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R-852 - 11

microwave cooking or dual cooking.

2. The peak reverse voltage of D2 of the rectifier goes

beyond the rated peak reverse voltage 1.7 KV in the
voltage doubler circuit.

3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-

age winding of the high voltage transformer.

5. The large electric currents beyond 8A flow through the

primary winding of the high voltage transformer.

6. The fuse F2 F8A blows by the large electric currents.
7. The power supplying to the high voltage transformer is

cut off.

The noise filter assembly prevents radio frequency inter-
ference that might flow back in the power circuit.

The turntable motor rotates the turntable.

The fan motor drives a blade which draws external cool air.
This cool air is directed through the air vanes surrounding
the magnetron and cools the magnetron. This air is chan-
nelled through the oven cavity to remove steam and
vapours given off from heating food. It is then exhausted
through the exhausting air vents of the oven cavity.

The convection motor drives the convection fan and
provide the heated air.

The grill heating element is provided to brown the food and
is located on the top of the oven cavity.

The grill heating element is provided to brown the food and
is located at the base of the oven cavity.

This oven is designed with a hot air heating system where
food is heated by forced circulation of the hot air produced
by the grill heaters.
The air heated by the grill heating elements is circulated
through the convection passage provided on the outer
casing of the oven cavity by means of the convection fan
which is driven by the convection motor. It is then enters
the inside of the oven through the vent holes provided on
the back side of the oven. Next, the hot air heats the food
on the turntable and leaves the oven cavity through the
vent in the oven cavity rear wall.
In this way, the hot air circulates inside the oven cavity to
raise its temperature and, at the same time, comes into
contact with the food being cooked.
When the temperature inside the oven cavity reaches the
selected temperature, the heating elements are de-ener-
gized. When the temperature inside the oven cavity drops
below the selected temperature, the heating elements are
energized again. In this way, the inside of the oven cavity
is maintained at approximately the selected temperature.
When the convection time reaches “0”, the heating ele-

ments are de-energized and the convection fan stops
operating and the oven shuts off. At that time if the cavity
air temperature has rised above120˚C, the fan motor
remains rotating. Automatically the fan motor will be shut
down at low temperature (less than 105˚C).

The oven will stop its operation when there is a fire in the
oven cavity in microwave cooking condition.
LSI measures the voltage across the temperature meas-
urement circuit intermittently within 32-seconds time base
since the oven is started in microwave cooking condition.
The oven will stop its operation when the difference of the
voltage is more than 0.781 volts in microwave cooking
condition.

1. Within a 32-seconds base, the thermistor is energized

for 2 seconds.  At that time, the voltage across the
temperature measurement circuit is measured.

2. The oven carries out the procedure above again. If the

second voltage is 0.781V higher than first voltage, LSI
judges it  is a fire in the oven cavity and stop the oven.

3. When LSI judges it  is a fire in the oven cavity, LSI will

switch off the relays to high voltage transformer and fan
motor and LSI stops countring down.

1. If the temperature of the thermistor does not rise to

more than 40˚C after 4 minutes and 15 seconds from
when the oven is started in convection, grill (top and
bottom grills) or dual cooking mode, the oven is turned
off.

2. When the thermistor or the wire harness to the thermis-

tor is opened, the oven is turned off after 4 minutes and
15 seconds because this condition is same as above.

THERMISTOR

Sensing 

Voltage

ON

OFF

ON

OFF

30

32 (sec.)

Sensing the voltage across temperature measurement circuit.

Door

Oven Cavity

Grill Heating
Element

Bottom Grill 
Heating Element

Convection
Passage

Cooling
Fan

Convection
Motor

Convection
Fan

Turntable

Insulation

Turntable
Motor

R-852 - 12

When troubleshooting the microwave oven, it is helpful to follow the Sequence of Operation in performing the checks.
Many of the possible causes of trouble will require that a specific test be performed. These tests are given a procedure
letter which will be found in the “Test Procedure” section.

IMPORTANT: If the oven becomes inoperative because of a blown fuse F2 (F8A) in the monitored latch switch - monitor

switch circuit, check the monitored latch switch and monitor switch before replacing the fuse F2 (F8A).

R-852 - 13

Fuse F2 F8A blows when the door is opened.

Home fuse blows when power cord is plugged into wall outlet.

Fuse F1 20A blows when power cord is plugged into wall outlet.

Nothing appears in display when power cord is plugged into wall outlet 

and the door is opened and closed.

Display does not operate properly when STOP key is touched.

Oven lamp does not light when door is opened. (Display operates.)

Oven does not start when the START key is touched. (Display operates.)

Oven lamp does not light  and turntable motor does not operate.

Fan motor does not operate. (Oven lamp lights.)

Convection fan motor does not operate. (Oven lamp lights.)

Turntable motor does not operate. (Oven lamp lights.)

Oven or any electrical parts (except fan motor) does not stop when 

cooking time is 0 or STOP  key is touched.

Oven stops after 4 minutes and 15 seconds since START key is 

touched. (Except Microwave and Dual Cook modes)

Display operates properly but all electrical parts do not operate.

Oven goes into cook cycle but shuts down before end of cooking cycle.

After cooking, the temperature of oven cavity is higher than 105

˚C but 

fan motor does not operate.

Oven seems to be operating but little or no heat is produced in oven 

load. (Microwave power control is set at HIGH)

Oven does not seem to be operating properly during variable cooking 

condition except HIGH cooking condition.

Oven goes into cook cycle but shuts down before end of cooking cycle.

