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Model
R-872M (serv.man5)
Pages
47
Size
1.02 MB
Type
PDF
Document
Service Manual
Brand
Device
Home Appliance / Microwave Oven / Complete
File
r-872m-sm5.pdf
Date

Sharp R-872M (serv.man5) Service Manual ▷ View online

R-872M - 9
NOISE FILTER
The noise filter assembly prevents radio frequency inter-
ference that might flow back in the power circuit.
TURNTABLE MOTOR
The turntable motor rotates the turntable.
FAN MOTOR
The fan motor drives a blade which draws external cool air.
This cool air is directed through the air vanes surrounding
the magnetron and cools the magnetron. This air is chan-
nelled through the oven cavity to remove steam and
vapours given off from heating food. It is then exhausted
through the exhausting air vents of the oven cavity.
CONVECTION MOTOR
The convection motor drives the convection fan and
provide the heated air.
TOP GRILL HEATING ELEMENT
The grill heating element is provided to brown the food and
is located on the top of the oven cavity.
BOTTOM GRILL HEATING ELEMENT
The grill heating element is provided to brown the food and
is located at the base of the oven cavity.
CONVECTION COOKING SYSTEM
This oven is designed with a hot air heating system where
food is heated by forced circulation of the hot air produced
by the grill heaters. The air heated by the grill heating
elements is circulated through the convection passage
provided on the outer casing of the oven cavity by means
of the convection fan which is driven by the convection
motor. It is then enters the inside of the oven through the
vent holes provided on the back side of the oven. Next, the
hot air heats the food on the turntable and leaves the oven
cavity through the vent in the oven cavity rear wall. In this
way, the hot air circulates inside the oven cavity to raise its
temperature and, at the same time, comes into contact
with the food being cooked. When the temperature inside
the oven cavity reaches the selected temperature, the
heating elements are de-energized. When the tempera-
ture inside the oven cavity drops below the selected
temperature, the heating elements are energized again. In
this way, the inside of the oven cavity is maintained at
approximately the selected temperature. When the con-
vection time reaches “0”, the heating elements are de-
energized and the convection fan stops operating and the
oven shuts off. At that time if the cavity air temperature has
rised above120˚C, the fan motor remains rotating. Auto-
matically the fan motor will be shut down at low tempera-
ture (less than 105˚C).
Figure D-2. Convection Cooking System
FIRE SENSING FEATURE
The oven will stop its operation when there is a fire in the
oven cavity in microwave cooking condition.
LSI measures the voltage across the temperature meas-
urement circuit intermittently within 32-seconds time base
since the oven is started in microwave cooking condition.
The oven will stop its operation when the difference of the
voltage is more than 0.781 volts in microwave cooking
condition.
1. Within a 32-seconds base, the thermistor is energized
for 2 seconds.  At that time, the voltage across the
temperature measurement circuit is measured.
2. The oven carries out the procedure above again. If the
second voltage is 0.781V higher than first voltage, LSI
judges it  is a fire in the oven cavity and stop the oven.
3. When LSI judges it  is a fire in the oven cavity, LSI will
switch off the relays to high voltage transformer and fan
motor and LSI stops countring down.
OPEN JUDGE BY THERMISTOR
1. If the temperature of the thermistor does not rise to
more than 40˚C after 4 minutes and 15 seconds from
when the oven is started in convection, grill (top and
bottom grills) or dual cooking mode, the oven is turned
off.
2. When the thermistor or the wire harness to the thermis-
tor is opened, the oven is turned off after 4 minutes and
15 seconds because this condition is same as above.
THERMISTOR
Sensing 
Voltage
ON
OFF
ON
OFF
30
32 (sec.)
Sensing the voltage across temperature measurement circuit.
Door
Oven Cavity
Grill Heating
Element
Bottom Grill 
Heating Element
Convection
Passage
Cooling
Fan
Convection
Motor
Convection
Fan
Turntable
Insulation
Turntable
Motor
FUNCTION OF IMPORTANT COMPONENTS
  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.
R-872M - 10
  TROUBLESHOOTING GUIDE
Fuse F8A blows when the door is opened.
Home fuse blows when power cord is plugged into wall outlet.
Fuse 15A blows when power cord is plugged into wall outlet.
