R-734 (serv.man2). Part 1 Text (1-30) - Sharp Home Appliance Service Manual (repair manual). Page 17

R-734 - 17

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
Switch  Unit, and troubleshooting by unit replacement is described according to the symptoms
indicated.
1.  Switch Unit.        Note : Check switch unit lead wire harness connection before replacement.

The following symptoms indicate a defective switch unit. Replace the switch unit.
a) When touching the keys, a certain key produces no signal at all.
b) When touching a key, two figures or more are displayed.
c) When touching the keys, sometimes a key produces no signal.

2.  Control Unit

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

2-1 In connection with keys.

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

2-2  In connection with indicators

a) At a certain digit, all or some dots do not light up.
b) At a certain digit, brightness is low.
c) Only one indicator does not light.
d) The corresponding dots 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.

CARRY OUT 3D CHECKS.

Before carrying out the following tests make sure the heating element is cool completely.

1. Resistance of heating element.

Disconnect the wire leads to the heating element to be tested.  Using ohmmeter with low resistance
range. Check the resistance across the terminals of the heating element as described in the
following table.

Table: Resistance of heating element

Parts name

Resistance

Grill heating element

Approximately 56 

Ω

2. Insulation resistance.

Disconnect the wire leads to the heating element to be tested. Check the insulation resistance
between the element terminal and cavity using a 500V - 100M

Ω

 insulation tester.  The insulation

resistance should be more than 10 M

Ω

 in the cold start.

If the results of above test 1 and/or 2 are out of above specifications, the heating element is probably
faulty and should be replaced.

CARRY OUT 4R CHECKS.