathode to
anode, the cathode must be heated and a potential
difference must exist between the two. This is
accomplished by heating the cathode with 3.15 VAC
(from the filament winding of the high voltage trans-
former), and applying a negative 4000 VDC (from
the voltage doubler circuit) to the cathode.
• Originally, the electrons would travel in a straight
line from the cathode to the anode (see Fig. 9).
However, with the addition of a permanent magnet
surrounding the anode creating a magnetic field, the
electrons travel an orbital path between the cathode
and anode (see Fig. 10). As the electrons approach
the anode, their orbital path takes them past small
resonant cavities that are part of the anode. The
passing notion of the electrons induces electron cur-
rent to oscillate in the resonant cavities at the very
high frequency or 2,450 megahertz. This RF energy
is then transferred to the antenna.
TESTING MAGNETRON TUBE
• Disconnect power, remove the wrapper, and dis-
charge the capacitor.
• Remove the two leads from the magnetron ter-
minals.
• Connect the ohmmeter between one terminal of the
magnetron and the outer case of the magnetron. If
the ohmmeter reads infinity, go to below. If the ohm-
meter reads less than infinity, the magnetron is
shorted.
• Connect an ohmmeter across the terminals of the
magnetron. The ohmmeter should read less than
one ohm. If the ohmmeter reads over one ohm or
infinity, the tube is defective.
MAGNETRON WITHOUT MAGNETIC FIELD
ELECTRONS
ANODE
MAGNETRON WITH MAGNETIC FIELD
ANTENNA
ELECTRONS
ANODE
– 19 –
ANTENNA
CATHODE
Fig. 9
2,450 MHz.
CATHODE
PERMANENT
MAGNET
Fig. 10