Sharp R-7A55 Service Manual page 19

PROCEDURE
LETTER
N THERMOSTAT TEST
CARRY OUT 3D CHECKS.
Disconnect connector B from CPU unit. Measure the resistance of thermistor with an ohmmeter. Connect
the ohmmeter leads to Pin No's B5 and B6 of the thermistor harness.
If the merter does not indicate above resistance, replace the thermistor.
CARRY OUT 4R CHECKS.
K TOUCH CONTROL PANEL ASSEMBLY TEST
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 troubleshooting by 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 Panel
The following symptoms indicate a defective control unit. Before replacing the control unit,
perform the Key unit test (Procedure L) 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.
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.
L KEY UNIT TEST
If the display fails to clearwhen the STOP pad is depressed, first verify the flat ribbon 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 andopen 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 jamper wire between the pins that correspond to the STOP 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.
TEST PROCEDURES (CONT'D)
Room Temperature
20˚C - 30˚C
Note: Check key unit ribbon connection before replacement.
COMPONENT TEST
Resistance
Approximately 326 kΩ - 175 kΩ
17
R-7A55(W)/(B)
R-7E45(W)/(B)