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Model
R-872M (serv.man2)
Pages
18
Size
439.63 KB
Type
PDF
Document
Service Manual
Brand
Device
Home Appliance / Microwave Oven / Part 1 Contents, General Information, Appearance view, Operation sequence, Function of important components, Troubleshooting guide, Test procedures, Touch control panel assemblyPage 1-17
File
r-872m-sm2.pdf
Date

Sharp R-872M (serv.man2) Service Manual ▷ View online

R-872M - 6
STARTkey is touched, the following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY3 is energized and the main supply
voltage is applied to the top grill heating element .
4. Now, the food is cooked by the top grill heating ele-
ment.
BOTTOM GRILL (Figure O-3b)
In this condition the food is cooked by bottom grill heating
element energy. Programme the desired cooking time by
touching the TIME keys and touh the GRILL key twice.
When the STARTkey is touched, the following operations
occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY4 is energized and the main supply
voltage is applied to the bottom grill heating element .
4. Now, the food is cooked by the bottom grill heating
element .
TOP AND BOTTOM GRILL (Figure O-3c)
In this condition the food is cooked by top and bottom grill
heating elements energy. Programme the desired cook-
ing time by touching the TIME keys and touh the GRILL
key three times. When the STARTkey is touched, the
following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY4 is energized and the main supply
voltage is applied to the bottom grill heating element.
4. The relay RY3 is energized and the main supply
voltage is applied to the top grill heating element.
5. Now, the food is cooked by the top and bottom grill
heating elements.
CONVECTION COOKING CONDITION
PRE-HEATING (Figure O-4)
Programme the desired convection temperature by touch-
ing CONVECTION TEMPERATURE key. When the
STARTkey is touched, the following operations occur:
1. The coil shut-off relays RY1  and RY6 are energized,
the oven lamp, cooling fan motor and turntable motor
are turned on.
2. The coil shut-off relays RY3  and RY4 are energized by
control unit and the main supply voltage is added to the
top and bottom heating elements.
3. When the oven temperature reaches the selected pre-
heat temperature,  the following operations occur:
3-1. The coil shut-off relays RY3  and RY4 are energized
by control unit temperature circuit and thermistor,
opening the circuit to the top and bottom grill heating
elements.
3-2. The oven will continue to function for 30 minutes,
turning the top and bottom heating elements on and
off, as needed to maintain the selected pre-heat
temperature. The ovenwill shut-down completely af-
ter 30 minutes.
NOTE: The fan motor will be turned on and off at
preheating. And the convection motor will be turned
off at  preheating. See "Power setting" on page 9.
CONVECTION COOKING (Figure O-4)
When the pre-heat temperature is reached, a beep signal
will sound indicating that the holding the temperature has
been reached in the oven cavity. Open the door and place
the food to cooked in the oven. Program desired cooking
time by touching the TIME keys and program convection
temperature by touching the CONVECTION TEMPERA-
TURE key . When the START key is touched, the following
operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor, turntable motor and
convection motor are energized.
NOTE: When the convection temperature 40˚C is se-
lected, the convection motor is not energized.
3. The relays RY3  and RY4 are energized (if the cavity
temperature is lower than selected temperature) and
the main supply voltage is applied to the top and
bottom heating elements to return to the selected
cooking temperature.
4. Upon completion of the cooking time, the audible sig-
nal will sound, and the oven lamp, turntable motor,
cooling fan motor, convection motor, top and bottom
heating elements are de-energized. At the end of
convection cycle, if the cavity air temperature rise
above 120˚C, the circuit to the relay RY6 will be main-
tained (by the thermistor circuit) to continue operation
of the cooling fan motor until temperature drops below
105˚C, at that time the relay RY6 will be de-energized,
turning off the fan motor.
DUAL COOKING CONDITION
MICROWAVE AND CONVECTION  (Figure O-5a)
Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILL key
once. Select the microwave power level by touching the
MICROWAVE POWER LEVEL key. And select the con-
vection temperature by touching the CONVECTION TEM-
PERATURE key.
