Sharp R-870B Service Manual Download Page 6

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R-870B

OPERATION SEQUENCE

OFF CONDITION

Closing the door activates the 1st. latch switch and 2nd.
interlock relay control switch.

IMPORTANT:
When the oven door is closed, the contacts COM-NC
of the monitor switch must be open. When the mi-
crowave oven is plugged in a wall outlet (220V 50Hz),
the line voltage is supplied to the noise filter and the
control unit.

Figure O-1 on page 34

1. The oven display will show " SHARP MICRO- WAVE

OVEN " .

2. Press the STOP/CLEAR pad. The oven display will

show " : ".

3. Set the clock as follows.

3-1. Press the CLOCK pad once.
3-2. Enter the time of day by perssing the number

pads.

3-3. Start the clock by pressing the CLOCK pad.

NOTE:
1. If you do not set the clock, " : " will appear on the

display. When the operation of the oven is finished, " :
" will appear on the display instead of the time of day.

2. The oven can be also used when the clock is not set.
3. When the oven door is opened, the oven lamp does not

come on.

MICROWAVE COOKING CONDITION

100% (HIGH) COOKING
Enter a desired cooking time by pressing the number pads
and start the oven by pressing START pad.

Function sequence Figure O-2 on page 34

CONNECTED COMPONENTS

RELAY

Convection motor

RY1

High voltage transformer

RY2

Grill heater (Top)

RY3

Bottom heater

RY4

Fan motor,

RY5

Oven lamp, Turntable motor

RY6

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 RY2 + RY5 + RY6 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 oven door is opened during a cooking cycle,

the switches come to the following condition.

Switch

Contact

Condition
During

Oven Door

Cooking

Open(No cooking)

1st. latch switch

COM-NO

Closed

Opened

2nd. interlock relay
control switch

COM-NO

Closed

Opened

Monitor Switch

COM-NC

Opened

Closed

The circuit to the high voltage transformer, fan motor, oven
lamp and turntable motor are cut off when the 1st. latch
switch and 2nd. interlock relay control switch are made
open. Shown in the display is remaining time.
6. MONITOR SWITCH CIRCUIT

The monitor switch is mechanically controlled by the
oven door, and monitors the operation of the 1st. latch
switch and the 2nd. interlock relay RY2.

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 the 2nd. interlock relay RY2
remain closed, the fuse M10A will blow, because the
monitor switch is closed and a short circuit is caused.

70% (MEDIUM HIGH), 50% (MEDIUM), 30% (ME-
DIUM LOW), 10% (LOW COOKING)

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the high voltage
transformer intermittently within a 32-second time base
through the relay contact which is coupled with the cur-
rent-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
Select preheating temperature by pressing the PREHEAT
pad. When the START pad is touched, the following
operations occur:

Figure O-3 on page 35

1. The coils of shut-off relays RY1+RY5+RY6 are ener-

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

2. The coil of heater relays RY3+RY4 are energized by

the CPU unit and the main supply voltage is added to

100%

32 sec. ON

Approx. 100%

70%

24 sec. ON