LG LMV1976 Service Manual

MICROWAVE OVEN

SERVICE MANUAL

MODEL : LMV1976ST

CAUTION

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

website: http://www.lgeservice.com

CAUTION

SAFETY PRECAUTIONS

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.

• Proper operation of the microwave ovens requires that the magnetron be assembled to the

wave guide 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.

• Routine service safety procedures should be exercised at all times.

• Untrained personnel should not attempt service without a thorough review of the test

procedures and safety information contained in this manual.

FOREWORD

Read this Manual carefully. Failure to adhere to or observe the information in this Manual may result in exposing
yourself to the Microwave Energy normally contained within the oven cavity.

TABLE OF CONTENTS

(Page)

SAFETY PRECAUTIONS ......................................................................................................... Inside front page

SPECIFICATIONS.......................................................................................................................................... 1-1

CAUTIONS ..................................................................................................................................................... 2-1

INSTALLATIONS............................................................................................................................................ 3-1

OPERATING INSTRUCTIONS....................................................................................................................... 4-1

CONTROL PANEL ................................................................................................................................. 4-1

CONTROL PANEL INSTRUCTIONS ..................................................................................................... 4-2

OVERALL CIRCUIT DIAGRAM...................................................................................................................... 5-1

SCHEMATIC DIAGRAM......................................................................................................................... 5-1

MATRIX CIRCUIT FOR TOUCH KEY BOARD ...................................................................................... 5-2

GENERAL INFORMATION FOR SERVICE ................................................................................................... 6-1

GENERAL PRECAUTIONS IN USE ...................................................................................................... 6-1

TRIAL OPERATION ............................................................................................................................... 6-1

FEATURES AND SPECIFICATIONS FEATURES................................................................................. 6-1

SERVICE INFORMATION ............................................................................................................................. 7-1

PRECAUTIONS AND REPAIR SERVICE TIPS..................................................................................... 7-1

MICROWAVE LEAKAGE TEST ............................................................................................................. 7-2

POWER OUTPUT MEASUREMENT ..................................................................................................... 7-3

DISASSEMBLY INSTRUCTIONS .......................................................................................................... 7-4

INTERLOCK SYSTEM .......................................................................................................................... 7-12

INTERLOCK CONTINUITY TEST.......................................................................................................... 7-14

TEST AND CHECKOUT PROCEDURES AND TROUBLE SHOOTING................................................ 7-15

A. TEST PROCEDURES................................................................................................................ 7-15

B. CHECKOUT PROCEDURES..................................................................................................... 7-18

C. TROUBLE SHOOTING.............................................................................................................. 7-21

EXPLODED VIEW .......................................................................................................................................... 8-1

REPLACEMENT PARTS LIST ............................................................................................................... 8-8

SCHEMATIC DIAGRAM OF PCB .......................................................................................................... 8-10

PRINTED CIRCUIT BOARD .................................................................................................................. 8-11

PCB PARTS LIST................................................................................................................................... 8-12

SPECIFICATIONS

Rated Power Consumption 1,500W maximum (
Microwave Output

1000W (IEC 60 705)

Microwave oven+Cook top lamps+Ventilation fan

Adjustable 100W through 1000W, 10 steps
Frequency 2,450 MHz +50 MHz
Power Supply 120 VAC, 60 Hz
Rated Current 13 Amp. (Microwave oven+Cook top lamps+Ventilation fan)
Magnetron Cooling Forced Air Cooling
Rectification Rectification Voltage Double Half-Wave
Door Sealing Choke System
Safety Devices Magnetron Thermal Fuse:

+

Open at 109

_

°

4

°

C

C

Oven Cavity Thermostat:

+

_

°

C

Open at 110

5°C
Fuse(20A)
Primary Interlock Switch
Secondary Interlock Switch
Interlock Monitor

Magnetron 2M248J
High Voltage Capacitor Capacitor: 1.0µF, 2.1 KV AC
High Voltage Diode Diode; 350mA, 9.0 KV
Cook top Lamp 125 V, 30 W
Cavity Lamp 125 V, 30 W
Timer Digital, up to 99 mim. 99 sec. (in each cooking stage)
Tray Tempered Safety Glass

Overall Dimensions 29
Oven Cavity Size 22

15

/16”(W)x167/16”(H)x155/8”(D)

3

/8”(W)x10”(H)x1419/32”(D)

Effective Capacity of Oven Cavity 1.9 Cu.ft.
Accessories Owner’s Manual & Cooking Guide, Installation Manual,

Exhaust Adapter, Exhaust Damper, Mounting Kit and Two
Filters, Rotating Ring Assembly, Metal Rack.

