C: High Voltage Rectifier Test; D: High Voltage Capacitor Test; E: Switch Test; F: Thermistor Test - Sharp R-890SLM Service Manual

R-890SLM

[3] C: HIGH VOLTAGE RECTIFIER TEST

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 kΩ 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 recti-
fier. 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.

[4] D: HIGH VOLTAGE CAPACITOR TEST

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 10MΩ 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 MΩ 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.

[5] E: SWITCH TEST

CARRY OUT 3D CHECKS.
Isolate the switch to be tested and using an ohmmeter check between the terminals as described in the following table.
Plunger Operation
Released
Depressed
If incorrect readings are obtained, make the necessary switch adjustment or replace the switch.
CARRY OUT 4R CHECKS.

[6] F: THERMISTOR TEST

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.
Room Temperature
20°C - 30°C
If the meter does not indicate above resistance, replace the thermistor.
CARRY OUT 4R CHECKS.
Service_R-890SLM.indd 16
Table: Terminal Connection of Switch
Common terminal to Normally open terminal
Open circuit
Short circuit
Resistance
Approximately 359.9 kΩ - 152 kΩ
8 - 2
A B
D2 D1
ASYMMETRIC
RECTIFIER
C
Common terminal to Normally close terminal
Short circuit
Open circuit
HIGH VOLTAGE
RECTIFIER
4/25/09 10:55:15 AM