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R-953/963M - 7

1. When the AH sensor cooking condition is started, the

coil of shut-off relays (RY1+RY6) are energized, the
oven lamp and cooling fan motor are turned on, but the
power transformer is not turned on.

NOTE: The oven should not be operated on AH SENSOR

COOKING immediately after plugging in the unit.
Wait two minutes before cooking on AH SENSOR
COOKING CONDITION.

2. After about 16 seconds, the cook relay (RY2) is ener-

gized. The power transformer is turned on, microwave
energy is produced and first stage is started. The 16
seconds is the cooling time required to remove any
vapour from the oven cavity and sensor. (Figure O-2)

NOTE: During this first stage, do not open the door or

touch STOP/CLEAR pad.

3. When the sensor detects the vapour emitted from the

food, the display switches over to the remaining cooking
time and the timer counts down to zero.
 At this time, the door may be opened to stir food, turn it
or season, etc.

NOTE: In case where a small quantity of food is cooked,

the oven will stop without displaying the remaining
cooking time.

4. When the timer reaches zero, an audible signal sounds.

The shut-off relay (RY1+RY6) and cook relay (RY2)
are de-energized and the power transformer, oven
lamp, etc. are turned off.

5. Operating the door or touching the STOP/CLEAR pad,

the time of day will reappears on the display and the
oven will revert to an OFF condition.

R-953/963M - 8

The door can be opened by pushing the open button on the
control panel. When the open button is pushed, the open
lever is 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 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 20A 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 switch
off all the electrical parts. The defective 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
cooking. If the temperature rises above 170˚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 temperature of
155˚C, the contacts of the thermal cut-out will close again.

This thermal cut-out protects the convection motor against
the overheat. If the temperature of the thermal cut-out
rises above 170˚C because the convection fan is inter-
rupted, the ventilation openings are obstructed or the
other abnormal matter occurs, the thermal cut-out opens
and switches off the convection heating element and the
other electrical parts. When the cools itself down to the
operating temperature of 155˚C, the contacts of the ther-
mal cut-out will close again.

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

Latch hook

Door

Stop switch

Latch
Heads

Monitor switch

Monitored
latch switch

Tab

R-953/963M - 9

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.)
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

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
provides the heated air.

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

The convection heating element escalated at the rear of
the oven cavity.  It is intended to heat air driven by the
convection fan. The heated air is kept in the oven and
force-circulated and reheated by the convection heating
element.

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

D2

D1

ASYMMETRIC
RECTIFIER

HIGH VOLTAGE 
RECTIFIER

THERMISTOR

Sensing 

Voltage

ON

OFF

ON

OFF

30

32 (sec.)

Sensing the voltage across temperature measurement circuit.

Door

Oven Cavity

Grill Heating
Elements

Convection
Passage

Cooling
Fan

Convection
Motor

Convection
Fan

Turntable

Turntable Motor

Roller Stay

Convection
Heating Element

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
raisen above 120˚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 0.39 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 0.39V 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 countering down.

R-953/963M - 10

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.

Damper position is set automatically by damper motor,
damper switch and motor cam.
These components are operated by a signal that judges
if microwave cooking or convection cooking operation is
selected by the CPU unit.

Microwave Cooking:
Damper is in the open position, because a portion of
cooling air is channelled through the cavity to remove
steam and vapours given off from the heating foods.
It is then exhausted at the top of the oven cavity into a
condensation compartment.

Convection Cooking:
Damper is in the closed position, so that no hot air will be
allowed to leak out the oven cavity.

Damper Operation
1. When power supply cord is plugged in or when the

control unit resumes after energy save mode finishes:

1-1. When power supply cord is plugged in, a signal is

sensed in the control unit, and operates shut-off relay
(RY8).

1-2. Contacts of shut-off relay (RY8) close, the damper

motor is energized, opening the damper door.

1-3. When the damper is moved to the open position by

the damper cam, damper switch is closed (ON posi-
tion).

1-4. The signal of damper switch is re-sensed in the

control unit and shut-off relay (RY8) is turned off.

1-5. The rated voltage to the damper motor is stopped and

the motor turns off.

2. When oven is microwave cooking:

Damper is in the open position

3. When oven is convection cooking:
3-1 Damper motor is energized right after the oven is

started.

3-2. When damper is in the closed position (damper

switch is OFF), its signal is sensed by the control unit,
and shut-off relay (RY8) is de-energized.

3-3. The damper is held in the closed position during the

convection cooking operation.

3-4. At the end of the convection cooking, when the fan

motor stops, shut-off relay (RY8) is energized, and
the damper is returned to the open position.

NOTE: If the damper door is not in the proper position,

closed during convection, grill or dual, or open
during microwave, the control unit will stop oven
operation after 1 minute.

4. Operation of damper is shown below.

Cooking Mode

Operation of Damper

Microwave cooking

Open

Convection cooking

Closed

Grill; during backed up with

convection heating element

Closed

Grill; after convection heating

element backed up has stopped

Open

Dual (Microwave and Convection)

Closed

Dual  (Microwave and Grill)

Open

Fir sensing condition

Closed

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.