R-98STMA (serv.man6). Function Of Important Components - Sharp Home Appliance Service Manual (repair manual). Page 2

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.