DOWNLOAD LG MB-4322AH Service Manual ↓ Size: 944.9 KB | Pages: 40 in PDF or view online for FREE

4-4

•  As the door is closed, the contact of MONITOR

SWITCH opens. This switch creates the short circuit to
blow fuse during operation under abnormal condition.
(ie, should the contacts of primary and secondary
switch fail to open the circuit)

•  The latches are secured by latch board. The oven light

turns on while the oven is in operation.

•  The contacts of the timer switch changes to ON

position when the timer knob is rotated.
(While power control knob is set at “Full Power”)

•  The contacts of primary and secondary switch close

the circuit.

•  Input Volts AC. is applied to the high voltage

transformer through power control switch as shown by
the solid line.

•  Turntable rotates.
•  The fan motor rotates and cools the magnetron by

blowing the air (coming from the intake holes on the
baseplate) over the magnetron.

•  The air is also directed into the oven to exhaust the

vapor in the oven through the upper plate.

•  Timer starts rotating.
•  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
diode and charging of the high voltage capacitor.
The negative DC voltage is then applied to the filament
of the magnetron.

•  Both primary switch and secondary switch open to cut

off the primary winding voltage to the high voltage
transformer to stop microwave oscillation.

•  Timer, turntable and fan motor stop.
•  As the door is opened, if the contact of primary switch

fail to open, the fuse opens due to the large current
surge caused by the monitor switch activation which in
turn stops magnetron, oscillation.

•  The contacts of the timer switch change to ON

position.

•  The contacts of the grill select switch close the circuit.
•  Input volts AC is applied to the grill heater through

thermostat-grill as shown by the solid line.

•  The fan motor and turntable rotates.

FUSE

PRIMARY

SWITCH

SECONDARY

SWITCH

MONITOR

SWITCH

L

N

H.V.T

TIMER

SWITCH

POWER

CONTROL

SWITCH

FUSE

PRIMARY

SWITCH

SECONDARY

SWITCH

L

N

H.V.T

TIMER

SWITCH

POWER

CONTROL

SWITCH

MONITOR

SWITCH

PRIMARY

SWITCH

TIMER

SWITCH

GRILL

SELECT
SWITCH

L

N

THERMOSTAT

GRILL

GRILL 

HEATER

L

G -Y

N

E

and should

repair any 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.

• 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 
4 mW/cm

2

. Always start measuring of an unknown field

to assure safety for operating personnel from radiation
leakage.

5-1

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

• TESTER(VOLTS-DC, AC., Ohmmeter)
• Microwave survey meter

- Holaday HI-1500

HI-1501

- Narda     8100

8200

• Inch scale
• 600 cc non conductive material beaker (glass or plastic),

inside diameter: approx. 8.5 cm(3

1

/2

in.)

• 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)

5-2

• 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. (Circled area of below Fig.)

- Around the magnetron
- The waveguide

• 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 removedless than

5 mW/cm.sq. Leakage for a fully assembled
oven (Before the latch switch (primary) is
interrupted) with the door in a slightly opened
position-less than 2 mW/cm.sq.

• 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.

• 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

than 4 mW/cm.sq. after determining that all parts are in
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
manufacturer.

5-3

1) Disconnect the power supply cord from the outlet.

2) Remove the screws from the rear and along side

edges of the case.

The outer case must be moved backward to be lifted

off.

1) Remove the outer case.
2) Disconnect two terminals, and remove one screw of

the earth terminal.

(refer to page 2-1)

1) Disconnect the leadwire from the PCB SUB ASS’Y.
2) Remove the screws for the earth and securing the

control panel.

3) Lift control panel ASS’Y from the oven by the tab

unhooked.

• 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 5°K.

• The microwave power output P in watts is calculated

from the following formula :

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.

WATER LOAD

TURNTABLE

P =

• T

2

: Temperature after heating

• T

0

: Temperature after bown

• M: Weight of bowl

4187 x (

∆

T) + 0.55 X (T

2 

- T

0

) X M

T