R-4G75M (serv.man7). Complete - Sharp Home Appliance Service Manual (repair manual). Page 6

4

Closing the door activates all door interlock switches (pri-
mary latch switch, 2nd latch switch and stop switch).

IMPORTANT
When the oven door is closed, the monitor switch contacts
COM - NC must be open.
When the microwave oven is pluged in a wall outlet (240V
50Hz), the line voltage is supplied to the point A3+A5 in the
control unit.

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

touch the STOP/CLEAR pad.

3. “88:88” appears in the display and the time counts up

every minute.

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

comes on at this time.

Enter a desired cooking time with the touching Time pad and
start the oven with touching AUTO MINUTE/START pad.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

Power transformer

RY2

1. The line voltage is supplied to the primary winding of the

power 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 voltagedoubling 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 channeled 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 single tone is heard and

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

5. When the door is opened during a cook cycle, the switches

come to the following condition.

opened after the cooking cycle has been interrupted,
because the relay RY1 stays closed. Shown in the
display is the remaining time.

6. MONITOR SWITCH CIRCUIT

The monitor switch is mechanically controlled by oven
door, and monitors the operation of the primary switch.

6-1

When the oven door is open during or after the cycle of

cooking a program, the primary latch and stop switches
must open their contacts first. After that the contacts
(COM - NC) of the monitor switch can be closed and
then contacts of the 2nd latch switch can be opened.

6-2

When the oven door is closed, the contacts

(COM - NC) of the monitor switch must be opened and
contacts (COM - NO) of the 2nd latch switch must be
closed first. After that contacts of the primary latch and
stop switches are closed.

6-3

When the oven door is opened and the contacts of the

primary latch switch remain closed, the fuse M8A will
blow, because the monitor switch is closed and a short
circuit is caused.

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the power 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.

SETTING

CONDITION

DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Primary latch switch

COM-NO

Closed

Open

Monitor switch

COM-NC

Open

Closed

2nd latch switch

COM-NO

Closed

Open

Stop switch

COM-NO

Closed

Open

The circuits to the power transformer, fan motor and
turntable motor are cut off when the primary latch switch,
2nd latch switch and stop switch are made open.
The oven lamp remains on even if the oven door is

32 sec. ON

24 sec. ON

HIGH

LOW

Approx. 100%

Approx. 70%

Approx. 50%

Approx. 30%

Approx. 10%

24 sec. ON

8 sec. OFF

18 sec. ON

12 sec. ON

6 sec. ON

14 sec. OFF

20 sec. OFF

26 sec. OFF

MEDIUM

MEDIUM HIGH

MEDIUM LOW

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 grill heater energy.
Program desired cooking time and grill mode by touching
TIME pads and GRILL pad. When the START pad is touched,
the following operations occur:
1. The relay RY1 and RY3 are energized.
2. The numbers of the digital readout start the count down

to zero.

3. Then the grill heating elements, turntable motor, oven

lamp and fan motor are energized.

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

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