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Model
R-754M (serv.man4)
Pages
40
Size
625.74 KB
Type
PDF
Document
Service Manual
Brand
Device
Home Appliance / Microwave Oven / Complete
File
r-754m-sm4.pdf
Date

Sharp R-754M (serv.man4) Service Manual ▷ View online

R-754M - 9
TROUBLESHOOTING GUIDE
When troubleshooting the microwave oven, it is helpful to follow the Sequence of Operation in performing the checks.
Many of the possible causes of trouble will require that a specific test be performed. These tests are given a procedure
letter which will be found in the “Test Procedure” section.
IMPORTANT: If the oven becomes inoperative because of a blown fuse (F8A) in the monitored latch switch - monitor
switch circuit, check the monitored latch switch and monitor switch before replacing the fuse (F8A).
Special fuse 20A blows when power 
cord is plugged into wall outlet.
Fuse F8A blows when the door is 
opened.
Oven lamp does not light when door is 
opened. (Display operates.)
Home fuse blows when power cord is 
plugged into wall outlet.
Nothing appears in display when power 
cord is plugged into wall outlet.
Display does not operate properly when 
STOP/CLEAR key is pressed.
Oven does not start when the START 
key is pressed. (Display operates.)
Oven lamp does not light. (Turntable 
motor operates.)
Fan motor does not operate. (Oven lamp 
lights.)
Turntable motor does not operate. (Oven 
lamp lights.)
Oven or electrical parts do not stop 
when cooking time is 0 or STOP/CLEAR 
key is pressed.
Oven goes into cook cycle but shuts 
down before end of cooking cycle.
Oven stops as soon as the START key 
is pressed. (Except for Grill Cooking)
Oven seems to be operating but little or 
no heat is produced in oven load. 
(Microwave power control is set at 100P)
Oven does not seem to be operating 
properly during variable cooking 
condition except 100P cooking condition.
Grill heating element does operate.
Oven seems to be operating but little or 
no heat is produced in oven load. 
(Microwave power does not seem to be 
generated properly)
Grill heating element does not operate
OFF
CONDITION
COOKING
CONDITION
(COMMON)
MICROWAVE
COOKING
CONDITION
GRILL 
COOKING
CONDITION
DUAL 
COOKING
CONDITION
CONDITION
PROBLEM
MAGNETRON
HIGH VOLTAGE  TRANSFORMER
H.V. RECTIFIER ASSEMBLY
HIGH VOLTAGE CAPACITOR
MONITORED LATCH SWITCH
MONITOR SWITCH 
STOP SWITCH 
THERMAL CUT-OUT (MAG.) 
THERMAL CUT-OUT (OVEN)
SPECIAL FUSE 20A 
FUSE F8A 
NOISE FILTER
TURNTABLE MOTOR
FAN MOTOR
GRILL HEATING ELEMENT
POWER SUPPLY CORD
OVEN LAMP 
SHORTED WIRE HARNESS
OPENED WIRE HARNESS
MIS-ADJUSTMENT SWITCHES
KEY UNIT
CONTROL PANEL
RELAY RY1
RELAY RY3
RELAY RY2
FOIL PATTERN ON P.W.B.
A B C D E E E F F G H
I
J
J K
L
M
N
N
N
O
TEST PROCEDURE
POSSIBLE CAUSE
AND
DEFECTIVE PARTS
R-754M - 10
B
HIGH VOLTAGE  TRANSFORMER TEST
WARNING:
High voltage and large currents are present at the secondary winding and
filament winding of the high voltage transformer. It is very dangerous to work
near this part when the oven is on. NEVER make any voltage measurements
of the high-voltage circuits, including the magnetron filament.
A
MAGNETRON TEST
CARRY OUT 3D CHECKS.
Isolate the magnetron from high voltage circuit by removing all leads connected to filament terminal.
To test for an open circuit filament use an ohmmeter to make a continuity test between the magnetron
filament terminals, the meter should show a reading of less than 1 ohm.
To test for short filament to anode condition, connect ohmmeter between one of the filament terminals
and the case of the magnetron (ground). This test should be indicated an infinite resistance. If a low
or zero resistance reading is obtained then the magnetron should be replaced.
MICROWAVE OUTPUT POWER
The power output of this oven is rated using the method specified by  IEC 60705. Full details of how
to curry out this procedure can be found in the Sharp Technical Training notes which is available from
Sharp Parts Centre (part number SERV-LITMW01).
The IEC60705 procedure must be carried out using laboratory-type procedures and equipment.
These requirements make the procedure unsuitable for routine performance checks.
