Midea MD927, MPD8320 Schematic
MICROWAVE OVEN
MODEL:
M720C -PM0C
FAA
(700W 220 V 50Hz)
SERVICE MANUAL
CAUTION
BEFORE SERVICING THE UNIT，READTHE SAFETY PRECAUTIONS IN THIS
MANUAL
SAFETY PRECAUTIONS
This device is to be serviced only by properly qualified service personnel.
Consult the service manual for proper service procedures to assure continued safety operation and for precautions to
be taken to avoid possible exposure to excessive microwave energy.
PRECAUTIONS TO BE OBSERVED BEFORE AND
DURING SERVICING TO AVOID POSSIBLE
EXPOSURE TO EXCESSIVE MICROWAVE ENERGY
A) Do not operate or allow the oven to be operated with the door open.
B) Make the following safety checks on all ovens to be serviced before activating the magnetron or other
microwave source, and make repairs as necessary; (1) interlock operation, (2) proper door closing, (3)
seal and sealing surfaces (arcing, wear, and other damage), (4) damage to or loosening of hinges and
latches, (5) evidence of dropping or abuse.
C) Before turning on microwave power for any service test or inspection within the microwave generating
compartments, check the magnetron, wave guide or transmission line, and cavity for proper alignment,
integrity, and connections.
D) Any defective or misadjusted components in the interlock, monitor, door seal, and microwave generation
and transmission systems shall be repaired, replaced, or adjusted by procedures described in this manual
before the oven is released to the owner.
E) A microwave leakage check should be performed on each oven prior to release to the owner.
CAUTION
MICROWAVE RADIATION
DO NOT BECOME EXPOSED TO RADIATION FROM THE MICROWAVE GENERATOR
OR OTHER PARTS CONDUCTING MICROWAVE ENERGY.
TABLE OF CONTENTS
（page）
SAFETY PRECAUTIONS------------------------------------
CAUTIONS------------------------------------------------------------------------------------------------1-1
THE HEATING PRINCIPLE OF MICROWAVE--------------------------------------------------2-1
INSTALLATIONS----------------------------------------------------------------------------------------3-1
OPERATING INSTRUCTIONS----------------------------------------------------------------------4-1
SCHEMATIC DIAGRAM--------------------------------------------------------------------------4-1
CIRCUIT DESCRIPTION-------------------------------------------------------------------------4-2
SERVICE INFORMATION-----------------------------------------------------------------------------5-1
TOOLS AND MEASURING INSTRUMENTS------------------------------------------------5-1
MICROWAVE LEAKAGE TEST----------------------------------------------------------------5-2
MEASUREMENT OF MICROWAVE POWER OUTPUT---------------------------------5-3
TROUBLE SHOOTING PROCEDURES------------------------------------------------------5-4
COMMON BREAKDOWN AND MEANS OF REPAIRING-------------------------------5-8
---------------------Inside front cover
I M PORTA N T T H I NG S TO DO P R IO R TO CR I T IC A L PA R T S E RV I CING -------------------------5-9
SAFETY CHECKS AND TESTS AFTER SERVING--------------------------------------5-10
EXPLODED VIEW---------------------------------------------------------------------------------------6-1
REPLACEMENT PARTS LIST-----------------------------------------------------------------------6-1
CAUTIONS
Unlike other appliances, the microwave oven
is
high-voltage and high-current equipment.
Though it is free from danger in ordinary use,
extreme care should be taken during repair.
• DO NOT operate on a 2-wire extension cord during
repair and use.
• NEVER TOUCH any oven components or wiring during
operation.
• BEFORE TOUCHING any parts of the oven, always
remove the power plug from the outlet.
• For about 30 seconds after the oven stops, an electric
charge remains in the high voltage capacitor. When
replacing or checking, you must discharge the high
voltage capacitor by shorting across the two terminals
with an insulated screwdriver.
• Remove your watches whenever working close to or
replacing the Magnetron.
• DO NOT touch any parts of the control panel circuit. A
resulting static electric discharge may damage this
P.C.B.
• NEVER operate the oven with no load.
• NEVER injure the door seal and front plate of the oven
cavity.
• NEVER put iron tools on the magnetron.
• NEVER put anything into the latch hole and the
interlock switches area.`
1-1
MICROWAVE RADIATION
Personnel should not be exposed to the
microwave energy which may radiate from the
magnetron or other microwave generating
device if it is improperly used or connection.
All input and output microwave connections,
waveguide, flange and gasket must be secure
never operate the device without a microwave
energy absorbing load attached.
Never look into an open waveguide or antenna
while the device is energized.
• Proper operation of the microwave oven
requires that
the magnetron be assembled to the waveguide
and
cavity. Never operate the magnetron unless it is
properly installed.
• Be sure that the magnetron gasket is
properly
installed around the dome of the tube
whenever
installing the magnetron.
THE OVEN IS TO BE SERVICED
ONLY
BY PROPERLY QUALIFIED SERVICE
PERSONNEL.
THE HEATING PRINCIPLE OF MICROWAVE
Microwave is one kind of radio wave whose wave length is very short, frequency is very high. Therefore, it is called ultrahigh
frequency electromagnetic wave. Microwave can heat food mainly r esult in the mutual affect of the food in the microwave field and
microwave field itself.
