Specifications

Ver SG 9/200 22 Sharp Electronics Corporation
MAGNETRON SECTION
In order for the service technician to comprehend the service procedures, and their
limitations, they must have a understanding of the operation of the magnetron tube.
Refer to the next page for the following explanation.
The magnetron is made of a cathode, an anode, and an antenna.
The cathode is made of a substance which will have many free electrons in its molecular
structure. Usually this substance is a man made element which combines barium and
strontium oxide together. Most manufacturers just refer to the oxide coating on the
cathode as “rare earth”. The cathode is then encircled with an anode which is constructed
to create resonate cavities within its interior. The resonate cavities are then connected to
an antenna. The next requirement for manufacturing is to evacuate the air space between
the cathode and anode creating a perfect vacuum. A heater is then placed on the cathode
which shares one of its terminals with the high voltage lead. Magnets are then placed
around the outer side of the anode. The magnetron tube is now ready for operation.
The first requirement for the tube to function requires us to get the free electrons of the
rare earth element off the cathode, suspending them in the vacuum around the cathode.
This is accomplished by pushing current through the heater which is on the cathode. The
current is pushed by a relatively small alternating voltage, usually around 3 volts AC. As
the cathode heats the electrons begin moving. Much like water is in solid state (ice)
when cold, as heat is applied the molecules will begin movement until they actually leave
the mass in the form of steam. We usually refer to the process of electron movement
from the cathode to the vacuum area as the boiling off process although the electrons do
not experience the change of state which we often relate to when we think of boiling.
The next requirement is to get the electron mass to move away from the cathode with a
certain degree of velocity. This is accomplished by putting a high voltage charge on the
cathode. The charge is usually about 4000 volts negative, direct current. The anode is
then connected directly to ground. The movement of electrons is from negative to
positive, the large negative charged electrons see ground as positive which will cause
them to accelerate outward toward the anode.
The next step for the tube to operate is the magnetic flux generated from the permanent
magnets on the magnetron, which will restrict the electron movement. The electrons can
not strike the anode directly, they must move past the resonate cavities of the anode. As
the cloud of electrons pass a cavity they will create movement of the electrons within the
anode material (usually a copper alloy).