R-98STMA (serv.man8). Test Procedures - Sharp Home Appliance Service Manual (repair manual). Page 2

R98STMAA

8 – 2

3. Also, the high voltage transformer has the thermal cut-out in the primary coil. The thermal cut-out will open when the temperature of the thermal

cut-out in the primary coil reaches approximately 150

°C. The thermal cut-out resets automatically at 130°C. If an ohmmeter indicates an open cir-

cuit under normal condition, replace the high voltage transformer because the primary coil (thermal cut-out) has opened. An open primary coil
(thermal cut-out) indicates overheating of the high voltage transformer. Check for restricted air flow to the high voltage transformer, especially the
ventilation opening.

4. CARRY OUT 4R CHECKS.

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 B+C 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.

ASYMMETRIC RECTIFIER TEST

CARRY OUT 3D CHECKS.

Isolate the high voltage rectifier assembly from the HV circuit. The asymmetric rectifier can be
tested using an ohmmeter set to its highest range across the terminals A+B of the asymmetric
rectifier and note the reading obtained. Reverse the meter leads and note this second reading.
If an open circuit is indicated in both directions then the asymmetric rectifier is good. If the asymmetric rectifier is shorted in either direction, then the
asymmetric rectifier is faulty and must be replaced with high voltage rectifier. When the asymmetric rectifier is defective, check whether magnetron,
high voltage rectifier, high voltage wire or filament winding of the high voltage transformer is shorted.

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.

CARRY OUT 3D CHECKS.

1. Isolate the high voltage capacitor from the circuit.

2. Continuity check must be carried out with measuring instrument which is set to the highest resistance range.

3. A normal capacitor shows continuity for a short time (kick) and then a resistance of about 10M

Ω after it has been charged.

4. A short-circuited capacitor shows continuity all the time.

5. An open capacitor constantly shows a resistance about 10 M

Ω because of its internal 10MΩ resistance.

6. When the internal wire is opened in the high voltage capacitor shows an infinite resistance.

7. The resistance across all the terminals and the chassis must be infinite when the capacitor is normal.

If incorrect reading are obtained, the high voltage capacitor must be replaced.

CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

Isolate the switch to be tested and using an ohmmeter check between the terminals as described in the following table.

If incorrect readings are obtained, make the necessary switch adjustment or replace the switch.

CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

Disconnect the connector B from CPU unit. Measure the resistance of the thermistor with an ohmmeter. Connect the ohmmeter leads to Pin No's C1
and C3 of the thermistor harness.

If the meter does not indicate above resistance, replace the thermistor.

CARRY OUT 4R CHECKS.

Table: Terminal Connection of Switch

Plunger Operation

Common terminal to Normally open terminal

Common terminal to Normally close terminal

Released

Open circuit

Short circuit 

Depressed

Short circuit 

Open circuit.

Room Temperature

Resistance

20

°C - 30°C

Approximately 359.9 k

Ω - 152 kΩ

D2

D1

ASYMMETRIC

RECTIFIER

HIGH VOLTAGE

RECTIFIER