R-21FBSTM - Sharp Home Appliance Service Manual (repair manual). Page 6

R-21FBSTM/R-22FBSTM - 6

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.

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.

6-2

When the oven door is closed, the contacts
(COM - NC) of the monitor switch (SW2) must be
opened first. The contacts (COM - NO) of the monitored
latch switch (SW1) and stop switch (SW3) are closed
after.

6-3

When the oven door is opened and the contacts of the
monitored latch switch (SW1) remain closed, the fuse
F1 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 vari
contact. The following levels of microwave power are given.
SETTING

Closing the door activates the door interlock switch (moni-
tored latch 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 (230-
240V 50Hz), the noise filter is energised.

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

comes on at this time.

Enter a desired cooking time  by rotating the time/weight
knob and start the oven by touching START pad.

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 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 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 + RY3 + RY4 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.

CONDITION

DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Monitor switch

COM-NC

Open

Closed

COM-NO

Closed

Open

Monitored 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 monitored latch
switch 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 stays closed.  Shown
in the display is the remaining time.

6. MONITOR SWITCH CIRCUIT

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

Oven lamp, Turntable motor

RY1

Power transformer

RY3

Fan motor

RY4

32 sec. ON

Approx. 70% = 560 Watts

100% = 800 Watts

HIGH

MEDIUM HIGH

MEDIUM

MEDIUM LOW

LOW

24 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

Approx. 50% = 400 Watts

18 sec. ON

Approx. 30% = 240 Watts

12 sec. ON

Approx. 10% = 80 Watts

6 sec. ON