R-757M (serv.man6). Operation sequence and function of important components - Sharp Home Appliance Service Manual (repair manual)

R-757M - 6

Closing the door activates all door interlock switches
(monitored latch switch and stop switch).

1. The display flashes "88:88".
2. To set the any programmes or set the clock, you must

first press the STOP/CLEAR key.

3. "  :  " appears in the display.

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

comes on at this time.

100P COOKING
Enter a desired cooking time by pressing TIME keys. And
choose the 100P microwave power by pressing MICRO-
WAVE POWER LEVEL key. And then start the oven by
pressing the START key.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

Grill heating element

RY2

High voltage transformer

RY3

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 wavelength 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 + RY3 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 cooking cycle, the

switches come to the following condition.

     Condition
During

Oven Door

 Switch

Contact

Cooking

Open(No cooking)

Monitored latch switch COM-NO

Closed

Opened

Stop switch

COM-NO

Closed

Opened

COM-NO

Closed

Opened

Monitor Switch

COM-NC

Opened

Closed

The circuit to the high voltage transformer, fan motor
and turntable motor are cut off when the monitored
latch switch and stop switch are 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
remaining time.

6. MONITOR SWITCH CIRCUIT

The monitor switch is mechanically controlled by the
oven door, and monitors the operation of the moni-
tored latch switch.

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

cycle of a cooking program, the monitored latch
switch and stop switch must open their contacts first.
After, the contacts (COM-NC) of the monitor switch
will close.

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 monitored latch switch and the
stop switch are closed.

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

monitored latch switch remain closed, the fuse F2
F8A will blow, because the monitor switch is closed
and a short circuit is made.

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 RY3. The following levels of microwave
power are given.

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.

In this condition the food is cooked by the grill heater
energy. Programme the desired cooking time by pressing
TIME keys, and choose the grill mode by pressing GRILL
key once. When the  START key is pressed, the following
operations occur:

1. The relay RY1 and RY2 are energized.

2. The numbers on the digital readout start the count

down to zero.

3. Then oven lamp, cooling fan motor, turntable motor

and grill heating element are energized.

4. Now, the food is grilled by the grill heating element.
5. Upon completion of the selected cooking time, audible

signal sounds, and contacts of relays are opened, then
the activated components as items 3 are de-energized.

100P

32 sec. ON

70P

Approx. 70%=630 Watts

100%=900 Watts

24 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

50P

Approx. 50%=450 Watts

18 sec. ON

30P

Approx. 30%=270 Watts

12 sec. ON

10P

Approx. 10%=90 Watts

6 sec. ON

R-757M - 7

The door can be opened by pulling the door handle. When
the door handle is pulled, the latch head is moved upward,
and released from the latch hook, now the door can be
opened.

Basis cooking time

Reduced power output

Cooking mode

 (minutes)

(%)

Microwave (100%)

20

70

Grill

30

50

DUAL (Micro
10%-70%) + Grill

30

50 (Grill)

NOTE:
1. If the multiple sequence cooking is carried out in the

same mode, the basis cooking time is calculated from
the first.

2. Even if the cooking is stopped by the STOP/CLEAR

key or opening the door, the basis cooking time is
calculated from the first.

3. If the same cooking mode is repeated within 1 minute

and 15 seconds, the basis cooking time is calculated
from the first.

4. If the same manu of automatic cooking is repeated

within 1 minute and 15 seconds, the power output of
the microwave or the grill will be reduced to 70% after
20 minutes when the oven is started at first.

In this condition the food is cooked by both microwave
energy and grill heating element energy simultaneously.
Enter the desired cooking time by pressing TIME keys.
And choose the dual grill mode by pressing DUAL GRILL
key. And choose the microwave power level by pressing
DUAL GRILL or MICROWAVE POWER LEVEL key. When
the  START key is pressed, the following operations occur:

1. The relays RY1,RY2 and RY3 are energized.

2. The numbers on the digital read-out start the count

down to zero.

3. Then following components are energized.

High voltage transformer

Grill heating element

Magnetron

Turntable motor

H.V. capacitor

Fan motor

H.V. rectifier

Oven lamp

4. Now, the food is cooked by microwave and grill heating

element energy simultaneously.

5. Upon completion of the selected cooking time, audible

signal sounds, and contacts of relays are opened, then
the activated cpmponents as items 3 are de-energized.

6. When cooking in dual mode the microwave power is

limited to 70% (via the I.C.).

