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R-874M - 8

simultaneously.

Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILL three
times. Select the microwave power level by touching the
MICROWAVE POWER LEVEL key. When the STARTkey
is touched, the following operations occur:
1. The numbers on the digital readout start the count

down to zero.

2. The oven lamp, cooling fan motor and turntable motor

are energized.

3. The relay RY4 is energized and the main supply

voltage is applied to the bottom grill heating element.

4. The relay RY2 is energized and the microwave energy

is generated by magnetron.

5. Now, the food is cooked by microwave and bottom grill

simultaneously.

Following table shows each power setting of the top
heating element, bottom heating element, magnetron, fan
motor and convection motor on Convection cooking mode,
Dual (Micro + Convec.) mode and Convection preheating
mode.

POWER OUTPUT

ON TIME

OFF TIME

100%

48 sec.

0 sec.

90%

44 sec.

4 sec.

80%

40 sec.

8 sec.

70%

36 sec.

12 sec.

60%

32 sec.

16 sec.

50%

26 sec.

22 sec.

40%

22 sec.

26 sec.

30%

16 sec.

32 sec.

20%

12 sec.

36 sec.

10%

8 sec.

40 sec.

PIZZA
BREAKFAST
INSTANT COOK
AUTO COOK
AUTO DEFROST
Above functions are automatic cooking. They automati-
cally work out the correct cooking mode and cooking time
and/or cooking temperature. They will cook according to
the special cooking sequence.

After the same cooking mode is carried out for more than
the specified cooking time, the power output is automati-
cally reduced by turning the control relays on and off
intermittently, as shown in the table below. This is to
protect the oven door against temperature rising.

250˚C

70%

70%

70%

70%

10, 30, 50%

100%

100%

230˚C

60%

70%

50%

70%

10, 30, 50%

100%

100%

220˚C

40%

70%

40%

70%

10, 30, 50%

100%

100%

200˚C

40%

60%

40%

60%

10, 30, 50%

100%

100%

180˚C

40%

50%

40%

50%

10, 30, 50%

100%

100%

160˚C

20%

40%

20%

40%

10, 30, 50%

100%

100%

130˚C

20%

30%

20%

30%

10, 30, 50%

100%

100%

100˚C

20%

20%

20%

20%

10, 30, 50%

100%

100%

70˚C

10%

20%

10%

20%

10, 30, 50%

100%

100%

40˚C

10%

10%

10%

10%

10, 30, 50%

100%

0%

250˚C

100%

100%

60%

0%

230˚C

100%

100%

60%

0%

220˚C

100%

100%

60%

0%

200˚C

100%

100%

60%

0%

180˚C

100%

100%

60%

0%

160˚C

100%

100%

60%

0%

130˚C

100%

100%

60%

0%

100˚C

100%

100%

60%

0%

70˚C

100%

100%

60%

0%

40˚C

100%

100%

60%

0%

In grill cooking, convection cooking or dual cooking, the
top heater, bottom heater or magnetron operate whithin a
48 second time base. The following table is the ON / OFF
time ratio at each power output of the top heaters, bottom
heater or magnetron.

D

U

A

L

Specified cooking

Limited power

Time base

Cooking mode

time (minutes)

output (%)

(seconds)

Microwave (100%)

20

70

32

Top grill

30

50

48

Bottom grill

45

50

48

Top grill and

15 (Top)

50

48

Bottom grill

15(Bottom)

50

48

Micro. (100%)

20 (Micro.)

70

48

+ Top grill

15 (Grill)

50

48

Micro. (100%)

20 (Micro.)

70

48

+ Bottom grill

15 (Heater)

50

48

NOTE:
1. In case of Automatic operations, the limitations of

power output are not carried out.

2. In case that the stop key is touched or the oven door

is opened during cooking, the limitations of power
output are not carried out after the total cooking time
beyond the specified cooking time.

3. In case of the two or more same cooking modes  are

carried out,  the limitations of power output are not
carried out after the total cooking time beyond the
specified cooking time.

4. In case of the two or more different cooking modes  are

carried out,  the specified cooking time is started to
count from the point when the cooking mode is changed.

5. If the cooking mode has the power level display, the

power level is also displayed when the limitations of

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The door can be opened by pushing the open button on the
control panel. When the open button is pushed, the open
lever pushes the lower latch head on the door upward.
The upper latch head is linked with the  lower latch head,
so now, the door can be opened.

1. When the oven door is closed, the contacts (COM-NO)

of the switch must be closed. And the contacts (COM-
NC) must be opened.

