Sharp R-880B Service Manual page 16

R-880B
PROCEDURE
LETTER
NOTE: FOR MEASUREMENT OF THE RESISTANCE OF THE RECTIFIER, THE BATTERIES OF
THE MEASURING INSTRUMENT MUST HAVE A VOLTAGE AT LEAST 6 VOLTS, BE-
CAUSE OTHERWISE AN INFINITE RESISTANCE MIGHT BE SHOWN IN BOTH DIREC-
TIONS.
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
resistance range.
C. A normal capacitor shows continuity for a short time (kick) and then a resistance of about 10M after
it has been charged.
D. A short-circuited capacitor shows continuity all the time.
E. An open capacitor constantly shows a resistance about 10 M
resistance.
F. When the internal wire is opened in the high voltage capacitor shows an infinite resistance.
G. 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.
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.
F THERMAL CUT-OUT TEST
CARRY OUT 3D CHECKS.
Disconnect the leads from the terminals of the thermal cut-out. Then using an ohmmeter, make a continuity
test across the two terminals as described in the below.
Parts Name
Thermal cut-out 125˚C
Thermal cut-out 170˚C
If incorrect readings are obtained, replace the thermal cut-out.
An open circuit thermal cut-out (MG) indicates that the magnetron has overheated, this may be due
to resistricted ventilation, cooling fan failure or a fault condition within the magnetron or HV. circuit.
An open circuit thermal cut-out (OVEN) indicates that the food in the oven cavity may catch fire, this
may be due to over heating produced by improper setting of the cooking timer or failure of the control
panel.
CARRY OUT 4R CHECKS.
TEST PROCEDURES
Table: Terminal Connection of Switch
COM to NO
Open circuit
Short circuit
Table: Thermal Cut-out Test
Temperature of "ON"
condition (closed circuit).
(˚C)
This is not resetable type.
Cuts back in at 155˚C.
COMPONENT TEST
COM to NC
COM; Common terminal,
Short circuit NO;
Open circuit NC;
Temperature of "OFF"
condition (open circuit).
(˚C)
Above 125˚C
Above 170˚C
14
because of its internal 10M
Normally open terminal
Normally close terminal
Indication of ohmmeter
(When room temperature
is approx. 20˚C.)
Closed circuit
Closed circuit