LG MH6848G Service Manual

MICROWAVE OVEN

SERVICE MANUAL

MODEL: MH6848G

CAUTION

BEFORE SERVICING THE UNIT, READ THE SAFETY PRECAUTIONS IN THIS MANUAL.

Website: http://biz.lgservice.com

삼 흥

정 판

P/NO : MFL30190716

March, 2010

Printed in Korea

Internal Use Only

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 to verify compliance with the Federal Performance Standard should be

performed on each oven prior to release to the owner.

SAFETY PRECAUTIONS

CAUTION

MICROWAVE RADIATION

DO NOT BECOME EXPOSED TO RADIATION FROM THE MICROWAVE GENERATOR
OR OTHER PARTS CONDUCTING MICROWAVE ENERGY.

LGE Internal Use OnlyCopyright © 2007 LG Electronics. Inc. All right reserved.

Only for training and service purposes

(Page)

SAFETY PRECAUTIONS

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Inside front cover

SPECIFICATIONS

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1-1

CAUTIONS

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2-1

INSTALLATIONS

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3-1

OPERATING INSTRUCTIONS

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4-1

FEATURES

-----------------------------------------------------------------------------------------------------------------------

4-1

CONTROL PANEL

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4-1

OPERATING SEQUENCE

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4-2

SCHEMATIC DIAGRAM

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4-3

CIRCUIT DESCRIPTION

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4-4

SERVICE INFORMATION

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5-1

TOOLS AND MEASURING INSTRUMENTS

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5-1

MICROWAVE LEAKAGE TEST

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5-1

MEASUREMENT OF MICROWAVE POWER OUTPUT

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5-3

DISASSEMBLY AND ADJUSTMENT

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5-3

INTERLOCK CONTINUITY TEST

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5-7

COMPONENT TEST PROCEDURE

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5-8

TROUBLE SHOOTING

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5-12

EXPLODED VIEW

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6-1

REPLACEMENT PARTS LIST

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7-1

CONTENTS

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Only for training and service purposes

1-1

SPECIFICATIONS

This microwave oven is designed for household use only.
It is not recommended for commercial purposes.

DESCRIPTION

MH6848G

220 V~ 50 Hz

Single phase, 3 wire grounded

Microwave 1300 W

Grill 1000 W

Combination 2250 W

900 watt full microwave power (IEC60705)

2450 MHz

2M214 - 39F

0 ~ 90 min.

510 (W) x 310 (H) x 404 (D) mm

348 (W) x 242 (H) x 373 (D) mm

17.0 kg (approx.)

19.0 kg (approx.)

Microwave Power for Variable Cooking

Power level

max. ................................................ Full power throughout the cooking time

MED.-HIGH .................................... approx. 80% of Full power

MEDIUM ......................................... approx. 60% of Full power

DEFROST ...................................... approx. 40% of Full power

LOW/WARM ................................... approx. 20% of Full power

Grill, COMBI

Owner's manual

Glass turntable

Roller rest

Grill rack

ITEM

MODEL

Power Requirement

Power Output

Microwave Frequency

Magnetron

Timer

Outside Dimensions

Cavity Dimensions

Net Weight

Shipping weight

Control Complement

Rating Label Location

Accessories

Back side

LGE Internal Use OnlyCopyright © 2007 LG Electronics. Inc. All right reserved.

Only for training and service purposes

• 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.

• 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.

• 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.

2-1

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.

THE OVEN IS TO BE SERVICED ONLY
BY PROPERLY QUALIFIED SERVICE
PERSONNEL.

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.

Gasket

ANTENNA

COOLING FIN

MAGNETRON
CHASSIS GROUND

FILAMENT
TERMINALS

MAGNETRON

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Only for training and service purposes

INSTALLATIONS

3-1

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.

8. Do not touch the front glass during or after cooking
of the Grill and Combination mode.
This glass is very hot during heater operating.

9. Do not operate the oven at microwave and
combination mode with Grill rack placed in the
cavity when the oven is empty.

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

WARNING­THIS APPLIANCE
MUST BE EARTHED

IMPORTANT

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 ( ) or
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.

BEFORE YOU BEGIN, READ THE FOLLOWING INSTRUCTIONS COMPLETELY AND CAREFULLY.

10cm

The wires in this mains lead are colored in
accordance with the following code:

Green-and-yellow: Earth
Blue: Neutral
Brown: Live

LGE Internal Use OnlyCopyright © 2007 LG Electronics. Inc. All right reserved.

