Midea MDX930GXE Schematic
MICROWAVE OVEN
MODEL:
AG930AZJ-P00C
( 220V50Hz)
9 00W
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 operatio n 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) pr oper door closing, (3)
seal and sealing surfaces (arcing, wear, and other damage), (4) dam age 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 gene rating
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
SCHEMATIC DIAGRAM
CIRCUIT DESCRIPTION
GENERAL DETAILS
• The low voltage transformer supplies the necessary
voltage to the micom controller when power cord is
plugged in.
• When the door is closed, the primary switch is ON, the
secondary switch is ON, and the monitor switch opens
(contact COM and NO).
WHEN SELECTING COOKING POWER
LEVEL AND TIME
• The micom controller memorizes the function you set.
• The time you set appears in the display window.
• Each indicator light turns on to indicate that the stage
has been set.
WHEN TOUCHING THE START PAD
• The coil of the relay is energized by the micom
controller.
• Power input is supplied to the high voltage transformer
through the fuse to the primary switch and relay 2.
• Turntable rotates.
FUSE
L
N
MICOM CONTROLLER
• The fan motor rotates and cools the magnetron by
blowing the air (coming from the intake on the
baseplate)
• The air is also directed into the oven to exhaust the
vapor in the oven through the upper plate.
• Cooking time starts counting down.
• 3.2 volts AC is generated from the filament winding of
the high voltage transformer. This 3.2 volts is applied
to the magnetron to heat the magnetron filament
through two noise preventing choke coils.
• A high voltage of approximately 2100 volts AC is
generated in the secondary of the high voltage
transformer which is increased by the action of the high
voltage diode and charging of the high voltage
capacitor.
• The negative 4,000 Volts DC is applied to the filament
of the magnetron.
WHEN THE OVEN IS SET AT ANY LEVEL
EXCEPT MAXIMUM.
• The micom controller controls the ON-OFF time of relay
2 by the applied signal to vary the average output
PRIMARY
SWITCH
RELAY 2
MONITOR
SWITCH
TRANS-
FORMER
SECONDARY
SWITCH
power of microwave oven as POWER LEVEL.
WHEN THE DOOR IS OPENED DURING
COOKING
• Both the primary switch and relay 2 are cut off primary
winding voltage of the high voltage transformer.
• ON-OFF of relay 2 is coupled electrically with opening
and closing of the secondary switch.
• When the door is opened, the secondary switch is
opened and when the door is closed, the secondary
switch is closed.
• The cooking time stops counting down.
• Relay stops functioning.
• As the door is opened, if the contact of primary switch
and relay 2 and/or secondary switch fails to open, the
fuse opens due to the large current surge caused by
the monitor switch activation, which in turn stops
magnetron oscillation.
FUSE
L
N
MICOM CONTROLLER
PRIMARY
SWITCH
RELAY 2
MONITOR
SWITCH
FORMER
SECONDARY
SWITCH
TRANS-
WHEN TOUCHING THE START KEY
WITH THE GRILL COOKING FUNCTION
SELECTED
• The contacts of the primary switch and the secondary
switch close the circuit.
• A.C.voltage is applied to the grill heater through grill
thermostat as shown by the solid line.
GRILL
THERMOSTAT
L
L
G -Y
N
E
N
• Turntable rotates.
• The fan motor rotates.
• The air is also directed into the oven to exhaust the
vapor in the oven through the base plate and upper
plate.
TOP HEATER RELAY
GRILL
HEATER
4-2
SERVICE INFORMATION
TOOLS AND MEASURING INSTRUMENTS
NECESSARY TOOLS
Tools normally used for TV servicing are sufficient.
Standard tools are listed below.
• Diagonal pliers
• Long nose pliers
• Phillips screwdriver
• Flat blade screwdriver
• Wrench (size 5mm)
• Nutdriver (size 5mm)
• Adjustable wrench
• Soldering iron
• Solder
• Vinyl insulation tape
• Polishing cloth
MICROWAVE LEAKAGE TEST
CAUTIONS
• Be sure to check microwave leakage prior to
servicing the oven if the oven is operative prior to
servicing.