2. When the oven door is opened, the contacts (COM-

NO) of the switch must be opened. And the contacts
(COM-NC) must be closed.

1. When the oven door is closed, the contacts (COM-NO)

of the switch must be closed.

2. When the oven door is opened, the contacts (COM-

NO) of switch must be opened.

The monitor switch is activated (the contacts opened) by the
upper latch head on the door while the door is closed. The
switch is intended to render the oven inoperative by means of
blowing the fuse F8A when the contacts of the monitored
latch switch fail to open when the door is opened.
Function
1. When the door is opened, the contacts (COM-NC) of

monitor switch  close (to the ON condition) due to their
being normally closed and contacts (COM-NO) open.
At this time the contacts (COM-NO) of monitored latch
switch is in the OFF condition (contacts open) due to
their being normally open contact switches.

2. As the door goes to a closed position, the monitor

switch contacts (COM-NC) are opened and contacts
(COM-NO) closed and then contacts (COM-NO) of
monitored latch switch and stop switch are closed.(On
opening the door, each of these switches operate
inversely.)

3. If the door is opened and the monitored latch switch

contacts  (COM-NO) fail to open, the fuse F8A blows
immediately after closing of the monitor switch (COM-
NC) contacts.

CAUTION: BEFORE REPLACING A BLOWN FUSE F8A,

TEST THE MONITORED LATCH SWITCH
AND  MONITOR SWITCH FOR PROPER
OPERATION.  (REFER TO CHAPTER “TEST
PROCEDURE”).

If the wire harness or electrical components are short-

circuited, this fuse blows to prevent an electric shock or fire
hazard.

1. If the wire harness or electrical components are short-

circuited, this fuse blows to prevent an electric shock or
fire hazard.

2. The fuse also blows when the monitored latch switch

remains closed with the oven door open and when the
monitor switch contact (COM-NC) closes.

3. The fuse also blows when the asymmetric rectifier,

H.V. rectifier,.H.V. wire harness, H.V. capacitor,
magnetron or secondary winding of high voltage trans-
former is shorted.

T/C transformer converts A.C. line voltage into low voltage
to drive the control unit.

This thermal cut-out protects the magnetron against over-
heat. If the temperature goes up higher than 125˚C be-
cause the fan motor is interrupted or the ventilation open-
ings are blocked, the thermal cut-out will open and line
voltage to the high voltage transformer will cut off and
operation of the magnetron will be stopped. The defec-
tive thermal cut-out must be replaced with a new one.

This thermal cut-out protects the oven against the over-
heat during grill cooking, convection cooking or dual
(combination) cooking. If the temperature rises above
150˚C because the fan motor is interrupted, the air inlet
duct is blocked or the ventilation openings are obstructed,
the thermal cut-out opens and switches off all the electrical
parts. When the cools itself down to the operating tem-
perature of 130˚C, the contacts of the thermal cut-out will
close again.

The asymmetric rectifier is a solid state device that pre-
vents current flow in both directions.  It prevents the
overheating of the high voltage transformer by blowing the
fuseF8A when the high voltage rectifier is shorted.

The rated peak reverse voltage of D1 of the asymmetric
rectifier is 6 KV. The rated peak reverse voltage of D2 of
the asymmetric rectifier is 1.7 KV. D1 and D2 of the
asymmetric rectifier or high voltage rectifier are shorted
when the each peak reverse voltage goes beyond the
each rated peak reverse voltage. (The process of the
blowing the fuse F8A.)

D2

D1

ASYMMETRIC
RECTIFIER

HIGH VOLTAGE 
RECTIFIER

1. The high voltage rectifier is shorted by some fault when

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

DOOR

LATCH HOOK

MONITOR SWITCH

MONITORED
LATCH SWITCH

 STOP SWITCH

OPEN LEVER

LATCH
HEADS

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R-874M - 10

primary winding of the high voltage transformer.

6. The fuse 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-energized. When the tempera-
ture 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 con-
vection time reaches “0”, the heating elements 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. Auto-
matically the fan motor will be shut down at low tempera-
ture (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 counting 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

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 F8A in the monitored latch switch - monitor

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

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Fuse F8A blows when the door is opened.

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

Fuse 15A 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/CLEAR 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/CLEAR key is touched.

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

touched. (Except Microwave mode)

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

A

B

C

D

E

E

E

F

G

G

H

H

H

I

J

K

L

L

M

NO

O

OOOOO

P

TEST PROCEDURE

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

BLOCKED COOLING FAN

BLOCKED CINVECTION 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  F2 F8A  

FUSE F1 20A 

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     

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