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5

771M -

DOOR

LATCH HOOK

MONITOR SWITCH

MONITORED
LATCH SWITCH

STOP SWITCH

OPEN LEVER

LATCH
HEADS

This model incorporates a sensing feature which will stop
the oven's operation if there is a fire in the oven cavity
during microwave cooking.
This accomplished by the LSI repeatedly measures the
voltage across the temperature measurement circuit (ther-
mistor) during it's 32-seconds time base comparing the
obtained voltage measurements. If the most recent  volt-
age measured is 300mV grater than the previous voltage
measured, the LSI judges it as a fire in the oven cavity and
switches off the relays to the high voltage transformer and
fan motor. The LSI also stops counting down. Please refer
to the following section for a more detailed description.

Please refer to the timing diagrams below.

1. The thermistor operates within a 32-seconds time base

and it is energized for three (3) seconds and off for 29
seconds.  Two (2) seconds after the thermistor is
energized, the voltage across the temperature meas-
urement circuit is sampled by the LSI.

2. The above procedure is repeated. If the difference

between the first voltage measured (in step 1) and the
voltage measured when the procedure is repeated
(step 2) is greater than 300mV the LSI makes the
judgment that there is a fire in the oven cavity and will
switch off the relays to the high voltage transformer and
fan motor. The LSI also stops counting down.

CAUTION:

BEFORE REPLACING A BLOWN FUSE
F8A TEST THE MONITORED LATCH
SWITCH,  MONITOR SWITCH AND
MONITOR RESISTOR FOR PROPER
OPERATION.

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 proce-
dure letter which will be found in the “Test Procedure”
section.

IMPORTANT: If the oven becomes inoperative because

of a blown fuse F8A in the primary latch
switch - monitor switch circuit, check the
primary latch switch, monitor switch and
monitor resistor before replacing the fuse
F8A.

3. Once the fire sensor feature has shut the unit down, the

programmed cooking cycle may be resumed by  press-
ing the "START" pad or the unit may be reset by
pressing the "CLEAR" pad.

THERMISTOR

SENSING

VOLTAGE

ON

OFF

ON

OFF

0

29

64 (sec.)

3 sec.

Sensing the voltage across temperature mesure circuit.

1. If the temperature of the thermistor does not rise  to

more than 40˚C after 4 minutes and 15 seconds from
when the oven is started in convection, grill or dual
cooking (Microwave and Convection) mode, the oven
is turned off.

2. When the thermistor or the wire harness to the thermis-

tor is opened, the oven is turned off after 4 minutes and
15 seconds because this condition is same as above.

MONITORED

6

771M -

The power output of this oven is rated using the method specified by IEC-705.  Full details of how to carry out this
procedure can be found in the Sharp Technical Training notes which is available from Sharp Parts Centre
(part number SERV-LITMW01).

The IEC-705 procedure must be carried out using laboratory-type procedures and equipment.  These require-
ments  make the procedure unsuitable for routine performance checks.

Note: The following test method gives an indication of the output power only, it cannot be used to establish the
actual/ rated output power.  If the true output power is required, then the IEC-705 test method must be used.

Alternative simplified method:
1. Place 2 litres of cold water (between 12

°

C and 20

°

C) in a suitable container.

2. Stir the water and measure the temperature in 

°

C.  Note temperature as T1.

3. Place the container in the microwave and heat the water for 2 minutes on full power.
4. When the 2 minutes is completed, remove the container and stir the water.  Note the water temperature as T2.
5. Calculate the output power using the following formula:

R.F. Power Output = (T2 - T1) x 80.

This oven should be tested for microwave leakage on completion of any repair or adjustment, following the
procedure described in the Sharp Technical Training notes (part number SERV-LITMW01).  The maximum
leakage permitted in BS EN 60335-2-25 is 50W/m

2 

(equivalent to 5mW/cm

2

), however it is not normal to detect

any significant leakage, therefore, any leakage which is detected should be investigated.

It is essential that only leakage detectors with current calibration traceable to the National Physical Laboratories

are used.

Suitable leakage detectors :

CELTEC A100
APOLLO X1

7

771M -

The touch control panel consists of circuits including semiconductors such as LSI, ICs, etc. Therefore,
unlike conventional microwave ovens, proper maintenance can not be performed with only a
voltmeter and ohmmeter.
In this service manual, the touch control panel assembly is divided into two units, Control Unit and Key
Unit, and troubleshooting by replacement is described according to the symptoms indicated.
1.  Key Unit   Note : Check key unit ribbon connection before replacement.

