Sharp R-7H55 Service Manual Download Page 6

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R-7H55

OPERATING SEQUENCE

OFF CONDITION

Closing the door activates all door interlock switches
(1st 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 plugged in a wall outlet, rated
voltage is supplied to the point A1+A3 in the control unit.

Figure O-1 on page 33

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 and the time counts up every

minute.

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

comes on at this time.

MICROWAVE COOKING CONDITION

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

Function sequence Figure O-2 on page 33

CONNECTED COMPONENTS

RELAY

Oven lamp, Turntable motor (Power transformer)

RY1

Power transformer

RY2

Fan motor

RY4

1. Rated 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 second-
ary winding.

2. The filament winding voltage (3.3 volts) heats the mag-

netron 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 mag-

netron 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+RY4) go back to their home
position. The circuits to the oven lamp, power trans-
former, 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

Oven Door

Switch

Contact

Cooking

Open(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

The circuits to the power transformer and turntable
motor are cut off when the 1st latch switch and 2nd latch
switch are made open. The fan motor is cut off when the
stop switch is 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 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 latch switch.

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

of a cooking program, the contacts (COM-NC) of the
monitor switch must be closed and the contacts (COM-
NO) of the 1st latch switch, 2nd latch switch and stop
switch must be opened at the same time.

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

of the monitor switch must be opened and the contacts
(COM-NO) of the 1st latch switch, 2nd latch switch and
stop switch must be closed at the same time.

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

NO) of the 1st latch switch and the 2nd latch switch
remain closed and relay RY2 closes, the fuse M8A will
blow, because the contacts (COM-NC) of the monitor
switch and relay RY1 are closed and a short circuit is
caused.

MEDIUM HIGH, MEDIUM, MEDIUM LOW, LOW COOKING

When the microwave oven is preset for variable cooking
power, rated 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

NOTE: The ON/OFF time ratio does not exactly correspond

to the percentage of microwave power, because
approx. 2 seconds are needed for heating up the
magnetron filament.

CONVECTION COOKING CONDITION

PREHEATING CONDITION
Program desired convection temperature by touching the
CONVECTION pad and the temp. pad. When the START
pad is touched, the following operations occur:
1. The coil of shut-off relays RY1+RY5+RY4 are ener-

gized, the oven lamp, cooling fan motor, turntable motor
and convection motor are turned on.

32 sec. ON

24 sec. ON

18 sec. ON

12 sec. ON

6 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

HIGH

MEDIUM HIGH