Sharp R-9H56, Service Manual

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R-9H56
8
FUSE M8A 250V
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 upper latch switch and lower
latch switch remain closed with the oven door open and
when the monitor switch closes.
3. The fuse M8A also blows when asymmetric rectifier,
H.V. rectifier, H.V. wire harness, H.V. capacitor,
magnetron or secondary winding of power transformer is
shorted.
ASYMMETRIC RECTIFIER
The asymmetric rectifier is a solid state device that prevents
current flow ins both directions. And it prevents the tempera-
ture rise of the power transformer by blowing the fuse M8A
when the high voltage rectifier is shorted.
FAN MOTOR
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 va-
pours given off from the heating foods. It is then exhausted
through the exhausting air vents at the oven cavity.
CONVECTION HEATER
The convection heater is located 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 heater.
CONVECTION COOKING SYSTEM
This oven is designed with a hot air heating system where
food is not directly heated by the convection heater, but is
heated by forced circulation of the hot air produced by the
convection heater. The air heated by the convection heater
is circulated through the convection passage provided on
the outer casing of the oven cavity by means of the convec-
tion fan which is driven by the convection motor. It then
enters the inside of the oven through the vent holes provided
on the left 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 left side wall. Without leaving the oven,
this hot air is reheated by the convection heater, passes
through the convection passage and enters the inside of the
oven cavity again, in a continuing cycle. 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 convection heater is
de-energized. When the temperature inside the oven cavity
drops below the selected temperature, the convection heater
is 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 convection heater
is de-energized and the convection fan stops operating and
the oven shuts off. Upon completion of the cooking time, the
audible signal will sound, and oven lamp, turntable motor,
cooling fan motor and convection motor are de-energized.
At the end of the convection cycle, if the cavity air tempera-
ture is above 116˚C, the circuit to RY6 will be maintained (by
the thermistor circuit) to continue operation of the cooling
fan motor until the temperature drops below 116˚C, at which
time the relay will be de-energized, turning off the fan motor.
Relay RY5 will however, open as soon as the convection
cycle has ended, turning off the convection fan motor. This
will now cool and allow the damper door to open.
DAMPER OPEN-CLOSE MECHANISM
Usually, the damper is in the open position except during
convection cooking.
Damper position is set automatically by damper motor,
damper switch, motor cam and damper shaft.
These components are operated by a signal that judges if
microwave cooking or convection cooking operation is
selected by the CPU unit.
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 blowing the fuse M8A.) .
1. The high voltage rectifier is shorted by any causes when
microwave cooking.
2. The peak reverse voltage of D2 of the rectifier goes
beyond the rated peak reverse voltage 1.7 KV in the
voltagedoubler circuit.
3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high voltage
winding of the power transformer.
5. The large electric currents beyond 8A flow through the
primary winding of the power transformer.
6. The fuse M8A blows by the large electric currents.
7. The power supply to the power transformer is cut off.
THERMISTOR
The thermistor is a negative temperature coefficient type.
The temperature in the oven cavity is detected through the
resistance of the thermistor, and then the control unit
causes the heating element relay to operate, thus the
current to the heating element is turned ON/OFF.
MONITOR RESISTOR
The monitor resistor prevents the fuse M8A 250V bursting
when the fuse M8A 250V blows due to the operation of the
monitor switch.
TURNTABLE MOTOR
The turntable motor drives the turntable supporting plate to
rotate the turntable.
CONVECTION MOTOR
The convection motor drives the convection fan and pro-
vides the heated air.
D2 D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE RECTIFIER
A B
C