The cooking modes and cooking time are automatically
computed and selected based on the programmes.

In DUAL cooking, the magnetron operates within a 48
second time base. The following table is the ON / OFF time
ratio at each power output of the magnetron.

POWER OUTPUT

ON TIME

OFF TIME

100%

48 sec.

0 sec.

70%

36 sec.

12 sec.

50%

26 sec.

22 sec.

30%

16 sec.

32 sec.

10%

8 sec.

40 sec.

After the same cooking mode is carried out for more than
the basis cooking time, the power output is automatically
reduced by turning the control relays on and off intermit-
tently, as shown in the table below. This is to protect the
oven door against temperature rising.

1. When the oven door is closed, the contacts (COM-NO)

must be closed.

2. When the oven door is opened, the contacts (COM-

NO) must be opened.

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

must be opened and contacts (COM-NO) must be
closed.

2. When the oven door is opened, the contacts (COM-

NC) must closed and contacts (COM-NO) must  open.

3. If the oven door is opened and the contacts  (COM-NO)

of the monitored latch switch fail to open, the fuse (F8A)
blows simultaneously with closing the contacts (COM-
NC) of the monitor switch.

Latch Hooks

Stop Switch

Monitor 
Switch

Monitored
Latch 
Switch

R-757M - 8

CAUTION: BEFORE REPLACING A BLOWN FUSE  F8A,

TEST THE MONITORED LATCH SWITCH
AND  MONITOR SWITCH FOR PROPER
OPERATION.  (REFER TO CHAPTER “TEST
PROCEDURE”).

If the wire harness or electrical components are short-
circuited, this fuse blows to prevent an electric shock of
fire hazard.

This fuse also blows when monitored latch switch remains
closed with the oven door open and when the contact
(COM-NC) of monitor switch closes.
1. If the wire harness or electrical components are short-

circuited, this fuse blows to prevent an electric shock
or fire hazard.

2. The fuse also blows when the asymmetric rectifier,

H.V. rectifier,.H.V. wire harness, H.V. capacitor,
magnetron or secondary winding of high voltage trans-
former is shorted.

This thermal cut-out protects the magnetron against over-
heat. If the temperature goes up higher than 125˚C be-
cause the fan motor is interrupted or the ventilation open-
ings are blocked, the thermal cut-out will open and line
voltage to the high voltage transformer will cut off and
operation of the magnetron will be stopped. The defec-
tive thermal cut-out must be replaced with a new one.

The thermal cut-out located on the top of the oven cavity
is designed to prevent damaged to the oven if the food in
the oven catch fire due to over heating produced by
improper sitting of cooking time or  failure of control unit.
Under normal operation, the oven thermal cut-out remains
closed. However, when abnormally high temperature is
reached within the oven cavity, the oven thermal cut-out
will open at 145˚C, causing the oven to shut down. When
the thermal cut-out cools itself down to the operating
temperature of 115˚C, the contacts of the thermal cut-out
will close again.

The turntable motor rotates the turntable.

The fan motor drives a blade which draws external cool air.
This cool air is directed through the air vanes surrounding
the magnetron and cools the magnetron. This air is chan-
nelled through the oven cavity to remove steam and
vapours given off from heating food. It is then exhausted
through the exhausting air vents of the oven cavity.

The grill heating element is provided to brown the food and
is located on the top of the oven cavity.

The noise filter prevents radio frequency interference that
might flow back in to the power circuit.

The asymmetric rectifier is solid state device that prevents
current flow in both directions. And it prevents the tem-
perature rise of the high voltage transformer by blowing
the fuse F8A when the high voltage rectifier is shorted.

The rated peak reverse voltage of D1 of the asymmetric
rectifier is 6 KV The rated peak reverse voltage of D2 of the
asymmetric rectifier is 1.7 KV. D1 and D2 of the asymmet-
ric rectifier or high voltage rectifier are shorted when the
each peak reverse voltage goes beyond the each rated
peak reverse voltage. (The process of the blowing the fuse
F8A.)
1. The high voltage rectifier is shorted by some fault when

microwave cooking or dual cooking.

2. The peak reverse voltage of D2 of the rectifier goes

beyond the rated peak reverse voltage 1.7 KV in the
voltage doubler circuit.

3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-

age winding of the high voltage transformer.

5. The large electric currents beyond 8A flow through the

primary winding of the high voltage transformer.

6. The fuse F8A blows by the large electric currents.
7. The power supplying to the high voltage transformer is

cut off.