View Sharp R-8740 (serv.man6) Service Manual online
control panel. When the open button is pushed, the open lever
pushes lower latch head on the door upward. The upper latch
head is linked with the lower latch head, so now, the door can
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 F2 F8A when the contacts of the moni-
tored latch switch SW1 fail to open when the door is opened.
this fuse F1 20A 250V blows to prevent an electric shock or fire
monitor switch SW2 contact (COM-NC) closes.
secondary winding of high voltage transformer is shorted
drive the control unit.
grill cooking, convection cooking or dual (combination) cook-
ing. If the temperature rises above 150˚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 the all electrical parts.
When the oven cools itself down to the operating temperature
of 130˚C, the contacts of the thermal cut-out will close again.
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 TC2 will open and line voltage to the high
voltage transformer T will cut off and operation of the magnetron
MG will be stopped. The defective thermal cut-out must be
replaced with a new one.
the following operations occur:
1. The numbers on the digital readout start the count down to
mode). the oven chooses cooking mode (Microwave, Convec-
tion, Grill or Dual) and sets cooking time automatically accord-
ing to the pre-programmed information in IC-1 (LSI).
1. When the door is opened, the contacts (COM-NC) of
being normally closed and contacts (COM-NO) open. At
this time the monitored latch SW1 is in the OFF condition
(contacts open) due to their being normally open contact
NO) closed and then the monitored latch switch and stop
switch contacts close.(On opening the door, each of these
switches operate inversely.)
after closing of the monitor switch (COM-NC) contacts.
MONITOR SWITCH FOR PROPER OPERA-
(REFER TO CHAPTER “TEST PROCEDURE”).
is heated by forced circulation of the hot air produced by the
The air heated by the grill heaters 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 circulated inside 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 elements
are 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 heating elements
are de-energized and the convection fan stops operating and
the oven shuts off. At the high temperature (more than 104˚C),
the fan motor remains rotating. Automatically the fan motor will
be shut down at low temperature (less than 104˚C).
7. The power supplying to the high voltage transformer is cut
ence that might flow back in the power circuit.
This cool air is directed through the air vanes surrounding the
magnetron and cools the magnetron. This air is channeled
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.
the heated air
located on the top of the oven cavity.
located at the base of the oven cavity
current flow is both directions. And it prevents the tempera-
ture rise of the power transformer by blowing the fuse F2 F8A
when the high voltage rectifier is shorted.
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 F2
1. The high voltage rectifier is shorted by any causes when
voltage doubler circuit.
4. The large electric currents flow through the high voltage
oven cavity in microwave cooking condition.
LSI measures the voltage across the temperature measure-
ment circuit intermittently within a 32-second time base after
since the oven is started in microwave cooking condition.
The oven will stop its operation when the difference of the
voltages is more than 0.69 volts in microwave cooking
energized, the voltage across the temperature
measurement circuit is measured. And after 21 seconds
since the thermistor is cut off, the convection motor
operates for 10 seconds.
judges it is a fire in the oven cavity and stops the oven.
Because food cannot be cooked well by rotating the
convection fan at that time. After sensing the steam, the
sensing fire operation is started.
convection motor and LSI stops counting down. And then
the damper is closed so that the fresh air does not come
into the oven cavity.