DOWNLOAD Sharp R-753 (serv.man3) Service Manual ↓ Size: 445.09 KB | Pages: 24 in PDF or view online for FREE

4

753M -

A

MICROWAVE OUTPUT POWER

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).

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 70.

MICROWAVE LEAKAGE TEST

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

5

753M -

The touch control panel consists of circuits including semiconductors such as LSI, ICs, etc. Therefore,
unlike conventional microwave ovens, proper maintenance cannot 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 also the control unit is divided into two units, CPU Unit and Power Unit, and troubleshooting
by unit replacement is described according to the symptoms indicated.
1.  Key Unit

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 Unit

The following symptoms indicate a defective control unit. Replacing the control unit.

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.
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 problems 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.

If the display fails to clear when the STOP/CLEAR pad is depressed, first verify the flat ribbon cable is
making 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, disconnect 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 STOP/CLEAR pad making 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.

G 9

G10

G11

G12

G 1

G 2

G 3

G 4

G 5

G 6

G 7

G 8

AUTO

START

ROST

BEEF/

LAMB

MICROWAVE

POWER

LEVEL

AUTO

COOK

CHICKEN

PORTIONS

ROST
PORK

ROST

POULTRY

MINCED

MEAT

SOUP

LESS

KITCHEN

TIMER

DUAL

GRILL

Express

DEFROST

STOP/

CLEAR

CLOCK

Kg / Lb

10 min.

1 min.

10 sec.

START

AUTO

MINUTE

DOWN

MORE

UP

GRILL

KEY UNIT 

6

753M -

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

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

CARRY OUT 4R CHECKS.

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 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)

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, grill cooking or dual 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.

RELAY SYMBOL

OPERATIONAL VOLTAGE

CONNECTED COMPONENTS

RY1

APPROX. 18.0V D.C.

Oven lamp / Turntable motor / Cooling fan motor

RY2

APPROX. 24.0V D.C.

Grill heating element

RY3

APPROX. 18.0V D.C.

High voltage transformer

CN - A

AC

31

VRS1

(J1)

7

753M -

CARRY OUT 3D CHECKS.

Isolate the high voltage rectifier assembly from the HV circuit. The high voltage rectifier  can
be tested  using an ohmmeter set to its highest range. Connect the ohmmeter across the
terminal of the high voltage rectifier and note the reading obtained. Reverse the meter leads
and note this second reading.
The normal resistance is infinite in one direction and more than 100 k

Ω

 in the other direction.

CARRY OUT 4R CHECKS.

NOTE:

FOR MEASUREMENT OF THE RESISTANCE OF THE RECTIFIER, THE
BATTERIES OF THE MEASURING INSTRUMENT MUST HAVE A VOLTAGE AT
LEAST 6 VOLTS, BECAUSE OTHERWISE AN INFINITE RESISTANCE
MIGHT BE SHOWN IN BOTH DIRECTIONS.

OUTLINE OF TOUCH CONTROL PANEL

The touch control section consists of the following units.

(1) Key Unit
(2) Control Unit (The Control Unit consists of Power Unit and

CPU Unit).

The principal functions of these units and the signals com-
municated among them are explained below.

The key unit is composed of a matrix, signals generated in
the LSI are sent to the key unit through P20, P21, P22, P23,
P24, P25, P26 and P27.
When a key pad is touched, a signal is completed through
the key unit and passed back to the LSI through P43, P44,
P45 and P46 to perform the function that was requested.

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

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 voltages necessary in the control

unit from the AC line voltage.
In addition, the synchronizing signal is available in
order to compose a basic standard time in the clock
circuit.

Symbol

Voltage

Application

VC

-5.2V

LSI(IC1)

A circuit to drive the magnetron, grill heating element,
fan motor, turntable motor and  light the oven lamp.

The buzzer is responsive to signals from the LSI to emit
audible sounds (key 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