R-2498G (serv.man2). Part 1 Text(1-25) - Sharp Home Appliance Service Manual (repair manual). Page 6

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remaining time, but the program is cancelled if the oven
is not started within 3 minutes.

6. MONITOR SWITCH CIRCUIT

The monitor switch is mechanically controlled by oven
door, and monitors the operation of the 1st latch switch.

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

of a cooking program, the 1st, 2nd, 3rd latch and stop
switches 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 first. After that
the contacts (COM-NO) of the 1st, 2nd, 3rd latch and
stop switches must be closed.

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

1st latch switch remain closed, remains closed, the
fuse F6.3A 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 high voltage
transformers intermittently within a 48 second time base
through the contacts of the relays RY3+RY4.
The following levels of microwaves power are given.

48 sec. ON

100%

26 sec. ON

50%

12 sec. ON

20%

8 sec. ON

10%

0 sec. ON

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

spond to the percentage of microwave power, be-
cause approx. 3 seconds are needed for heating up
the magnetron filament.

Two magnetrons MG1+MG2 are equipped in order to get
higher microwave power output. The primary windings of the
high voltage transformers T1+T2 are connected so that each
magnetron can be oscillated alternatively according to the
frequency of the power supply. Refer to the Figure 1 and 2.

Closing the door activates all door interlock switches
(1st latch switch, 2nd latch switch, 3rd 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
plugged in a wall outlet (230 - 240  volts, 50Hz), the line
voltage is supplied to the point A5+A7 in the control panel.

1. The digital display shows      .   

0

NOTE: When the door is opened or after cooking, oven

lamp, blower motor and stirrer motors work for 1
minute.

Enter a desired cooking time with the turning TIMER CON-
TROL knob. And then push STARTbutton.

CONNECTED COMPONENTS

RELAY

Oven lamp/ Blower motor/ Stirrer motors

RY1

Power transformer  (front)

RY3

Power transformer (rear)

RY4

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 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 RY3+RY4 go back to their home position. The
circuits to the high voltage transformers. The relay RY1
remains and oven lamp, blower motor and stirrer motors
work for 1 minute.

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

Monitor switch

COM-NC

Open

  Closed

2nd latch switch

COM-NO

Closed

  Open

Stop switch

COM-NO

Closed

  Open

3rd latch switch

COM-NO

Closed

  Open

The circuits to the high voltage transformers are cut off
when the 1st latch, 2nd latch, 3rd latch and stop switches
are made open. The blower motor, stirrer motors and
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 the

T1: HIGH VOLTAGE
      TRANSFORMER

ASYMMETRIC

RECTIFIER

 H. V. RECTIFIER

C1: H.V. CAPACITOR

MG1: MAGNETRON (UPPER)

(REAR)

ASYMMETRIC

RECTIFIER

 H. V. RECTIFIER

C2: H.V. CAPACITOR

MG2: MAGNETRON (LOWER)

T2: HIGH VOLTAGE
      TRANSFORMER

(FRONT)