DOWNLOAD Sharp R-734 (serv.man4) Service Manual ↓ Size: 609.24 KB | Pages: 44 in PDF or view online for FREE

R-734 - 9

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 button.

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 with the ROTARY knob.
Then choose the microwave mode by pressing COOKING
MODE button once. And choose the 100P microwave
power with the ROTARY knob. And then start the oven by
pressing the START button.

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 SW2 is mechanically controlled
by the oven door, and monitors the operation of the
monitored latch switch SW1.

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 first. After  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. After
that the contacts of the monitored latch switch SW1
and the stop switch SW3 are closed.

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

monitored latch switch SW1 remain closed, the fuse
F2 F8A will blow, because the monitor switch SW2 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
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 with the
ROTARY knob, and choose the grill mode by pressing
COOKING MODE button twice. When the  START button
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-734 - 10

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 button 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 with the ROTARY knob.
And choose the dual grill mode by pressing COOKING
MODE button three times. Then choose the microwave
power with  ROTARY knob. When the  START button 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, magnetron operates whithin 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 be close and contacts (COM-NO) must be
open.

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

of the monitored latch switch SW1 fail to open, the fuse
F2 (F8A) blows simultaneously with closing the con-
tacts (COM-NC) of the monitor switch SW2.

Latch Hooks

Stop Switch
(SW3)

Monitor 
Switch
(SW2)
Monitored
Latch 
Switch
(SW1)

R-734 - 11

CAUTION: BEFORE REPLACING A BLOWN FUSE  F2

F8A, TEST THE MONITORED LATCH
SWITCH  SW1 AND  MONITOR SWITCH
SW2  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 or
fire hazard.

This fuse also blows when monitored latch switch SW1
remains closed with the oven door open and when the
contact (COM-NC) of monitor switch SW2  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 TC1 will open and
line voltage to the high voltage transformer T will cut off
and operation of the magnetron MG will be stopped.
The defective 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 catches fire due to over heating produced by
improper stting 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 noise filter prevents radio frequency interference that
might flow back in the power circuit.

The grill heating element is located on the top of the oven
cavity assembly. The grill heating element sends out heat
to grill foods.

The asymmetric rectifier is solid state device that prevents
current flow is both directions. And it prevents the tem-
perature rise of the high voltage transformer by blowing
the fuse F2 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
F2 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 F2 F8A blows by the large electric currents.
7. The power supplying to the high voltage transformer is

cut off.

D2

D1

ASYMMETRIC
RECTIFIER

HIGH VOLTAGE 
RECTIFIER

R-734 - 12

When troubleshooting the microwave oven, it is helpful to follow the Sequence of Operation in performing the checks.
Many of the possible causes of trouble will require that a specific test be performed. These tests are given a procedure
letter which will be found in the “Test Procedure” section.

IMPORTANT: If the oven becomes inoperative because of a blown fuse F2 (F8A) in the monitored latch switch - monitor

switch circuit, check the monitored latch switch and monitor switch before replacing the fuse F2 (F8A).

Special fuse F1 20A blows when power 
cord is plugged into wall outlet.

Fuse F2 F8A blows when the door is 
opened.

Oven lamp does not light when door is 
opened. (Display operates.)

Home fuse blows when power cord is 
plugged into wall outlet.

Nothing appears in display when power 
cord is plugged into wall outlet.

Display does not operate properly when 
STOP button is pressed.

Oven does not start when the START 
button is pressed. (Display operates.)

Oven lamp does not light. (Turntable 
motor operates.)

Fan motor does not operate. (Oven lamp 
lights.)

Turntable motor does not operate. (Oven 
lamp lights.)

Oven or electrical parts do not stop 
when cooking time is 0 or STOP  button 
is pressed.

Oven goes into cook cycle but shuts 
down before end of cooking cycle.

Oven stops as soon as the START 
button is pressed. (Except for Grill 
Cooking)

Oven seems to be operating but little or 
no heat is produced in oven load. 
(Microwave power control is set at 100P)

Oven does not seem to be operating 
properly during variable cooking 
condition except 100P cooking condition.

Oven goes into cook cycle but shuts 
down before end of cooking cycle.

Grill heating element does operate.

Oven seems to be operating but little or 
no heat is produced in oven load. 
(Microwave power does not seem to be 
generated properly)

Grill heating element does not operate.

OFF

CONDITION

COOKING

CONDITION

(COMMON)

MICROWAVE

COOKING

CONDITION

GRILL 

COOKING

CONDITION

DUAL 

COOKING

CONDITION

CONDITION

PROBLEM

MAGNETRON

HIGH VOLTAGE  TRANSFORMER

H.V. RECTIFIER ASSEMBLY

HIGH VOLTAGE CAPACITOR

MONITORED LATCH SWITCH

MONITOR SWITCH 

STOP SWITCH 

THERMAL CUT-OUT (MAG.) 

THERMAL CUT-OUT (OVEN)

SPECIAL FUSE 20A F1

FUSE F8A F2

NOISE FILTER

TURNTABLE MOTOR

FAN MOTOR

GRILL HEATING ELEMENT

POWER SUPPLY CORD

OVEN LAMP 

SHORTED WIRE HARNESS

OPENED WIRE HARNESS

MIS-ADJUSTMENT SWITCHES

SWITCH UNIT

CONTROL PANEL

RELAY RY1

RELAY RY3

RELAY RY2

FOIL PATTERN ON P.W.B.

A B C D E E E F F G H I

J J K

L

M

N

N

N

O

TEST PROCEDURE

POSSIBLE CAUSE

AND

DEFECTIVE PARTS