Sharp R-9H58, Service Manual

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R-9H58
8
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 temperature rise of the power transformer by blowing
the fuse M8A when the high voltage rectifier is shorted.
rounding the magnetron and cools the magnetron. This air
is channelled 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.
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 pro-
vided on the outer casing of the oven cavity by means of
the convection 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 en-
ters 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 main-
tained 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 temperature 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 how-
ever, 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.
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.
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 blow-
ing 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 burst-
ing 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
provides the heated air.
FAN MOTOR
The fan motor drives a blade which draws external cool
air. This cool air is directed through the air vanes sur-
D2 D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE RECTIFIER
A B
C