The following symptoms indicate a defective key unit. Replace the key unit.

a) When touching the pads, a certain pad produces no signal at all.
b) When touching a number pad, two figures or more are displayed.
c) When touching the pads, sometimes a pad produces no signal.

2. Control Panel

The following symptoms indicate a defective control unit. Before replacing the control unit.
perform the key unit test (Procedure N) to determine if control unit is faulty.

2-1  In connection with pads

a) When touching the pads, a certain group of pads do not produce a signal.
b) When touching the pads, no pads produce a signal.

2-2  In connection with indicators

a) At a certain digit, all or some segments do not light up.
b) At a certain digit, brightness is low.
c) Only one indicator does not light up.
d) The corresponding segments of all digits do not light up; or they continue to light up.
e) Wrong figure appears.
f) A certain group of indicators do not light up.
g) The figure of all digits flicker.

2-3  Other possible troubles caused by defective control unit.

a) Buzzer does not sound or continues to sound.
b) Clock does not operate properly.
c) Cooking is not possible.
d) Proper temperature measurement is not obtainned.

CARRY OUT 4R CHECKS.

If the display fails to clear when the STOP/CLEAR pad is depressed, first verify the flat ribbon
cable is marking good contact, verify that the door sensing switch (stop switch) operates properly;
that is the contacts are closed when the door is closed and open when the door is open. If the
door sensing switch (stop switch) is good, disconnecct the flat ribbon cable that connects the key
unit to the control unit and make sure the door sensing switch is closed (either close the door or
short the door sensing switch connecter). Use the Key unit matrix indicated on the control panel
schematic and place a jumper wire between the pins that correspond to the STOP/CLEAR pad
marking momentary contact. If the control unit responds by clearing with a beep the key unit is
faulty and must be replaced. If the control unit does not respond, it is a faulty and must be
replaced.  If a specific pad does not respond, the above method may be used (after clearing the
control unit) to determine if the control unit or key pad is at fault.

G11

G7

G5,9

G2

G4,12

G10

G6

G3,8

G1

KEY UNIT

PIZZA

GRILL

DUAL

GRILL

AUTO

COOK

OVEN

(C˚)

STOP/

CLEAR

AUTO

START/

CLOCK

START/

AUTO

MINUTE

BREAKFAST

AUTO

DEFROST

MICROW.

POWER

LEVEL

TIMER

Kg / Lb

8

771M -

CARRY OUT 3D CHECKS.

Remove the outer case and check voltage between Pin Nos. 1 and 3 of the 4 pin connector (A)  on
the control unit with an A.C. voltmeter.
The meter should indicate 230-240 volts, if not check oven circuit.

Relay Test
Check voltage at the relay coil with a D.C. voltmeter during the microwave cooking operation.
convection cooking operation or grill operation.

DC. voltage indicated .......... Defective relay.
DC. voltage not indicated .... Check diode which is connected to the relay coil. If diode is good,

control unit is defective.

RELAY SYMBOL

OPERATIONAL VOLTAGE

CONNECTED COMPONENTS

RY1

Approx. 24.0V D.C.

Oven lamp / Turntable motor

RY2

Approx. 18.0V D.C.

High voltage transformer

RY3

Approx. 18.0V D.C.

Grill heating element (Top grill)

RY4

Approx. 18.0V D.C.

Bottom heating element

RY5

Approx. 24.0V D.C.

Fan motor

CARRY OUT 4R CHECKS.

To protect the electronic circuits, this model is provided with a fine foil pattern added to the input circuit
on the PWB, this foil pattern acts as a fuse. If the foil pattern is open, follow the troubleshooting guide
given below for repair.
Problem: POWER ON, indicator does not light up.

CARRY OUT 3D CHECKS.

STEPS

OCCURANCE

CAUSE OR CORRECTION

1

The rated AC voltage is not present at

Check supply voltage and oven power cord.

Power terminal of CPU connector (CN-A).

2

The rated AC voltage is present at primary

Low voltage transformer or secondary circuit defective.

side of low voltage transformer.

Check and repair.

3

Only pattern at "a" is broken.

*Insert jumper wire J1 and solder.

(CARRY OUT 3D CHECKS BEFORE REPAIR)

4

Pattern at "a" and "b" are broken.

*Insert the coil RCILF2003YAZZ between "c" and "d".

(CARRY OUT 3D CHECKS BEFORE REPAIR)

NOTE: *At the time of these repairs, make a

visual inspection of the varistor for burn-
ing damage and examine the trans-
former with tester for the presence of
layer short circuit (check primary coil
resistance).
If any abnormal condition is detected,
replace the defective parts.

CARRY OUT 4R CHECKS.

(J1)

a

c

b

d

CN -A

RY1

D21

T1

VRS1

1