)

1-1

CAUTIONS

Gasket

ANTENNA

COOLING FIN

MAGNETRON
CHASSIS GROUND

FILAMENT
TERMINALS

MAGNETRON

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.

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.

•

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.

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

2-1

INSTALLATIONS

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

PRECAUTIONS ON INSTALLATION

A. Plug the power supply cord into a 120V AC, 60Hz,

single-phase power source with a capacity of 15A
or 20A.

B. Avoid placing the unit in a location where there is

direct heat or splashing water.
C. Install the unit on the mounting plate firmly.
D. Place the unit as far away as possible from TV,

radio, etc. to prevent interference.

GROUNDING INSTRUCTIONS

For personal safety, this appliance must be fully
grounded at all times.

In the event of an electrical short circuit, grounding
reduces the risk of electrical shock.
The plug must be plugged into an outlet that is
properly installed and grounded.

CAUTION

This unit is equipped with a 3-prong plug for your
safety. If the wall outlet is a grounded 3-hole type,
the unit will be grounded automatically.

Plug with Ground
Prong

Properly Polarized
and Grounded
Outlet

WARNING

Improper use of the grounding plug can result in a
risk of electric shock.
Do not, under any circumstances, cut or remove the
third ground prong from the power cord plug.

3-1

CONTROL PANEL

25

21

24

28

26

23

4

7

9

12

14

15

16

19

13

8

10

17

18

20

22

27

2

5

3

6

11

1

OPERATING INSTRUCTIONS

4-1

CONTROL PANEL INSTRUCTIONS

1. DISPLAY: The Display includes a clock and indicators to

tell you time of day, cooking time settings and cooking
functions selected.

2. POPCORN: Touch this pad when popping popcorn in

your microwave oven. The oven’s sensor will tell the oven
how long to cook depending on the amount of humidity it
detects from the popcorn.

3. POTATO: Touch this pad to cook potatoes. The oven’s

sensor will tell the oven how long to cook depending on
the amount of humidity it detects from the potato.

4. PIZZA: Touch this pad to cook pizza without selecting

cooking times and power levels.

5. COOK: Touch this pad to cook frozen entree, casserole,

and rice. The oven’s sensor will tell the oven how long to
cook depending on the amount of humidity coming from
the food.

6. REHEAT: Touch this pad to reheat casserole, dinner

plate, and soup/sauce. The oven’s sensor will tell the oven
how long to cook depending on the amount of humidity
coming from the food.

7. VEGETABLE: Touch this pad to reheat vegetables. The

oven’s sensor will tell the oven how long to cook
depending on the amount of humidity it detects from the
vegetables.

8. CUSTOM COOK: Touch this pad to recall one cooking

instruction previously programmed into memory.

9. EASY COOK: Touch this pad to set and start quickly at

100% power level.

10. AUTO DEFROST: Meat, poultry, fish. Touch this pad to

select food type and defrost food by weight.

11. TIME DEFROST: Touch this pad to defrost most other

frozen foods.

12. Q DEFROST: This pad provides you with the RAPID

defrosting method for 1.0 pounds frozen foods.

13. NUMBER: Touch number pads to enter cooking time,

power level, quantities, or weights.

14. KITCHEN TIMER: Touch this pad to set the kitchen
timer.

15. CLOCK: Touch this pad to enter the time of day.

16. LIGHT TIMER: Touch this pad to set the light timer.

17. MORE: Touch this pad to add ten seconds of cooking

time each time you press it.