• The service personnel should inform the
manufacture importer, or assembler of any certified
oven unit found to have a microwave emission
2
level in excess of 5 mW/cm
unit found to have excessive emission levels at no cost
to the owner and should ascertain the cause of the
excessive leakage. The service personnel should
instruct the owner not to use the unit until the oven has
been brought into compliance.
• If the oven operates with the door open, the service
personnel should:
- Tell the user not to operate the oven.
- Contact the manufacturer.
• The service personnel should check all surface and
vent openings for microwave leakage.
• Check for microwave leakage after every servicing. The
power density of the microwave radiation leakage
emitted by the microwave oven should not exceed
2
5 mW/cm
to assure safety for operating personnel from radiation
leakage.
. Always start measuring of an unknown field
and should repair any
5-1
NECESSARY MEASURING
INSTRUMENTS
• TESTER(VOLTS-DC, AC., Ohmmeter)
• Microwave survey meter
- Holaday HI-1710(A)
- Narda 8100
8200
• Inch scale
• 600 cc non conductive material beaker (glass or
plastic), inside diameter: approx. 8.5 cm(3
• Cylindrical and made of borosilicate glass vessel.
max. thickness: 3 mm
outside diameter: approx. 190mm
height: approx. 90mm
• Glass thermometer: 100°C or 212°F (1 deg scale)
1/2 in.)
MEASURING MICROWAVE ENERGY
LEAKAGE
• Pour 275±15 cc of 20±5°C(68±9°F) water in a
beaker
which is graduated to 600 cc, and place the beaker
on the center of the turntable.
• Set the energy leakage monitor to 2,450 MHz and
use it following the manufacturer's recommended
test procedure to assure correct result.
• When measuring the leakage, always use the 2-
inch (5 cm) spacer supplied with the probe.
• Operate the oven at its maximum output.
• Measure the microwave radiation using and
electromagnetic radiation monitor by holding the
probe perpendicular to the surface being measured
Move probe along shaded area
Probe scanning speed: Less than 2.5 cm/sec
MEASUREMENT WITH OUTER CASE
REMOVED
• When you replace the magnetron, measure for
microwave energy leakage before the outer case is
installed and after all necessary components are
replaced or adjusted.
Special care should be taken in measuring the
following parts.
- Around the magnetron
- The waveguide
WARNING : AVOID CONTACTING ANY
(Magnetron, H.V. Transformer,
H.V. Capacitor, H.V. Cable Ass’y,
H.V. Fuse)
HIGH VOLTAGE PARTS
MEASUREMENT WITH A FULLY
ASSEMBLED OVEN
• After all components, including the outer case, are fully
assembled, measure for microwave energy leakage
around the door viewing window, the exhaust opening,
and air inlet openings.
• Microwave energy leakage must not exceed the values
prescribed below.
NOTE: Leakage with the outer case removed less than
2
3 mW/cm
oven (Before the latch switch (primary) is
interrupted) with the door in a slightly opened
position-less than 1 mW/cm
. Leakage for a fully assembled
2
NOTES WHEN MEASURING
• Do not exceed meter full scale deflection.
• The test probe must be removed no faster than
1 inch/sec (2.5 cm/sec) along the shaded area,
otherwise a false reading may result.
• The test probe must be held with the grip portion of
the
handle.
A false reading may result if the operator's hand is
between the handle and the probe.
• When testing near a corner of the door, keep the
probe
perpendicular to the surface making sure the probe
horizontally along the oven surface, this may possibly
cause probe damage.
RECORD KEEPING AND NOTIFICATION
AFTER MEASUREMENT
• After adjustment and repair of any microwave energy
interruption or microwave energy blocking device,
record the measured values for future reference. Also
enter the information on the service invoice.
• The microwave energy leakage should not be more
2
than 1.0 mW/cm
good condition, functioning properly and genuine
replacement parts which are listed in this manual have
been used.
• At least once a year, have the electromagnetic energy
leakage monitor checked for calibration by its
. after determining that all parts are in
5-2
MEASUREMENT OF MICROWAVE POWER OUTPUT
• Microwave power output measurement is made with
the microwave oven supplied at its rated voltage and
operated at its maximum microwave power setting with
a load of (1000±5) g of potable water.
