Sharp R-880B Service Manual page 18

R-880B
PROCEDURE
LETTER
Check the resistance across the terminals of the heater as described in the following table.
2. Insulation resistance.
Disconnect the wire leads to the heater to be tested. Check the insulation resistance between the
heater terminal and cavity using a 500V - 100M insulation tester. The insulation resistance should
be more than 10 M in the cold start.
If the results of above test 1 and/or 2 are out of above specifications, the heater is probably faulty and
should be replaced.
CARRY OUT 4R CHECKS.
L THERMISTOR TEST
1. CARRY OUT 3D CHECKS.
2. Disconnect connector-D from the control unit. Measure the resistance of the thermistor with an
ohmmeter. Connect the ohmmeter leads to Pin No's D1 and D3.
3. If the meter does not indicate above resistance, replace the thermistor
4. CARRY OUT 4R CHECKS.
M 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 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 also the Control unit is divided into two units, CPU unit and Power 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 obtained.
TEST PROCEDURES
Table: Resistance of heater
Parts name
Top heaters
Bottom heater
Room Temperature
68˚F(20˚C) - 86˚F(30˚C)
COMPONENT TEST
Resistance
Approximately 24
Approximately 72
Resistance
Approx. 293k - 184K
16
x 2 = 48