Nothing appears in display when power cord is plugged into wall outlet 
and the door is opened and closed.
Display does not operate properly when STOP/CLEAR key is touched.
Oven lamp does not light when door is opened. (Display operates.)
Oven does not start when the START key is touched. (Display operates.)
Oven lamp does not light  and turntable motor does not operate.
Fan motor does not operate. (Oven lamp lights.)
Convection fan motor does not operate. (Oven lamp lights.)
Turntable motor does not operate. (Oven lamp lights.)
Oven or any electrical parts (except fan motor) does not stop when 
cooking time is 0 or STOP/CLEAR key is touched.
Oven stops after 4 minutes and 15 seconds since START key is 
touched. (Except Microwave mode)
Display operates properly but all electrical parts do not operate.
Oven goes into cook cycle but shuts down before end of cooking cycle.
After cooking, the temperature of oven cavity is higher than 105˚C but 
fan motor does not operate.
Oven seems to be operating but little or no heat is produced in oven 
load. (Microwave power control is set at HIGH)
Oven does not seem to be operating properly during variable cooking 
condition except HIGH cooking condition.
Oven goes into cook cycle but shuts down before end of cooking cycle.
Convection cooking mode does not heat.
Oven seems to be operating but the temperature in the oven cavity is 
lower or higher than preset one. 
Grill heating element does operate.
Oven goes into cook cycle but shuts down before end of cooking cycle.
Oven seems to be operating but little or no heat is produced in oven 
load. (Microwave power does not seem to be generated properly)
Oven seems to be operating but the temperature in the oven cavity is 
lower or higher than preset one. 
Top and Bottom Grill heating elements do not heat.
Top or Bottom Grill heating element does not heat.
OFF
CONDITION
COOKING
CONDITION
(COMMON MODE)
MICROWAVE
COOKING
CONDITION
GRILL COOKING
CONDITION
CONVECTION
COOKING
CONDITION
DUAL COOKING
CONDITION
(COMMON MODE)
DUAL COOKING
CONDITION
(MICRO./CONV.)
DUAL COOKING
CONDITION
(MICRO./GRILL)
CONDITION
PROBLEM
A B C
D E E E F G G H H H I
J K L L
M N O O O O O O O P
TEST PROCEDURE
POSSIBLE CAUSE
AND
DEFECTIVE PARTS
BLOCKED 
COOLING FAN
BLOCKED CINVECTION FAN
NO POWER AT WALL OUTLET
HOME FUSE OR BREAKER
MIS-ADJUSTMENT OF SWITCHES
BLOCKED VENTILATION OPENINGS
OPENED WIRE HARNESS
SHORTED WIRE HARNESS
OVEN LAMP OR SOCKET
POWER SUPPLY CORD
FOIL PATTERN ON P.W.B.
RELAY RY7
RELAY RY6
RELAY RY5
RELAY RY4
RELAY RY3
RELAY RY2
RELAY RY1
KEY UNIT
TOUCH CONTROL PANEL
TC TRANSFORMER
BOTTOM GRILL HEATING ELEMENT
TOP GRILL HEATING ELEMENT
FUSE F8A  
FUSE 15A 
NOISE FILTER
CONVECTION FAN MOTOR
FAN MOTOR
TURNTABLE MOTOR
THERMAL CUT-OUT 150
˚C TC2
THERMAL CUT-OUT 125
˚C TC1 
THERMISTOR
MONITOR SWITCH SW3
STOP SWITCH SW2
MONITORED LATCH SWITCH SW1
HIGH VOLTAGE CAPACITOR
H.V. HARNESS
H.V. RECTIFIER ASSEMBLY
HIGH VOLTAGE  TRANSFORMER
MAGNETRON     
R-872M - 11
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 705. 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-705 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 IEC705
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 2 minutes on fill 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 70.
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-872M - 12
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, BECAUSE
OTHERWISE AN INFINITE RESISTANCE MIGHT BE SHOWN IN BOTH DIRECTIONS.
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 99 
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.
A
B
C
D2
D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE RECTIFIER
  TEST PROCEDURES
PROCEDURE
LETTER
COMPONENT 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.
D
HIGH VOLTAGE CAPACITOR TEST
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