R-249 - Sharp Home Appliance Service Manual (repair manual). Page 6

4

Closing the door activates all door interlock switches
(1st. latch switch, 2nd. interlock relay control switch).

IMPORTANT
When the oven door is closed, the monitor switch contacts
(COM-NC) must be open.
When the microwave oven is plugged in a wall outlet, rated
voltage is supplied to the noise filter and 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 key.

3. "  :  " appears in the display.
NOTE: When the oven door is opened, the oven lamp

comes on at this time.

HIGH COOKING
Enter a desired cooking time with touching the TIME keys
and start the oven with touching START key.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

High voltage transformer

RY2

1. Rated 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 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 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 cook cycle, the

switches come to the following condition.

          CONDITION
DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

1st. latch switch

COM-NO

Closed

  Open

2nd. interlock relay
control switch

COM-NO

Closed

  Open

Monitor switch

COM-NC

Open

  Closed

The circuits to the high voltage transformer, fan motor
turntable motor and oven lamp are cut off when the 1st.
latch switch and 2nd. interlock relay control switch are
made open.

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 1st. latch switch
and 2nd. interlock relay.

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

cycle of a cooking program, the 1st. latch switch and
2nd. interlock relay control switch must open their
contacts first. After that the contacts (COM-NC) of the
monitor switch can be closed.

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 1st. latch switch and 2nd. interlock
relay control switch are closed.

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

the 1st latch switch and 2nd. interlock relay remain
closed. The fuse T6.3A will blow, because the moni-
tor switch is closed and a short circuit is caused.

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

SETTING;

NOTE: The ON/OFF time ratio does not exactly corre-

spond to the percentage of microwave power,
because approx. 3 seconds are needed for heat-
ing up the magnetron filament.

If the oven set for more than 20 minutes at 100% power
level, after the first 20 minutes the power level will automati-
cally adjust itself to 70% power to avoid overcooking.

100P

32 sec. ON

70P

Approx. 70%

100%

24 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

50P

Approx. 50%

18 sec. ON

30P

Approx. 30%

12 sec. ON

10P

Approx. 10%

6 sec. ON