Only for training and service purposes

4-1

OPERATING INSTRUCTIONS

FEATURES

CONTROL PANEL

1. MICRO: Used to select power level for microwave

cooking.

2. GRILL: Used to select the grill cooking.

3. COMBI: Used to select the combination cooking.

4. AUTO COOK: Used to cook the foods listed by one

touch.

5. AUTO DEFROST: Used to select the auto weight

defrost.

6. CLOCK: Used to set the time of day.

7. QUICK DEFROST: Used to select the auto weight

defrost.

8. MORE/LESS: Used to change cooking time.
WEIGHT: Used to select the weight of food.

9. COOK TIME: Used to cook food for a desired time.

10. STOP/CLEAR: Used to stop oven and clear all

entries except time of day.

11. START/QUICK START: To quickly set the

cooking time directly.

Oven Front Plate

Door Seal

Control Panel

Safety Door Lock System

Window Door Screen

Turntable

Grill Rack

Roller Rest

1

2
3

7

8

9

11

4

5

6

10

LGE Internal Use Only Copyright © 2007 LG Electronics. Inc. All right reserved.

Only for training and service purposes

4-2

OPERATING SEQUENCE

The following is a description of component functions
during oven operation.

1. SETTING THE CLOCK

NOTE: You can set 12 hour clock or 24 hour clock

optionally.

2. CANCEL FUNCTION

Touch the

STOP/CLEAR

pad whenever you need to
cancel an entry or a function currently in use.
The display will either return to the last item entered
or to the clock.

3. QUICK START

4. MICROWAVE COOKING

5. MULTI-STAGE MICROWAVE
COOKING

STAGE 1

STAGE 2

6. GRILL COOKING

7. COMBINATION COOKING

8. AUTO WEIGHT DEFROST

9. CHILD LOCK

This oven has a CHILD LOCK feature.
TO SET CHILD LOCK

• Touch the

Stop/Clear

pad

• Touch

STOP/CLEAR

pad L appear on

the display.

TO CANCEL CHILD LOCK

• Touch the

Stop/Clear

pad

• Touch

STOP/CLEAR

pad L disappears.

10. MORE / LESS

11. QUICK WEIGHT DEFROST

Stop/
Clear

Set

clock

Touch

start

Set the

correct hour

Set the

correct minute

Touch

start

Stop/Clear

Desired

power level

Start

Start

Desired

cooking time

Stop/Clear

Quick start

Stop/Clear Grill

Start

Desired

cooking Time

Stop/Clear

Desired

combi

category

Start

Desired

cooking Time

Stop/Clear

Desired

defrost

category

Start

Desired
cooking

Weight

Stop/Cle

Desired

power level

Desired

cooking time

Desired

power level

Desired

cooking time

Grill

or Combi

Desired

cooking time

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Only for training and service purposes

Stop/Clear

COOKING MORE

Stop/Clear

COOKING LESS

Stop/Clear Quick Defrost StartWeight (kg)

SCHEMATIC DIAGRAM

4-3

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Only for training and service purposes

4-4

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.

• 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

power of microwave oven as POWER LEVEL.

(refer to page 1-1)

• One complete cycle of relay 2 is 22 seconds.

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.

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.

• 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.

L

FUSE

H.V.

TRANS-

FORMER

RELAY 2

MICOM CONTROLLER

SECONDARY
SWITCH

PRIMARY

SWITCH

N

MONITOR

SWITCH

MONITOR

SWITCH

L

FUSE

H.V.

TRANS-

FORMER

RELAY 2

MICOM CONTROLLER

SECONDARY
SWITCH

PRIMARY

SWITCH

N

RELAY 3

L

N

GRILL

THERMOSTAT

GRILL

HEATER

L

G -Y

N

E

GRILL

CONTROL SWITCH

LGE Internal Use OnlyCopyright © 2007 LG Electronics. Inc. All right reserved.

Only for training and service purposes

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
level in excess of 5 mW/cm2and 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.

• 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
4 mW/cm2. Always start measuring of an unknown field
to assure safety for operating personnel from radiation

leakage.

MEASURING MICROWAVE ENERGY
LEAKAGE

• Pour 275±15cc 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
(5cm) 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
(1in/sec)

5-1

SERVICE INFORMATION

TOOLS AND MEASURING INSTRUMENTS

MICROWAVE LEAKAGE TEST

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

NECESSARY MEASURING INSTRUMENTS

• 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(31/2 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)

LGE Internal Use Only Copyright © 2007 LG Electronics. Inc. All right reserved.

Only for training and service purposes