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R-8740M  - 9

The door can be opened by pushing the open button on the
control panel. When the open button is pushed, the open lever
pushes 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

each switch must be closed.

2. When the oven door is opened, the contacts COM-NO of

each 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 F2 F8A when the contacts of the moni-
tored latch switch SW1 fail to open when the door is opened.

LATCH
HEADS

LATCH HOOK

MONITORED LATCH
SWITCH

STOP SWITCH

OPEN LEVER

DOOR OPEN BUTTON

MONITOR 
SWITCH

If the wire harness or electrical components are short-circuited,
this fuse F1 20A 250V 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 monitored latch switch SW1

remains closed with the oven door open and when the
monitor switch SW2  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 transformer is shorted

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

The thermal cut out protects the oven against overheat during
grill cooking, convection cooking or dual (combination) cook-
ing. 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 the all electrical parts.
When the oven cools itself down to the operating temperature
of 130˚C, the contacts of the thermal cut-out will close again.

This thermal cut-out protects the magnetron against overheat.
If the temperature goes up higher than 125˚C because the fan
motor is interrupted or the ventilation openings are blocked,
the thermal cut-out TC2 will open and line voltage to the high
voltage transformer T will cut off and operation of the magnetron
MG will be stopped. The defective thermal cut-out must be
replaced with a new one.

Program the desired cooking time and then touch the
"BOTTOM GRILL"   

   and MICROWAVE keys (led's will

light on the display panel). When the START key 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. Relay RY4 is energized and the main supply voltage is

applied to the bottom heater.

4. Relay RY2 is energized and the microwave energy is

generated by the magnetron.

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

simultaneously.

When the number keys are pressed (not in "MANUAL" 
mode). the oven chooses cooking mode (Microwave, Convec-
tion, Grill or Dual) and sets cooking time automatically accord-
ing to the pre-programmed information in IC-1 (LSI).

Function
1. When the door is opened, the contacts (COM-NC) of

monitor switch SW2 close (to the ON condition) due to their
being normally closed and contacts (COM-NO) open. At
this time the monitored latch SW1 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

SW2 contacts (COM-NC) are opened and contacts (COM-
NO) closed  and then the monitored latch switch and stop
switch contacts close.(On opening the door, each of these
switches operate inversely.)

3. If the door is opened and the monitored latch switch  SW1

contacts fail to open, the fuse F2 (F8A) blows immediately
after closing of the monitor switch (COM-NC) contacts.

CAUTION: BEFORE REPLACING A BLOWN FUSE F2 F8A

TEST THE MONITORED LATCH  SWITCH,
MONITOR SWITCH  FOR PROPER OPERA-
TION.
(REFER TO CHAPTER “TEST PROCEDURE”).

R-8740M  - 10

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 heaters 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 circulated inside 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 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 elements
are de-energized and the convection fan stops operating and
the oven shuts off. At the high temperature (more than 104˚C),
the fan motor remains rotating. Automatically the fan motor will
be shut down at low temperature (less than 104˚C).

Door

Oven Cavity

Grill Heating
Elements

Convection
Passage

Auxiliary
Fan

Convection
Motor

Convection
Fan

Insulator

Roller Stay

Turntable
Motor

Bottom 
Heater
Element

Turntable

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 interfer-
ence that might flow back in the power circuit.

The turntable motor drives the roller stay to rotate the turnta-
ble.

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 channeled
through the oven cavity to remove steam and vapours given
off from the heating foods. It is then exhausted through the
exhausting air vents at 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

The asymmetric rectifier is solid state device that prevents
current flow is both directions. And it prevents the tempera-
ture rise of the power transformer by blowing the fuse F2 F8A
when the high voltage rectifier is shorted.

D2

D1

ASYMMETRIC
RECTIFIER

HIGH VOLTAGE 
RECTIFIER

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 F2
F8A.)
1. The high voltage rectifier is shorted by any causes 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 voltage

winding of the high voltage transformer.

R-8740M  - 11

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 measure-
ment circuit intermittently within a 32-second time base after
since the oven is started in microwave cooking condition.
The oven will stop its operation when the difference of the
voltages is more than 0.69 volts in microwave cooking
condition.

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

for 3 seconds.  After 2 seconds since the thermistor is
energized, the voltage across the temperature
measurement circuit is measured. And after 21 seconds
since the thermistor is cut off, the convection motor
operates for 10 seconds.

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

second voltage is 0.69V higher than the first voltage, LSI
judges it is a fire in the oven cavity and stops the oven.

3. When sensor cooking, the sensing fire operation is not

carried out until the oven senses the steam from food.
Because food cannot be cooked well by rotating the
convection fan at that time. After sensing the steam, the
sensing fire operation is started.

4. When LSI judges it is fire in the oven cavity, LSI will switch

off the relays to the power transformer, fan motor and
convection motor and LSI stops counting down. And then
the damper is closed so that the fresh air does not come
into the oven cavity.

.

.

THERMISTOR

SENSING

VOLTAGE

ON

OFF

ON

OFF

0

20

64 (sec.)

3 sec.

2

30 32

10 SEC.

Sensing the voltage across temperature measurement circuit.

ON

OFF

CONVECTION

MOTOR

R-8740M  - 12

       PROBLEM

Fuse F2 F8A blows when the door is opened.

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

Fuse F1 15A blows when power cord is plugged into wall

outlet.

":0" does not appear in display when power cord is plugged

into wall outlet.

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 about 4 minutes since START key is touched.

(Except Microwave and Dual Cook modes)

Display operates properly but all electrical parts do not oper-

ate.

The oven stops 1 minute after starting.

Oven goes into cook cycle but shuts down before end of

cooking cycle.

After cooking, the temperature of oven cavity is higher than

104˚C but the 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. (Oven operates properly at HIGH)

Oven goes into cook cycle but shuts down before end of

cooking cycle.

Convection cooking mode does not heat.

Oven seems to be operate when the temperature in the oven

cavity is lower or higher than preset one.

Grill heating element does not heat.

Convection heating does not stop when the temperature of

oven cavity is higher than 210˚C or it stop to heat when the

temperature of oven cavity is lower than 210˚C

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 of oven cavity

is lower or higher than preset one.

Convection heating element does not heat.

Grill heating does not heat.

TEST PROCEDURE

OFF
CONDITION

DUAL COOKING
CONDITION
(MICRO./GRILL)

MAGNETRON

HIGH VOLTAGE  TRANSFORMER

H.V. RECTIFIER ASSEMBLY

H.V. HARNESS

HIGH VOLTAGE CAPACITOR

MONITORED LATCH SWITCH

STOP SWITCH

MONITOR SWITCH

THERMAL CUT-OUT 150

˚C TC1

THERMAL CUT-OUT 125

˚C TC2

TURNTABLE MOTOR

FAM NOTOR

CONVECTION FAN MOTOR

NOISE FILTER

 A   B   C

  D E E E G G H H H I

COOKING
CONDITION
(COMMON  MODE)

MICROWAVE
COOKING
CONDITION

CONVECTION
COOKING
CONDITION

DUAL COOKING
CONDITION
(COMMON  MODE)

DUAL COOKING
CONDITION
(MICRO./CONV.)

GRILL
COOKING
CONDITION

POSSIBLE CAUSE

AND

DEFECTIVE PARTS