Convection cooking mode does not heat.

Oven seems to be operating but the temperature in the oven cavity is 

lower or higher than preset one. 

Grill heating element does operate.

Oven goes into cook cycle but shuts down before end of cooking cycle.

Oven seems to be operating but little or no heat is produced in oven 

load. (Microwave power does not seem to be generated properly)

Oven seems to be operating but the temperature in the oven cavity is 

lower or higher than preset one. 

Top and Bottom Grill heating elements do not heat.

Top or Bottom Grill heating element does not heat.

OFF

CONDITION

COOKING

CONDITION

(COMMON MODE)

MICROWAVE

COOKING

CONDITION

GRILL COOKING

CONDITION

CONVECTION

COOKING

CONDITION

DUAL COOKING

CONDITION

(COMMON MODE)

DUAL COOKING

CONDITION

(MICRO./CONV.)

DUAL COOKING

CONDITION

(MICRO./GRILL)

CONDITION

PROBLEM

BLOCKED COOLING FAN

BLOCKED CONVECTION FAN

NO POWER AT WALL OUTLET

HOME FUSE OR BREAKER

MIS-ADJUSTMENT OF SWITCHES

BLOCKED VENTILATION OPENINGS

OPENED WIRE HARNESS

SHORTED WIRE HARNESS

OVEN LAMP OR SOCKET

POWER SUPPLY CORD

FOIL PATTERN ON P.W.B.

RELAY RY7

RELAY RY6

RELAY RY5

RELAY RY4

RELAY RY3

RELAY RY2

RELAY RY1

KEY UNIT

TOUCH CONTROL PANEL

TC TRANSFORMER

BOTTOM GRILL HEATING ELEMENT

TOP GRILL HEATING ELEMENT

FUSE F8A F2

FUSE 20A F1

NOISE FILTER

CONVECTION FAN MOTOR

FAN MOTOR

TURNTABLE MOTOR

THERMAL CUT-OUT 150˚C TC2

THERMAL CUT-OUT 125˚C TC1 

THERMISTOR

MONITOR SWITCH SW3

STOP SWITCH SW2

MONITORED LATCH SWITCH SW1

HIGH VOLTAGE CAPACITOR

H.V. HARNESS

H.V. RECTIFIER ASSEMBLY

HIGH VOLTAGE  TRANSFORMER

MAGNETRON     

AB

C

D

EEE

F

G

G

H

H

H

I

J

K

L

L

M

N

O

O

O

O

O

O

O

P

TEST PROCEDURE

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

R-852 - 14

CARRY OUT 3D CHECKS.

Isolate the magnetron from high voltage circuit by removing all leads connected to filament terminal.

To test for an open circuit filament use an ohmmeter to make a continuity test between the magnetron
filament terminals, the meter should show a reading of less than 1 ohm.

To test for short filament to anode condition, connect ohmmeter between one of the filament terminals
and the case of the magnetron (ground). This test should be indicated an infinite resistance. If a low
or zero resistance reading is obtained then the magnetron should be replaced.

The following test procedure should be carried out with the microwave oven in a fully assembled
condition (outer case fitted). Microwave output power from the magnetron can be measured by way
of IEC 705, i.e. it is measured by how much power the water load can absorb. To measure the
microwave output power in the microwave oven, the relation of calorie and watt is used. When P(W)
heating works for t(second), approximately P x t/4.187 calorie is generated. On the other hand, if the
temperature of the water with V(ml) rises 

∆

T (

°

C) during this microwave heating period, the calorie of

the water is V x 

∆

T.

Measuring condition:
1. Container

The water container must be a cylindrical borosilicate glass vessel having a maximum material
thickness of 3 mm and an outside diameter of approximately 190 mm.

2. Temperature of the oven and vessel

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

3. Temperature of the water

The initial temperature of the water is (10

±

2)

°

C.

4. Select the initial and final water temperature so that the maximum difference between the final water

temperature and the ambient temperature is 5K.

5. Select stirring devices and measuring instruments in order to minimize addition or removal of heat.
6. The graduation of the thermometer must be scaled by 0.1

°

C at minimum and be an accurate

thermometer.

7. The water load must be (1000

±

5) g.

8. “t” is measured while the microwave generator is operating at full power. Magnetron filament heat-

up time is not included.

NOTE: The operation time of the microwave oven is “t + 3” sec. (3 sec. is magnetron filament heat-up time.)

Therefore total heating time = 50 sec.

Measuring method:
1. Measure the initial temperature of the water before the water is added to the vessel.

(Example: The initial temperature T1 = 11

°

C)

2. Add the 1 litre water to the vessel.
3. Place the load on the centre of the shelf.
4. Operate the microwave oven at HIGH for the temperature of the water rises by a value 

∆

 T of

(10 

±

 2) K.

5. Stir the water to equalize temperature throughout the vessel.
6. Measure the final water temperature. (Example: The final temperature T2 = 21

°

C)

7. Calculate the microwave power output P in watts from above formula.

NEVER TOUCH ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL
WHILE THE OVEN IS IN OPERATION.

The formula is as follows;

P x t / 4.187 = V x 

∆

 T

P (W) = 4.187 x V x 

∆

T / t

Our condition for water load is as follows:

Room temperature ........... around 20

°

C

Power supply Voltage .............. Rated voltage

Water load .................................. 1000 g

Initial temperature .............................. 10

±

2

°

C

Heating time .............................. 47 sec.
P = 90 x 

∆

T