LG MB-394, 594A Service Manual

MICROWAVE OVEN

SERVICE MANUAL

MODEL : MB-394A, MH-594A

CAUTION

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

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(3

/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

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. (Circled area of below Fig.)

- Around the magnetron

- The waveguide

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 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.

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

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.

WARNING : AVOID CONTACTING ANY

HIGH VOLTAGE PARTS (Magnetron, H.V. Transformer, H.V. Capacitor, H.V. Cable AssÕy, H.V. Circuit Protector)

5-3

¥ 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 Æ T of (10±2)¡K is measured, where T is the time

in seconds and ÆT 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

MEASUREMENT OF MICROWAVE POWER OUTPUT

P =

DISASSEMBLY AND ADJUSTMENT

A. OUTER CASE REMOVAL

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.

B. POWER SUPPLY CORD

1) Remove the outer case.

2) Disconnect two terminals, and remove one screw of the earth terminal.

CAUTION: DISCHARGE THE HIGH VOLTAGE

CAPACITOR BEFORE SERVICING

(refer to page 2-1)

C. CONTROL PANEL ASSEMBLY

1) Disconnect the leadwire from the PCB SUB ASSÕY.

2) Remove the screws for the earth and securing the control panel.

3) Lift control panel ASSÕY from the oven by the tab unhooked.

4187 X (DT) + 0.88 X (T2 - T0) X M

¥ T2: Temperature after heating ¥ T0: Temperature after bowl ¥ M: Weight of bowl

D. DOOR GROSS ASSEMBLY REMOVAL

1) Open the door.

2) Remove the choke cover cap very carefully with a flat-blade screwdriver.

CAUTION : Be careful not to damage door seal plate

by screwdriver.

3) Lift up and push the door.

NOTE:

1. After replacing the door, be sure to check that the

primary switch, monitor switch, and secondary switch operate normally.

2. After replacing the door, check for microwave energy

leakage with a survey meter. Microwave energy must be below the limit of 5 mW/cm. (with a 275 ml water load)

3. When mounting the door assembly to the oven

assembly, be sure to adjust the door assembly parallel to the chassis. Also adjust so the door has no play between the inner door surface and oven frame assembly. If the door assembly is not mounted properly, microwaves may leak from the clearance between the door and the oven.

E. HIGH VOLTAGE TRANSFORMER

REMOVAL

1) Discharge the high voltage capacitor.

2) Disconnect the leadwire from magnetron, high voltage transformer, and capacitor.

3) Remove the screw holding the high voltage transformer to the baseplate.

F. FAN MOTOR ASSEMBLY REMOVAL

1) Discharge the high voltage capacitor.

2) Disconnect the leadwire from fan motor, noise filter and high voltage capacitor.

3) Remove the two screws holding the the suction guide ASSÕY to the oven cavity and remove the high voltage diode earth screw.

4) Remove the screw of the capacitor bracket.

5) Remove the two screws holding the fan motor ASSÕY to the suction guide ASSÕY.

G. HIGH VOLTAGE CAPACITOR AND

DIODE REMOVAL

1) Discharge the high voltage capacitor.

2) Disconnect the leadwire from fan motor, noise filter and high voltage capacitor.

3) Remove the screw holding the suction guide ASSÕY to the oven cavity and remove the high voltage diode earth screw.

4) Remove the screw holding the high voltage capacitor bracket.

Remove choke cover cap

Remove door

H.V. Transformer

H.V. Diode

H.V. Capacitor

Suction Guide

Fan Motor ASS'Y

5-4

Door seal plate

5-5

H. AIR DUCT ASSEMBLY REMOVAL

1) Disconnect the leadwire from lamp.

2) Remove the mounting screw to Latch Board.

I. MAGNETRON REMOVAL

1) Disconnect the leadwire from the high voltage transformer and high voltage capacitor.

2) Carefully remove the mounting screws holding the magnetron and the waveguide.

3) Remove the magnetron ASSÕY until the tube is clear from the waveguide.

NOTE:

1. When removing the magnetron, make sure its dome does not hit any adjacent parts, or it may be damaged.

2. When replacing the magnetron, be sure to install the magnetron gasket in the correct position and be sure that the gasket is in good condition.

3. After replacing the magnetron, check for microwave leakage with a survey meter around the magnetron. Microwave energy must be below the

limit of 5 mW/cm2. (With a 275 ml. water load).

Make sure that gasket is rigidly attached to the

magnetron. To prevent microwave leakage,

tighten the mounting screws properly, making sure there is no gap between the waveguide and the magnetron.

J. REMOVING THE TURNTABLE MOTOR

1) Remove the turntable.

2) Remove the Rotating Ring.

3) Lay the unit down on its back.

4) Remove the turntable motor cover. The turntable base cover is easily removed by

pinching the six parts with a wire cutting.