18. LESS: Touch this pad to subtract ten seconds of cooking
time each time you press it.

19. HELP: Touch this pad to learn how to use each oven
function.

20. STOP/CLEAR: Touch this pad to stop the oven or to
clear all entries.

21. START: Touch this pad to start a function. If you open
the door after oven begins to cook, START again.

22. POWER LEVEL: Touch this pad to select a cooking
power level.

23. CUSTOM SET: Touch this pad to change the oven’s
default settings for sound, clock, display speed, defrost
weight, and demo mode operations.

24. HOLD WARM: Touch this pad to keep hot, cooked foods
warm in your microwave oven for up to 99 minutes 99
seconds.

25. PROGRAM: Touch this pad to set a cooking time.

26. LIGHT HI-LOW-OFF: Touch this pad to turn on the

cooktop/countertop light.

27. VENT HI-LOW-OFF: Touch this pad to turn the fan on or
off.

28. TURNTABLE ON-OFF: Touch this pad to turn off the
turntable. OFF will appear in the display.

ww

WARNING:

DO NOT RUN THE OVEN EMPTY WITHOUT FOOD IN IT.

4-2

A

B

C

OVERALL CIRCUIT DIAGRAM

SCHEMATIC DIAGRAM

5-1

MATRIX CIRCUIT FOR TOUCH KEY BOARD

5-2

GENERAL INFORMATION FOR SERVICE

GENERAL PRECAUTIONS IN USE

A. Never operate the unit when it is empty.

Operating the oven with no load may shorten the
life of the magnetron. Whenever cooking dry foods
(dried fish, bread, etc.) or a small amount of food,
be sure to put a glass of water into the cooking
compartment. The glass turntable may become hot
after operating, be careful when touching it.

B. Aluminum foil should be avoided because it will

disrupt cooking and may cause arcing. However,
small pieces may be used to cover some parts of
food to slow the cooking. Any aluminum foil used
should never be closer than 2.5 cm to any side wall
of the oven.

TRIAL OPERATION

After installation, the following sequences and results
should be checked carefully.

A. Put a container filled with water (about 1 liter) into

the oven, and close the door tightly.

B. Set cooking time for 10 minutes by touching “1”

and then “0” three times. “1, 0, 0, 0” appears in
the display window.

C. Touch the START key.

Make sure the cavity light comes on. The unit will
begin cooking and the display window will show the
time counting down by seconds.

D. After about 5 minutes, make sure the primary

interlock switch, the secondary interlock switch and
the interlock monitor switch operate properly by
opening and closing the door several times. Touch
the START key each time the door is closed.

E. Continue operating the unit. Four long beep sound

signal is heard when the time is up. The unit will

shut off automatically.
F. Confirm the water is hot.
G. Finally, measure the output power according to

“POWER OUTPUT MEASUREMENT” on

page 7-3.

(1) Primary interlock switch
(2) Secondary interlock switch
(3) Interlock monitor switch
(4) Choke system
(5) Magnetron thermal fuse
(6) Oven cavity thermostat

(Note: This thermostat located on the oven
cavity will open and stop the unit from operation
only if a high temperature is reached, such as,
a fire created by overcooking food.)

B. Any one of 10 power output levels ranging 100W to

1000W can be selected by the touch control and
electronic computer system.

FEATURES AND SPECIFICATIONS
FEATURES

A. The safety systems incorporated in this model are:

6-1

SERVICE INFORMATION

PRECAUTIONS AND REPAIR SERVICE TIPS

PRELIMINARY

A. SINCE NEARLY 2,100 VOLTS EXISTS IN SOME

CIRCUITS OF THIS UNIT REPAIRS SHOULD BE
CARRIED OUT WITH GREAT CARE.

The filament leads of magnetron carry High
Voltage with respect to ground. Extreme caution
must be exercised. Never plug the unit into a
power source to determine which component is
defective in high voltage section.

B. TO AVOID POSSIBLE EXPOSURE TO

MICROWAVE ENERGY LEAKAGE, THE
FOLLOWING PRECAUTIONS MUST BE TAKEN
BEFORE SERVICING.

(1) Before the power is applied:

(a) Make sure the primary interlock switch, the

secondary interlock switch and the interlock
monitor switch operate properly by opening
and closing the door several by opening and
closing the door several times.

