R-8740 (serv.man17). Complete - Sharp Home Appliance Service Manual (repair manual). Page 9

R-8740M  - 9

The door can be opened by pushing the open button on the
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
be opened.

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

each switch must be closed.

2. When the oven door is opened, the contacts COM-NO of

each 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 F2 F8A when the contacts of the moni-
tored latch switch SW1 fail to open when the door is opened.

LATCH
HEADS

LATCH HOOK

MONITORED LATCH
SWITCH

STOP SWITCH

OPEN LEVER

DOOR OPEN BUTTON

MONITOR 
SWITCH

If the wire harness or electrical components are short-circuited,
this fuse F1 20A 250V blows to prevent an electric shock or fire
hazard.

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 monitored latch switch SW1

remains closed with the oven door open and when the
monitor switch SW2  contact (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

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

The thermal cut out protects the oven against overheat during
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.

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 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.

Program the desired cooking time and then touch the
"BOTTOM GRILL"   

   and MICROWAVE keys (led's will

light on the display panel). When the START key is touched,
the following operations occur:
1. The numbers on the digital readout start the count down to

zero.

2. The oven lamp, cooling fan motor and turntable motor are

energized.

3. Relay RY4 is energized and the main supply voltage is

applied to the bottom heater.

4. Relay RY2 is energized and the microwave energy is

generated by the magnetron.

5. Now the food is cooked by microwave and bottom grill

simultaneously.

When the number keys are pressed (not in "MANUAL" 
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).

Function
1. When the door is opened, the contacts (COM-NC) of

monitor switch SW2 close (to the ON condition) due to their
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
switches.

2. As the door goes to a closed position, the monitor switch

SW2 contacts (COM-NC) are opened and contacts (COM-
NO) closed  and then the monitored latch switch and stop
switch contacts close.(On opening the door, each of these
switches operate inversely.)

3. If the door is opened and the monitored latch switch  SW1

contacts fail to open, the fuse F2 (F8A) blows immediately
after closing of the monitor switch (COM-NC) contacts.

CAUTION: BEFORE REPLACING A BLOWN FUSE F2 F8A

TEST THE MONITORED LATCH  SWITCH,
MONITOR SWITCH  FOR PROPER OPERA-
TION.
(REFER TO CHAPTER “TEST PROCEDURE”).