R-98STMA (serv.man6). Function Of Important Components - Sharp Home Appliance Service Manual (repair manual)

R98STMAA

6 – 1

R98STMAA

Service Manual

 

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.

Figure D-1. Door Open Mechanism

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

switch must be closed. And the contacts (COMNC) 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.

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 (con-
tacts 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 clos-
ing of the monitor switch (COMNC) contacts.

CAUTION: BEFORE REPLACING A NOISE FILTER (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 shortcircuited, this
fuse 20A blows to prevent an electric shock of fire hazard.

1. If the wire harness or electrical components are shortcircuited, 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 con-
tact (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.

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

°C

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
will open and switch off all the electrical parts. The defective thermal
cut-out must be replaced with a new one.

°C

This thermal cut-out protects the oven against the overheat 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 cut-out cools
itself down to the operating temperature of 155

°C, the contacts of the

thermal cut-out will close again.

°C

This thermal cut-out protects the convection motor against overheat-
ing. If the temperature of the thermal cut-out rises above 170

°C

because the convection fan is interrupted, 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 cut-out cools itself down to the operating
temperature of 155

°C, the contacts of the thermal cut-out will close

again.

The asymmetric rectifier is solid state device that prevents current flow
in both directions. And it prevents the temperature rise of the high volt-
age transformer by blowing the fuse F8A when the high voltage recti-
fier is shorted.

Latch hook

Door

Stop switch

Latch
Heads

Monitor switch

Monitored
latch switch

Tab

R98STMAA

6 – 2

the asymmetric rectifier is 1.7 KV. D1 and D2 of the asymmetric recti-
fier or high voltage rectifier are shorted when the each peak reverse
voltage goes beyond the each rated peak reverse voltage. (The pro-
cess 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 voltage 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 interference 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 channelled 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 elements are provided to brown the food and are
located on the top of the oven cavity.

The convection heating element situated 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 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 temperature inside
the oven cavity drops below the selected temperature, the heating ele-
ments 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 deenergized and
the convection fan stops operating and the oven shuts off. At that time
if the cavity air temperature has risen above 120

°C, the fan motor

remains rotating. Automatically the fan motor will be shut down at low
temperature (less than 105

°C).

Figure D-2. Convection Cooking System

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 measurement 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 sec-

onds. At that time, the voltage across the temperature measure-
ment circuit is measured.

2. The oven carries out the procedure above again. If the second volt-

age 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
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 or dual cooking mode, the oven is turned off.

2. When the thermistor or the wire harness to the thermistor is

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

D2

D1

ASYMMETRIC
RECTIFIER

HIGH VOLTAGE
RECTIFIER

Door

Oven Cavity

Grill Heating
Elements

Convection
Passage

Cooling
Fan

Convection
Motor

Convection
Fan

Turntable

Turntable Motor

Roller Stay

Convection
Heating Element

THERMISTOR

Sensing

Voltage

ON

OFF

ON

OFF

Sensing the voltage across temperature measurement circuit.

R98STMAA

6 – 3

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.

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 cav-
ity into a condensation compartment.

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

1. When power supply cord is plugged in or when the control unit

resumes after energy save mode finishes:

1) When power supply cord is plugged in, a signal is sensed in the

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

2) Contacts of shut-off relay (RY8) close, the damper motor is

energized, opening the damper door.

3) When the damper is moved to the open position by the damper

cam, damper switch SW4 is closed (ON position).

4) The signal of damper switch is re-sensed in the control unit and

shut-off relay (RY8) is turned off.

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:

1) Damper motor is energized right after the oven is started.

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) The damper is held in the closed position during the convection

cooking operation.

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

Open

Fire sensing condition Closed

Closed