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4

743M -

The power output of this oven is rated using the method specified by IEC-60705.  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-60705 procedure must be carried out using laboratory-type procedures and equipment.  These requirements
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-60705 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

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743M -

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 in one unit and troubleshooting by unit
replacement is described according to the symptoms indicated.
Control Panel.

The following symptoms indicate a defective control unit.

1.  Tact Switch.

The following symptoms indicate a defective tact switch. Replace the tact switch.

a) When touching a tact switch, a certain tact switch produces no signal at all.
b) When touching a tact switch, sometimes a tact switch produces no signal.

2. In connection with tact switches.

a) When touching a tact switch, a certain group of tact switch do not produce a signal.
b) When touching a tact switch, no tact switch produce a signal.

3. Display problems.

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.
h)  When touching a tact switch, the control unit does not respond.

4. Other possible problems caused by defective control unit.

a) Buzzer does not sound or continues to sound.
b) Cooking is not possible.

1. Disconnect the oven from the power supply.
2. Discharge the high voltage capacitor.
3. Remove the control unit from the control panel.
4. By using an ohmmeter, check the tact switch operation.
5. When the tact switch is not depressed, an ohmmeter should indicate an open circuit. When the tact

switch is depressed, an ohmmeter should indicate a short circuit. If improper operation is indicated,
the tact switch is probably defective and should be checked.

1. CARRY OUT 3D CHECKS.
2. Disconnect the leads from the primary of the high voltage transformer. Make sure that the leads

remain isolated from other oven components and chassis. (Use insulation tape if necessary.)

3. Close the door.
4. Reconnect the supply.
5. Check the voltage between Pin Nos. 1 and 3 of the 3 pin connector (A) on the control unit with an

A.C. voltmeter. The meter should indicate 230-240 volts, if not check oven circuit.

6. RY1, RY2 and RY3 TEST
   These are operated by D.C. voltage. Check voltage at the relay coil with a D.C. voltmeter during the

microwave cooking operation or grill cooking 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.

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743M -

To protect the electronic circuits, this model is provided with a fine foil pattern added to the primary 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 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".

See NOTE : 2.

(CARRY OUT 3D CHECKS BEFORE REPAIR)

NOTE : 1

*At the time of these repairs, make a
visual inspection of the varistor for
burning damage and examine the
transformer with tester for the pres-
ence of layer short-circuit (check pri-
mary coil resistance). If any abnor-
mal condition is detected, replace
the defective parts.

NOTE : 2

The hole "C" is not prepared for the
coil on this PWB. So, insert the coil
instead of J1 at first. Then solder the
one lead wire of the coil at "d". And
solder the other lead wire of the coil
at the Pin No.1 of the connector CN-
A.

CARRY OUT 4R CHECKS.

VRS1

D20

WH - 3

A

5

1

AC

CN - A

OL/FM
/TTM

(J1)

D21

RY1

See NOTE : 2

RELAY SYMBOL

OPERATIONAL VOLTAGE

CONNECTED COMPONENTS

RY1

APPROX. 24.0V D.C.

Oven lamp / Turntable motor / Cooling fan motor

RY2

APPROX. 18.0V D.C.

Grill heater

RY3

APPROX. 18.0V D.C.

High voltage transformer

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OUTLINE OF TOUCH CONTROL PANEL

The touch control section consists of the following circuits as
shown in the touch control panel circuit.

The principal functions of these circuits and their related
signals are explained below.

Signals generated in the LSI are sent to the tact switches
through SEG0, SEG1, SEG2, and SEG3.
When a tact switch is touched, a signal is completed through
the tact switch and passed back to the LSI through P20 and
P23 to perform the function that was requested.

Control unit consists of LSI, ACL circuit, indicator circuit,
power source circuit, relay circuit, buzzer circuit, synchroniz-
ing signal circuit and back light circuit.

This LSI controls the tact switch strobe signal, relay
driving signal for oven function and indicator signal.

This circuit generates a signal which resets the LSI to the
initial state when power is supplied.

This circuit consists of 4-digits, 12-segments and 3-
common electrodes using a Liquid Crystal Display.

This circuit generates voltage necessary in the control
unit from the AC line voltage.

Symbol

Voltage

Application

VC

- 5V

LSI(IC1)

To drive the magnetron, grill heater, fan motor, turntable
motor and  light the oven lamp.

The buzzer is responsive to signals from the LSI to emit
audible sounds (tact switch touch sound and completion
sound).

The power source synchronizing signal is available in
order to compose a basic standard time in the clock
circuit. It accompanies a very small error because it
works on commercial frequency.

A switch to "tell" the LSI if the door is open or closed.

A circuit to drive the back light (Light emitting diodes
LED1-LED4)