Under the affect of microwave field, the thermal effect mechanism produced from the mutual affect of the microwave and the food
includes two aspects: One is Dielectric loss of polar molecule, the other is conductive loss of ion.
Usually, food is constitute of organism (plant and animal).The organism is formed by all kinds of polar water molecule, polar
protein molecule, and all sorts of salt ion. The center of gravity of the positive and negative charge in the molecule is not c oincide.
In normal condition, the molecule is in irregular order due to its thermal action, thus the food do not appear polarity (FIG.11a).Under the action of outer electric field, the positive end of the polar molecule tre nd to the negative electric field, the negative
end of polar molecule trend to the positive electric field, and somewhat arrange in order through the direction of the electric
field(FIG.1-1c).This phenomenon usually be called “TORQUE POLARITY”. When the outer electric field apply for the opposit e
polarity, the polar molecule then arrange an opposite direction order accordingly (FIG.1-1b).If the direction of the outer electric field
changed repeatedly, the polar molecule would repeatedly sway accordingly. During the swaying, it understands that the polar
molecule would produce heat due to somewhat similar friction among them.
When the electric field is applied for ultrahigh frequent microwave field from the outside, its direction would chang e tens billion
times per second, so do the molecule. This kind of molecule swaying producing similar frictional heat from the interference and
block of the action strength among the molecule, and changed to microscopic microwave heating. Microwave heating not only
concerned the nature of the matter itself, but also closely connected with the electric field strength and frequency. When the
frequency is low, the molecule swaying rate and the acute degree of the mutual friction among the molecule is low, and would
produce much heat. When the frequency is too high, as the swing of the polar molecule is with rotating inertia, it made the swing
do not in line with the changing rhythm of the electric field because of the friction drag, thus, actually lowed the polar molecule
swaying speed. The friction dragging degree is concerning a bout the ma gneto electric wave frequency, polar molecule shape, and
the matter’s sticky degree. To different matter’s molecule, there are different special frequency zone. Those ado absorb microwave
energy from these zone are most capable to turn microwave energy to heat energy.
Apart from the above said action, there is another action which is electric ion under t he action of microwave field, act fiercely
accompanied with the acceleration of electric field. The positive ion transfer to the negative polarity of the field while the negative
ion do opposite. Accompanying with the changing electric field, the electric ion changing according ly. During the transferring, heat
produced with the crash among the ion. This kind of action take the main effect to those microwave heating of high salt molecule.
No matter it is the polar molecule swaying or the ion transferring, they both are turning the microwave energy which the heatin g
matter got from the microwave field to hear energy. From the analysis of theory, we can draw such a conclusion that the power
which a unit of volume matter absorbed from the microwave field as the following formula:
Pa=K·E· F·Er·tgδ
Pa Stands for the power the heated matter absorbed from the microwave field.
K Stands for a constant
E Stand for microwave field strength
F Stands for the microwave frequency.
Er Stands for relative dielectric constant of the heated matter.
tgδStands for loss angle tangent of the heated matter
2-1
INSTALLATIONS
BEFORE YOU BEGIN, READ THE FOLLOWING INSTRUCTIONS COMPLETELY
INSTALLING
1. Empty the microwave oven and clean inside it with
a soft, damp cloth. Check for damage such as
misaligned door, damage around the door or dents
inside the cavity or on the exterior.
2. Put the oven on a counter, table, or shelf that is
strong enough to hold the oven and the food and
utensils you put in it. (The control panel side of the
oven is the heavy side. Use care when handling.)
3. Do not block the vent and the air intake openings.
Blocking vent or air intake openings can cause
damage to the oven and poor cooking results.
Make sure the microwave oven legs are in place to
ensure proper air flow.
4. The oven should not be installed in any area where
heat and steam are generated, because they may
damage the electronic or mechanical parts of the
unit.
Do not install the oven next to a conventional
surface unit or above a conventional wall oven.
5. Use microwave oven in an ambient temperature
less than 104°F(40°C).
6. Place the microwave oven on a sturdy and flat
surface at least 10 cm(4 inches) from the wall.
7. Place the microwave oven as far away as possible
from TV, RADIO, COMPUTER, etc., to prevent
interference.
EARTHING INSTRUCTIONS
This microwave oven is designed to be used in a fully
earthed condition.
It is imperative, therefore, to make sure it is properly
earthed before servicing
WARNING-
THIS APPLIANCE
MUST BE EARTHED
IMPORTANT
The wires in this mains lead are colored in
accordance with the following code:
Green-and-yellow: Earth
Blue: Neutral
Brown: Live
As the colors of the wires in the mains lead of this
appliance may not correspond with the colored
markings identifying the terminals in your plug,
proceed as follows.
The wire which is colored green-and-yellow must be
connected to the terminal in the plug which is marked
with the letter E or by the earth symbol (
colored green or green-and-yellow.
The wire which is colored blue must be connected to
the terminal in the plug which is marked with the letter
N or colored black.
The wire which is colored brown must be connected
to the terminal in the plug which is marked with the
letter L or colored red.
) or
3-1
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