• The water is contained in a cylindrical borosilicate glass
vessel having a maximum material thickness of 3 mm
and an outside diameter of approximately 190mm.
• The oven and the empty vessel are at ambient
temperature prior to the start of the test.
• The initial temperature (T1) of the water is (10±2)°C It
is measured immediately before the water is added to
the vessel. After addition of the water to the vessel,
the load is immediately placed on the center of the
turntable which is in the lowest position and the
microwave power switched on.
• The time T for the temperature of the water to rise by a
value . T of (10±2)°K is measured, where T is the time
in seconds and .T is the temperature rise. The initial
and final water temperatures are selected so that the
maximum difference between the final water
temperature and the ambient temperature is 2°K.
• The microwave power output P in watts is
calculated
approximately from the following formula :
P=4187 x (△T)/T
is measured while the microwave generator is
operating at full power. Magnetron filament heat-up
time is not included. (about 3 sec)
• The water is stirred to equalize temperature
throughout
the vessel, prior to measuring the final water
temperature.
• Stirring devices and measuring instruments are
selected in order to minimize addition or removal of
heat.
5-3
TROUBLE SHOOTING PROCEDURES
Before overhauling a microwave oven, you should judge the breakdown and the cause correctly, then you can repair it with
corresponding ways. The overhauling must be proceed in order, any hasty conclusion is not recommendable, otherwise
overworking would be done when repair. The microwave oven may occur compound breakdown due to all kinds of different
reasons, thus, when overhaul, they all should be taken into consideration. Special attention must be given to the microwave
leakage and the electric insulation when examine because they may do harmful to the repairing staff.
Ⅰ.MEANS OF THE BREAKDOWN EXAMINING
How to examine a microwave oven with breakdown? A better means which demonstrated in practical operating are through
inspecting and listening. On the basis of large amounts of perceptual knowledge, you can judge and analysis the breakdown
quickly and correctly.
1. Inspection.
Inspect whether the oven shape is disordered and where is the disordered position, if any. It is normal if the outer case
disordered a little, but abnormal if the oven, the door disordered, the door hook broken, the door crooked, or there are too
much looseness between the door and the oven after the door is closed.
2. Listening.
Listening to the voice of the oven operating and the noise of the fan after it conducted. Minor “wen wen” noise, cycling “kala”
noise and “shishi” noise should be consider as normal. But it is abnormal if the following noise occurred:
(1) Sound “wenwen” noise.
(2) Long time “shishi” noise.
(3) Strike voice like “pipa pipa”.
Ⅱ.SPOT EXAMINING STEPS OF THE MICROWAVE OVEN.
1. Examine the microwave insulating resistance.
Measure the insulating resistance with a multi meter or a mega ohmmeter. The value should not less than 2 mega ohm.
Otherwise, part examination should be taken at once. Such as checking whether the motor, the thermal cutout, the
transformer or the capacitor are electricity leaking.
2. Examination of the resistance value of the microwave oven.
Close the door, set the time (the oven is at operating condition but the power plug haven’t been pl ugged in), measure the t wo
feet(L-N)of the power plug with Rx1 grade of a multimeter, the resistance value should be about 22 ohm.
If open circuit occurred, then you must check whether the 10 A fuse is broken、the primary winding of the transformer is open
circuit、the thermal cutout is open circuit or not, you must check whether the interlock device is put through or all the plugs
are connected well. If short circuit occurred, you should check whether the primar y winding of the power transformer is shortcircuited or part short-circuited.
3. Examination of microwave leakage
Measure the microwave leakage with a microwave leakage Measure. Place a graduat e of 275ml water at the middle of the
glass tray of the oven (FIG.5-1). Close the door, power set high, time set to 3 minutes, press the starting button to operate the
oven. After rectified the microwave leakage measure, measure around the door crack, those hole position of the window and
the air vent at four sides of the oven with the probe of the measure. When meas ure, the moving speed of the probe should
not exceed 2.5cm per second, and the measuring direction should be the same with the outing direction of the microwave
leakage (FIG.5-2).
When measuring, the ultimate value of microwave leakage of all the measured position should not exceed 1.0 mili watt/cm
or should be considered as abnormal.
FIG.5-1
275ml
Glass tray
FIG.5-2
5-4
2
,
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