R-757M (serv.man17). Complete - Sharp Home Appliance Service Manual (repair manual). Page 6

R-757M - 6

Closing the door activates all door interlock switches
(monitored latch switch and stop switch).

1. The display flashes "88:88".
2. To set the any programmes or set the clock, you must

first press the STOP/CLEAR key.

3. "  :  " appears in the display.

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

comes on at this time.

100P COOKING
Enter a desired cooking time by pressing TIME keys. And
choose the 100P microwave power by pressing MICRO-
WAVE POWER LEVEL key. And then start the oven by
pressing the START key.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

Grill heating element

RY2

High voltage transformer

RY3

1. The line voltage 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 wavelength 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 + RY3 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 door is opened during a cooking cycle, the

switches come to the following condition.

     Condition
During

Oven Door

 Switch

Contact

Cooking

Open(No cooking)

Monitored latch switch COM-NO

Closed

Opened

Stop switch

COM-NO

Closed

Opened

COM-NO

Closed

Opened

Monitor Switch

COM-NC

Opened

Closed

The circuit to the high voltage transformer, fan motor
and turntable motor are cut off when the monitored
latch switch and stop switch are made open. The oven
lamp remains on even if the oven door is opened after
the cooking cycle has been interrupted, because the

relay  RY1  stays closed. Shown in the display is
remaining time.

6. MONITOR SWITCH CIRCUIT

The monitor switch is mechanically controlled by the
oven door, and monitors the operation of the moni-
tored latch switch.

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

cycle of a cooking program, the monitored latch
switch and stop switch must open their contacts first.
After, the contacts (COM-NC) of the monitor switch
will close.

6-2. When the oven door is closed, the contacts (COM-

NC) of the monitor switch must be opened. After that
the contacts of the monitored latch switch and the
stop switch are closed.

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

monitored latch switch remain closed, the fuse F2
F8A will blow, because the monitor switch is closed
and a short circuit is made.

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the high voltage
transformer intermittently within a 32-second time base
through the relay contact which is coupled with the cur-
rent-limiting relay RY3. 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.

In this condition the food is cooked by the grill heater
energy. Programme the desired cooking time by pressing
TIME keys, and choose the grill mode by pressing GRILL
key once. When the  START key is pressed, the following
operations occur:

1. The relay RY1 and RY2 are energized.

2. The numbers on the digital readout start the count

down to zero.

3. Then oven lamp, cooling fan motor, turntable motor

and grill heating element are energized.

4. Now, the food is grilled by the grill heating element.
5. Upon completion of the selected cooking time, audible

signal sounds, and contacts of relays are opened, then
the activated components as items 3 are de-energized.

100P

32 sec. ON

70P

Approx. 70%=630 Watts

100%=900 Watts

24 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

50P

Approx. 50%=450 Watts

18 sec. ON

30P

Approx. 30%=270 Watts

12 sec. ON

10P

Approx. 10%=90 Watts

6 sec. ON