Sharp R-4H55 Service Manual page 15

R-4H55
R-4X55
R-4X55B
TEST PROCEDURES
PROCEDURE
LETTER
COMPONENT TEST
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 mor than 100 kR in the other direction.
CARRY OUT* CHECKS
ASYMMETRIC RECTIFIER TEST
ASYMMETRIC
CARRY OUT 3D CHECKS.
RECTIFIER
Isolate the high voltage rectifier assembly from the HV circuit. The asymmetric rectifier can be tested
using an ohmmeter se? to its highest range. Connect the ohmmeter 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 an
asymmetric rectifier is shorted in either direction, then the the asymmetric rectifier is probably faultly
and must be replaced with the high voltage rectifier. When the asymmetric rectifier is defective, check
whether magnetron, high voltage rectifier, high voltage wire or filament winding of the power trans-
former 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.
D HIGH VOLTAGE CAPACITOR TEST
CARRY OUT 3D CHECKS.
A. Isolate the high voltage capacitor from the circuit.
B. Continuity check must be carried out with measuring instrument which is set to the highest resis-
tance range.
C. A normal capacitor shows continuity for a short time (kick) and then a resistance of about 10 MR
after
it
has been charged.
D. A short-circuited capacitor shows continuity all the time.
E. An open capacitor constantly shows a resistance of about 10 MR because of its internal 10 MQ
resistance.
F. When the internal wire is opened in the high voltage capacitor, the capacitor shows an infinite
resistance.
G. The resistance across all the terminals and the chassis must be inifinte when the capacitor is nor-
mal.
If incorrect readings are obtained, the high voltage capacitor must be replaced.
CARRY OUT $FJ CHECKS
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.
Table: Terminal Connection of Switch
Plunger Operation COM to NO COM to NC
Released O.C. S.C.
Depressed S.C.
O.C.
COM; Common terminal
NO; Normally open terminal
NC; Normally closed terminal
S.C.; Short circuit
O.C.; Open circuit
I
If incorrect readings are obtained, make the necessary switch adjustment or replace the switch.
CARRY OUT 4R CHECKS.
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