NOTE: The following test method gives an indication of the output power only, it cannot be used to
establish the actual/rated output power. If the true output power is required, then the IEC60705
test method must be used.
Alternative simplified method:
1. Place 2  litres of cold water (between 12
°
C and 20
°
C) in a suitable container.
2. Stir the water and measure the temperature in 
°
C. Note temperature as T1.
3. Place the container in the microwave and heat the water for 1 minute and 40 seconds on full power.
4. When the 40 seconds is completed, remove the  container and stir the water. Note the water
temperature as T2.
5. Calculate the output power using the following formula:
R.F. Power Output = (T2 - T1) x 90.
NOTE: The result from this test should be within the allowance of 2000cc alternative method.
(
±
10%).
MICROWAVE LEAKAGE TEST
This oven should be tested for microwave leakage on completion of any repair or adjustment, following
the procedure described in the Sharp Technical Training notes (part number SERV-LITMW01). The
maximum leakage permitted in BS EN 60335-2-25 is 50W/m
(equivalent to 5W/m
2
), however it is not
normal to detect any significant leakage, therefore, any leakage which is detected should be
investigated.
It is essential that only leakage detectors with current calibration traceable to National Physical
Laboratories are used.
Suitable leakage detectors: CELTEC A100
APOLLO X1
NEVER TOUCH ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL
WHILE THE OVEN IS IN OPERATION.
TEST PROCEDURES
PROCEDURE
LETTER
COMPONENT TEST
R-754M - 11
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 k
 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 probably 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, BE-
CAUSE OTHERWISE AN INFINITE RESISTANCE MIGHT BE SHOWN IN BOTH DIREC-
TIONS.
CARRY OUT 3D CHECKS.
Disconnect the leads to the primary winding of the high voltage  transformer. Disconnect the filament
and secondary winding connections from the rest of the HV circuitry. Using an ohmmeter, set on a low
range,  it is possible to check the continuity of all three winding. The following readings should be
obtained:-
a. Primary winding ................................... approximately 2 
b. Secondary winding .......................... approximately 130 
c. Filament winding .........................................  less than 1 
If the readings obtained are not stated as above, then the high voltage transformer is probably faulty
and should be replaced.
CARRY OUT 4R CHECKS.
TEST PROCEDURES
PROCEDURE
LETTER
COMPONENT TEST
D2
D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE 
RECTIFIER
A
B
C
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
 because of its internal 10M
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.
R-754M - 12
G
BLOWN SPECIAL FUSE 20A
CARRY OUT 3D CHECKS.
If the special fuse 20A is blown, there is a shorts or grounds in electrical parts or wire harness. Check
them and replace the defective parts or repair the wire harness.
CARRY OUT 4R CHECKS.
CAUTION:
Only replace
 special 
fuse 
20A with
 the correct value replacement.
H
BLOWN FUSE F8A
CARRY OUT 3D CHECKS.
1. If the fuse F8A  is blown when the door is opened, check the monitored latch switch  monitor switch.
2. If the fuse F8A is blown by incorrect door switching replace the defective switch(es) and the  fuse
F8A.
3. If the fuse F8A is blown, there could be shorts in the asymmetric rectifier or there is a ground in wire
harness. A short in the asymmetric rectifier may be occurred due to short or ground in H.V. rectifier,
magnetron, high voltage transformer or H.V. wire. Check them and replace the defective parts or
repair the wire harness.
CARRY OUT 4R CHECKS.
CAUTION:
Only replace fuse 
F8A
 with the correct value replacement.
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.
Table: Thermal Cut-out Test
Temperature of "ON"
Temperature of "OFF"
Indication of ohmmeter
        Parts Name
 condition  (closed circuit).
condition (open circuit).
(When room temperature
(˚C)
(˚C)
is approx. 20˚C.)
Thermal cut-out 125˚C
This is not resetable
Above 125˚C
Closed circuit
Thermal cut-out 145˚C
Below 115˚C.
Above 145˚C
Closed circuit
E
SWITCH TEST
Table: Terminal Connection of Switch
Plunger Operation
COM to NO
COM to NC
Released
Open circuit
Short circuit
Depressed
Short circuit
Open circuit
COM; Common terminal,
NO;
Normally open terminal
NC;
Normally close terminal
If incorrect readings are obtained, make the necessary switch adjustment or replace the switch.
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.
TEST PROCEDURES
PROCEDURE
LETTER
COMPONENT TEST
F
THERMAL CUT-OUT TEST
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 oven cavity has cought fire, this may be due
to over heating produced by omproper setting of the cooking time or failure of the control panel.
CARRY OUT 4R CHECKS.
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