5) Disconnect the leadwire from the turntable motor

terminals.

6) Remove the screw securing the turntable motor to

the oven cavity ASSÕY

7) After repairing the motor, rotate the removed

turntable motor cover.

8) Fit the turntable motor coverÕs projecting part to the

base plate slit.

NOTE:

1. Remove the wire lead from the turntable motor

VERY CAREFULLY.

2. Be sure to grasp the connector, not the wires, when

removing.

Waveguide

Magnetron

Magnetron Gasket

Magnetron

Dome

Waveguide  Bracket

Wire Leads

Turntable Motor

K. PCB ASSEMBLY REMOVAL

1) Remove the control panel assembly from the cavity. (Refer to control panel assembly removal on previous page.)

2) Remove screws which hold the PCB SUB ASSÕY to the control panel.

3) Disconnect the flat cable from the PCB SUB ASSÕY and take off the PCB SUB ASSÕY.

L. INTERLOCK SYSTEM

1) INTERLOCK MECHANISM The door lock mechanism is a device which has

been specially designed to eliminate completely microwave activity when the door is opened during cooking and thus to prevent the danger resulting from the microwave leakage.

2) MOUNTING OF THE PRIMARY/MONITOR/ SECONDARY SWITCHES TO THE LATCH BOARD

3) INSTALLATION AND ADJUSTMENT OF THE LATCH BOARD TO THE OVEN ASSEMBLY

¥ Mount the latch board to the oven assembly. ¥ Adjust the latch board in the arrow direction so that

oven door will not have any play in it when the door is closed.

¥ Tighten the mounting screw.

¥ Check for play in the door by pushing the door

release button. Door movement should be less than 0.5 mm. (1/64 inch)

Don't push the door release button while making

adjustment. Make sure that the latch moves smoothly after adjustment are completed and that the screws are tight. Make sure the primary, monitor, and secondary switches operate properly by following the continuity test procedure.

5-6

Key Membrane

Control Panel

PCB Sub ASS'Y

FIG. 1

ADJUSTMENT DIRECTION

PRIMARY SWITCH

MONITOR SWITCH

SECONDARY SWITCH

CAUTION: CHECK THE CORRECT POSITION

MAGNETRON

H.V. TRANSFORMER

H.V. CAPACITOR

PRIMARY

CIRCUIT PROTECTOR

H.V. DIODE

A. PRIMARY INTERLOCK SWITCH TEST

When the door release button is depressed slowly with the door closed, an audible click should be heard at the same time or successively at intervals. When the button is released slowly, the latches should activate the switches with an audible click. If the latches do not activate the switches when the door is closed, the switches should be a adjusted in accordance with the adjustment procedure. Disconnect the wire lead from the primary switch. Connect the ohmmeter leads to the common (COM) and normally open (NO) terminal of the switch. The meter should indicate an open circuit in the door open condition. When the door is closed, the meter should indicate a closed circuit. When the primary switch operation is abnormal, make the necessary adjustment or replace the switch only with the same type of switch.

B. SECONDARY INTERLOCK SWITCH TEST

Disconnect the wire lead from the secondary switch. Connect the ohmmeter leads to the common (COM) and normally open (NO) terminals of the switch. The meter should indicate a open circuit in the door open condition. When the door is closed, meter should indicate an closed circuit. When the secondary switch operation is abnormal, make the necessary adjustment or replace the switch only with the same type of switch.

C. MONITOR SWITCH TEST

Disconnect the wire lead from the monitor switch. Connect the ohmmeter leads to the common (COM) and normally closed (NC) terminals of the switch. The meter should indicate closed circuit in the door open condition. When the door is closed, meter should indicate an open circuit. When the monitor switch operation is abnormal, replace with the same type of switch.

NOTE: After repairing the door or the interlock system, it is necessary to do this continuity test before operating the oven.

5-7

WARNING : FOR CONTINUED PROTECTION AGAINST EXCESSIVE RADIATION

EMISSION, REPLACE ONLY WITH IDENTICAL REPLACEMENT PARTS.

TYPE NO. SZM-V 16-FA-63 OR VP-533A-OF FOR PRIMARY SWITCH TYPE NO. SZM-V 16-FA-62 OR VP-532A-OF FOR MONITOR SWITCH TYPE NO. SZM-V 16-FA-63 OR VP-533A-OF FOR SECONDARY SWITCH

COMPONENTS TEST PROCEDURE RESULTS

SWITCHES Check for continuity of the Door Door (Wire leads removed) switch with an Ohm-meter open closed

Primary Switch

Monitor Switch

NOTE : After checking for the continuity of switches, make sure that are

connected correctly.

INTERLOCK CONTINUITY TEST

Secondary Switch

COM

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