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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  - 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  - 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