LG MC-785BC Service Manual ▷ View online
22
COMPONENTS
TEST PROCEDURE
RESULTS
RELAY 3,4, OF P.C.B
(Disconnect the 7 pin
connector from P.C.B)
See Schematic Dagram on
page 9)
(Disconnect the 7 pin
connector from P.C.B)
See Schematic Dagram on
page 9)
RELAY 1,2,3 OF P.C.B
(Wire leads removed.)
RY1 : Microwave
RY2 : Convection
RY3 : Turn Table,
(Wire leads removed.)
RY1 : Microwave
RY2 : Convection
RY3 : Turn Table,
Fan Motor,
Roti-Motor
(For MC-785BC only)
Oven Lamp
Roti-Motor
(For MC-785BC only)
Oven Lamp
RY4 : Circulation Motor
RY5 : Grill
RY5 : Grill
MEMBRANE KEY
Check continuity between
switch terminals, by pressing an
appropriate pad on key board.
The contacts assignment of the
respective pads on the key
board is as shown left figures.
switch terminals, by pressing an
appropriate pad on key board.
The contacts assignment of the
respective pads on the key
board is as shown left figures.
Cooking Start
OFF
CN2
1
3
5
7
Relay 3
Relay 4
Relay 2
Relay 5
Relay 1
1
2
3
4
5
6
7
8
9
2
3
4
5
6
7
8
9
10
11
11
1
2
3
4
5
6
7
8
9
10
11
2
3
4
5
6
7
8
9
10
11
Membrane
Panel
P.C.B
Cooking Start
OFF
NOTE : A MICROWAVE ENERGY LEAKAGE TEST MUST ALWAYS BE PERFORMED WHEN THE UNIT IS
SERVICED FOR ANY REASON.
MAKE SURE THE WIRE LEADS ARE CORRECT POSITION.
WHEN REMOVING THE WIRE LEAD FROM THE PARTS, BE SURE TO GRASP THE
CONNECTOR, NOT THE WIRES.
MAKE SURE THE WIRE LEADS ARE CORRECT POSITION.
WHEN REMOVING THE WIRE LEAD FROM THE PARTS, BE SURE TO GRASP THE
CONNECTOR, NOT THE WIRES.
Trouble Shooting Guide
23
CAUTIONS
1. Check grounding before checking for other trouble.
2. Be careful of the high voltage circuit.
3. When checking the continuity of the switches or transformer, disconnect one lead wire from these parts and
then check continuity without turning the power source on.
4. Do not touch any part of the circuitry on the control circuit board, since static electric discharge may damage
this control circuit.
Always touch yourself to ground while working on this circuit to discharge any static charge built up in your
body.
Always touch yourself to ground while working on this circuit to discharge any static charge built up in your
body.
5. First operate the microwave oven following the correct operating.
6. If the oven become inoperative because of a fuse, check the primary, the secondary and the monitor switches
before replacing the fuse.
CONDITION
The fuse blows when power
cord is plugged into wall
receptacle.
cord is plugged into wall
receptacle.
Shorted lead wise assy or
wire hardness.
wire hardness.
Defective monitor switch or
out of adjustment.
out of adjustment.
Test procedure switches.
": 0" does not appear in
displasy window when
power cord is first plugged
into wall outlet.
displasy window when
power cord is first plugged
into wall outlet.
Microwave oven plug is not
inserted tightly.
inserted tightly.
No power at outlet.
Check home fuse.
Replace or repair wiring.
Test procedure tact switch
key.
key.
Defective tact switch key of
P.C.B.
P.C.B.
Display does not operate
properly when CLOCK is
touched.
properly when CLOCK is
touched.
Open wire in power cord,
wiring harness, or wiring
between control panel
assembly units.
wiring harness, or wiring
between control panel
assembly units.
Insert microwave oven plug
securely.
securely.
Replace lead wire or check
and repair wire hardness.
and repair wire hardness.
CAUSE
TEST PROCEDURE OR
CORRECTION
Blown fuse.
Test procedure fuse.
(TROUBLE 1) Oven does not operate all.
OFF CONDITION
24
CONDITION
No input can be
programmed.
programmed.
Some inputs can not be
programmed.
programmed.
Display fixes some figure and
can not accept any input.
can not accept any input.
Set time does not count
down when touched START.
down when touched START.
Display shows a number or
figure different from one
touched.
figure different from one
touched.
Random programming when
pressing other than touch
keys.
pressing other than touch
keys.
Defective tact switch key of
P.C.B.
P.C.B.
Loose connection.
Connect them tightly.
Defective secondary switch
or out of adjustment
or out of adjustment
Defective tact switch key of
P.C.B.
P.C.B.
Test procedure tact switch
key of P.C.B.
key of P.C.B.
Test procedure secondary
switch.
switch.
Test procedure tact switch
key of P.C.B.
key of P.C.B.
CAUSE
TEST PROCEDURE OR
CORRECTION
Incorrect touch key.
Touch key again after
closing.
closing.
Loose connection.
Test tact switch key of
P.C.B.
P.C.B.
(TROUBLE 2) Oven does not at all even thought MOTOR and P.C.B program.
25
CONDITION
Oven does not go into a
cook cycle when START key
is touched.
cook cycle when START key
is touched.
Primary and secondary
interlock switches defective
or out of adjustment.
interlock switches defective
or out of adjustment.
Defective relay 1 of P.C.B.
Test procedure relay 1 of
P.C.B.
P.C.B.
Test procedure primary and
secondary switches.
secondary switches.
CAUSE
TEST PROCEDURE OR
CORRECTION
Defective tact switch key of
P.C.B.
P.C.B.
Test procedure tact switch
key of P.C.B.
key of P.C.B.
Open or loose wiring to
above components.
above components.
Check and repair wiring.
Ouput power is too low.
Defective fan motor.
Low AC input voltage.
Use the microwave oven at
adequate line voltage.
adequate line voltage.
Test procedure fan motor.
Food temperature is too low.
This may not be a defect.
It is possible that the food
should be cooked for a
longer period.
It is possible that the food
should be cooked for a
longer period.
Defective relay 3 of P.C.B.
Test procedure relay 3 of
P.C.B.
P.C.B.
Uneven cooking.
Inconsistent intensity of
microwave by their
characteristics.
microwave by their
characteristics.
Food does not turn during
cook cycle.
cook cycle.
Food or cookware extending
oven edges of turntable
prevents turning rearrange
oven edges of turntable
prevents turning rearrange
1. Wrap the thinner part with
aluminum foil.
2. Use plastic wrap or with a lid.
3. Stir once or twice while
3. Stir once or twice while
cooking soup, cocoa or milk
Turntable motor does not
operate.
operate.
Refer to preceding turntable
motor problem.
motor problem.
Output is too high when you
set lower power level.
set lower power level.
Defective relay 1 of P.C.B.
High AC input voltage.
Use the microwave oven at
adequate line voltage.
adequate line voltage.
Test procedure relay 1 of
P.C.B.
P.C.B.
(TROUBLE 3) Oven does not operating microwave cooking
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