(b) Make sure the perforated screen and the

dielectric choke of the door are correctly and
firmly mounted.

(2) After power is applied:

(a) Make sure the interlock switch mechanism

is operating properly by opening and closing
the door.

(b) Check microwave energy leakage must bebelow

the limit of 5 mW/cm

(All service adjustments should be made for

minimum microwave energy leakage

readings).

2

.

(3) Do not operate the unit until it is completely

repaired, if any of the following conditions exist.

The unit must not be operated.
(a) The door does not close firmly.
(b) The hinge is broken.
(c) The door seal is damaged.
(d) The door is bent or warped, or there is any

other visible damage on the unit that may

cause microwave energy leakage.
NOTE: Always keep the seal clean.
(e) Make sure that there are no defective parts

in the interlock mechanism.
(f) Make sure that there are no detective parts

in the microwave generating and transmission

assembly (especially waveguide).

(4) The following items should be checked after the

unit is repaired:

(a) The interlock monitor switch is connected

correctly and firmly.
(b) The magnetron gasket is properly positioned

and mounted.
(c) The waveguide and the oven cavity are intact.

(no microwave energy leakage)
(d) The door can be properly closed and the

safety switches work properly.
(e) The unit must stop when the door is opened or

the time is up.

The unit must not be operated with any of the above
components removed or by-passed.

7-1

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 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 and CDRH

(Center for Devices and Radiological Health)

immediately.

NOTE: Address on CDRH

Office of Compliance(HFZ-312)
Center for Devices and Radiological
Healthe 1390, Piccard Drive,
Rockville. MD 20850

• 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. ( 1 in/sec)

EQUIPMENT

• TESTER (VOLTS-DC, AC, Ohmmeter)

• Microwave survey meter

- Holaday HI-1500
HI-1501

- Narda 8100

8200

• 600 cc non conductive material beaker (glass or
plastic), inside diameter: approx. 8.5 cm (3

• Glass thermometer: 100°C or 212°F (1 deg scale)

1

/2 in.)

7-2

MEASUREMENT WITH THE OUTER CASE

WATER LOAD

TURNTABLE

REMOVED

(1) 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 HIGH

VOLTAGE PARTS.

NOTE WHEN MEASURING

(1) Do not exceed meter full scale deflection.
(2) The test probe must be removed no faster than

1 inch/sec (2.5cm/sec) along the shaded area,
otherwise a false reading may result.

(3) 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.

(4) 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.

MEASUREMENT WITH A FULLY
ASSEMBLED OVEN

(1) After all components, including the outer

panels, are fully assembled, measure for
microwave energy leakage around the door
viewing window, the exhaust opening and air
inlet openings.

(2) Microwave energy leakage must not exceed the

values prescribed below.

NOTES:

Leakage with the outer panels removed - less
than 5 mW/cm
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

2

.

POWER OUTPUT MEASUREMENT

(1)

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.

(2)

The water is contained in a cylindrical borosilicate
glass vessel having a maximum material thickness of

1/8” (3 mm) and an outside diameter of
approximately 7.6” (190mm).

(3)

The oven and the empty vessel are at ambient
temperature prior to the start of the test.

(4)

The initial temperature (T1) of the water is (10±2)°C
(50°F) 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.

(5)

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.

RECORD KEEPING AND NOTIFICATION
AFTER MEASUREMENT

(1) 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.

(2) Should the microwave energy leakage not be

more than 2 mW/cm2after determining that all
parts are in good condition, functioning properly
and genuine replacement parts which are listed in
this manual have been used.

(3) At least once a year, have the electromagnetic

2

.

energy leakage monitor checked for calibration
by its manufacturer.

(6)

The microwave power output P in watts is
calculated from the following formula :

P =

is measured while the microwave generator is
operating at full power. Magnetron filament heat-up
time is not included. (about 3 sec)

(7)

The water is stirred to equalize temperature
throughout the vessel, prior to measuring the final
water temperature.

(8)

Stirring devices and measuring instruments are
selected in order to minimize addition or removal of
heat.

4187 x (∆T)

T

7-3