Sharp R-8740 (serv.man17) Service Manual ▷ View online
R-8740M - 13
J K M M N
FUSE 20A
FUSE F2 F8A
GRILL HEATING ELEMENT (TOP)
GRILL HEATING ELEMENT (BOTTOM)
THERMISTOR
POWER SUPPLY CORD
SHORTED WIRE HARNESS
OPENED WIRE HARNESS
OVEN LAMP OR SOCKET
TC TRANSFORMER
TOUCH CONTROL PANEL
BLOCKED VENTILATION OPENINGS
WRONG OPERATION
MIS-ADJUSTMENT OF SWITCHES
HOME FUSE OR BREAKER
NO POWER AT WALL OUTLET
BLOCKED CONVECTION FAN
BLOCKED COOLING FAN
RELAY (RY1, 2, 3, 4, 5, 7)
FOIL PATTERN
KEY UNIT
O
P Q R
TROUBLESHOOTING GUIDE
R-8740M - 14
PROCEDURE
LETTER
COMPONENT TEST
TEST PROCEDURES
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.
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.
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 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 IEC-60705 procedure must be carried out using laboratory-type procedures and equipment.
These requirements make the procedure unsuitable for routine performance checks.
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.
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 2 minutes on full power.
4. When the 2 minutes is completed, remove the container and stir the water. Note temperature as T2.
5. Calculate the output power using the following formula:
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 2 minutes on full power.
4. When the 2 minutes is completed, remove the container and stir the water. Note temperature as T2.
5. Calculate the output power using the following formula:
R.F. Power Output = (T2 - T1) x 80.
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
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
2
(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.
are used.
Suitable leakage detectors:
CELTEC A100
APOLLO X1
APOLLO X1
NEVER TOUCH ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL WHILE
THE OVEN IS IN OPERATION.
THE OVEN IS IN OPERATION.
R-8740M - 15
A
B
C
D2
D1
ASYMMETRIC
RECTIFIER
RECTIFIER
HIGH VOLTAGE RECTIFIER
C
HIGH VOLTAGE RECTIFIER ASSEMBLY TEST
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.
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.
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 windings. The following readings should be obtained:-
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 windings. The following readings should be obtained:-
If the reading obtained are not stated above, then the high voltage transformer is probably faulty and
should be replaced.
should be replaced.
CARRY OUT 4R CHECKS
a. Primary winding
1.9 ohms approximately
b. Secondary winding
122 ohms approximately
c. Filament winding
less than 1 ohm
WARNING: High voltages and large currents are present at the secondary winding and
filament winding 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.
the oven is on. NEVER make any voltage measurements of the high-voltage
circuits, including the magnetron filament.
B
HIGH VOLTAGE TRANSFORMER TEST
TEST PROCEDURES
PROCEDURE
LETTER
COMPONENT TEST
R-8740M - 16
Isolate the high voltage rectifier assembly from the HV circuit. The asymmetric 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 direction then the asymmetric rectifier is good. If an 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 power transformer is shorted.
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 direction then the asymmetric rectifier is good. If an 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 power 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 OF AT LEAST 6 VOLTS,
BECAUSE OTHERWISE AN INFINITE RESISTANCE MIGHT BE SHOWN IN BOTH
DIRECTIONS.
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 resistance
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 10MΩ 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 MΩ because of its internal 10MΩ
E. An open capacitor constantly shows a resistance about 10 MΩ because of its internal 10MΩ
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.
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.
following table.
COM; Common terminal, NO; Normally open terminal NC; Normally close terminal
S.C.; Short, O.C.; Open circuit
S.C.; Short, O.C.; Open circuit
Table: Terminal Connection of Switch
Plunger Operation
COM to NO
COM to NC
Released
O.C.
S.C.
Depressed
S.C.
O.C.
If incorrect readings are obtained, replace the switch.
CARRY OUT 4R CHECKS.
TEST PROCEDURES
PROCEDURE
LETTER
COMPONENT TEST
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