R-8740 (serv.man17). Complete - Sharp Home Appliance Service Manual (repair manual). Page 7

R-8740M  - 7

Closing the oven door activates all door interlock switches
(Monitored latch switch and stop switch.

IMPORTANT:
When the oven door is closed, the contacts COM-NC of the
monitor switch SW2 must be open and the contacts (COM-
NO) must be closed. When the microwave oven is plugged
in a wall outlet (230V / 50Hz), 230 volts A.C. is supplied to
the point A9 + A11 in the control unit.

1. The display flashes ".0"
2. To set any programmes or set the clock, you must first

"put the oven in MANUAL MODE .  Press the MANUAL

key for 6-10 seconds. (oven will 'beep').  Press the

MANUAL key once more.  The oven is now in manual
mode (this will be indicated in the display window, above
the         symbol).

NOTE: When the oven door is opened, the oven lamp

comes on at this time.

HIGH COOKING
Enter a desired cooking time with the number keys, press
the " MICROWAVE " 

  key, then press the START key 

.

CONNECTED COMPONENTS

RELAY

Oven lamp, Turntable motor

RY1

High voltage transformer

RY2

Fan motor

RY5

1. 230 volts A.C. is supplied to the primary winding of the

high voltage transformer. The voltage is converted to
about 3.3 volts A.C. output on the filament winding and
high voltage of approximately 2000 volts A.C. on the
secondary winding.

2. The filament winding voltage (3.3 volts) heats the

magnetron filament and the high voltage (2000 volts) is
sent to the voltage doubling circuit, where it is doubled to
negative voltage of approximately 4000 volts D.C..

3. The 2450 MHz microwave energy produced in the

magnetron generates a wave length of 12.24 cm. This
energy is channelled through the waveguide (transport
channel) into the oven cavity, where the food is placed to
be cooked.

4. When the cooking time is up, a signal tone is heard and

the relays RY1 + RY2 + RY5 go back to their home
position. The circuits to the oven lamp, high voltage
transformer, fan motor and turntable motor are cut off.

5. When the oven door is opened during a cooking cycle,

the switches come to the following condition.

Switch

Contact

          Condition
During

Oven Door

Cooking

Open(No cooking)

Monitored latch Switch COM-NO

Closed

Opened

Monitor Switch

COM-NO

Closed

Opened

Monitor Switch

COM-NC

Opened

Closed

Stop switch

COM-NO

Closed

Opened

The circuit to the high voltage transformer, is cut off when
the contact of relay RY2 contacts COM-NO of the monitored
latch switch, and monitor switch SW2 are made open. The

circuit to the fan motor is cut off when the relay RY5 is made
open. The circuit to the turntable motor is cut off when the
contacts COM-NO of the monitored latch switch are made
open. The relays RY2 and RY5 are made open when the
door is opened. The oven lamp remains on even if the oven
door is opened after the cooking cycle has been interrupted,
because the relay RY1  stay closed. Shown in the display is
remaining time.
6. MONITOR SWITCH CIRCUIT

The monitor switch SW2 is mechanically controlled by
the oven door, and monitors the operation of the
monitored latch switch SW1.

6-1. When the oven door is opened during or after the cycle

of a cooking program, the monitored latch switch SW1
and stop switch SW3 must open their contacts (COM-
NO) first. After that the contacts (COM-NC) of the
monitor switch SW2 can be closed and the contacts
(COM-NO) of monitor switch SW2 are made open.

6-2. When the oven door is closed. The contacts (COM-NC)

of the monitor switch SW2 must be opened and the
contacts (COM-NO) of monitor switch SW2 must be
closed.  After that the contacts of the monitored latch
switch SW1 and the stop switch SW3 are made closed.

6-3. When the oven door is opened and the contacts of the

monitored latch switch SW1 remain closed, the fuse F2
F8A will blow. Because the relay RY1 and monitor
switch SW2 are closed  and a short circuit is caused.

Program desired convection temperature by touching
"MANUAL", then the CONVECTION key. When the START key
is touched, the following operations occur:
1. The coil of shut-off relays RY1, RY5 and RY7 are energized,

the oven lamp, cooling fan motor, turntable motor and
convection motor are turned on.

2. The coil of shut-off relays RY3  and RY4 are energized by

the control unit and the main supply voltage is added to the

When the microwave oven is preset for variable cooking
power, the 230 volts A.C. power is supplied to the high voltage
transformer intermittently within a 32-second time base through
the relay contact which is coupled with the current-limiting relay
RY2. The following levels of microwave power are given.

Note: The On/Off time ratio does not exactly correspond to the

percentage of microwave power, because approx. 3
seconds  are needed for heating up the magnetron
filament.

32 sec. ON

24 sec. ON

18 sec. ON

12 sec. ON

6 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

HIGH

MEDIUM HIGH

MEDIUM

MEDIUM LOW

LOW

Approx. 70% = 630 Watts

100%

Approx. 50% = 450 Watts

Approx. 30% = 270 Watts

Approx. 10% = 90 Watts

 = 900 Watts