NOTE: The microwave power level can be selected from
90W, 270W and 450W.
When the STARTkey is touched, the following operations
occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor, turntable motor and
convection motor are energized.
3. The relay RY3 will be energized and the main supply
voltage is applied to the top grill heatier.
4. The relay RY4 is energized (if the cavity temperature is
lower than selected temperature) and the main supply
voltage is applied to the bottom heating elementer.
5. The relay RY2 is energized and the microwave energy
is generated by magnetron.
6. Now, the food is cooked by microwave and convection
energy simultaneously.
OPERATION SEQUENCE
R-872M - 7
MICROWAVE AND TOP GRILL  (Figure O-5b)
Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILLL key
twice. Select the microwave power level by touching the
MICROWAVE POWER LEVEL key. When the STARTkey
is touched, the following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY3 is energized and the main supply
voltage is applied to the top grill heating element.
4. The relay RY2 is energized and the microwave energy
is generated by magnetron.
5. Now, the food is cooked by microwave and top grill
simultaneously.
MICROWAVE AND BOTTOM GRILL  (Figure O-5c)
Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILL three
times. Select the microwave power level by touching the
MICROWAVE POWER LEVEL key. When the STARTkey
is touched, the following operations occur:
1. The numbers on the digital readout start the count
down to zero.
2. The oven lamp, cooling fan motor and turntable motor
are energized.
3. The relay RY4 is energized and the main supply
voltage is applied to the bottom grill heating element.
4. The relay RY2 is energized and the microwave energy
is generated by magnetron.
5. Now, the food is cooked by microwave and bottom grill
simultaneously.
POWER SETTING
Following table shows each power setting of the top
heating element, bottom heating element, magnetron, fan
motor and convection motor on Convection cooking mode,
Dual (Micro + Convec.) mode and Convection preheating
mode.
time ratio at each power output of the top heaters, bottom
heater or magnetron.
POWER OUTPUT
ON TIME
OFF TIME
100%
48 sec.
0 sec.
90%
44 sec.
4 sec.
80%
40 sec.
8 sec.
70%
36 sec.
12 sec.
60%
32 sec.
16 sec.
50%
26 sec.
22 sec.
40%
22 sec.
26 sec.
30%
16 sec.
32 sec.
20%
12 sec.
36 sec.
10%
8 sec.
40 sec.
AUTOMATIC COOKING
PIZZA
BREAKFAST
INSTANT COOK
AUTO COOK
AUTO DEFROST
Above functions are automatic cooking. They automati-
cally work out the correct cooking mode and cooking time
and/or cooking temperature. They will cook according to
the special cooking sequence.
LIMITATIONS OF POWER OUTPUT IN MANUAL
OPERATION
After the same cooking mode is carried out for more than
the specified cooking time, the power output is automati-
cally reduced by turning the control relays on and off
intermittently, as shown in the table below. This is to
protect the oven door against temperature rising.
NOTE:
1. In case of Automatic operations, the limitations of
power output are not carried out.
2. In case that the stop key is touched or the oven door is
opened during cooking, the limitations of power output
are not carried out after the total cooking time beyond
the specified cooking time.
3. In case of the two or more same cooking modes  are
carried out,  the limitations of power output are not
carried out after the total cooking time beyond the
specified cooking time.
4. In case of the two or more different cooking modes  are
carried out,  the specified cooking time is started to
count from the point when the cooking mode is changed.
5. If the cooking mode has the power level display, the
power level is also displayed when the limitations of
power output are carried out.
D
U
A
L
Specified cooking
Limited power
Time base
Cooking mode
time (minutes)
output (%)
(seconds)
Microwave (100%)
20
70
32
Top grill
30
50
48
Bottom grill
45
50
48
Top grill and
15 (Top)
50
48
Bottom grill
15(Bottom)
50
48
Micro. (100%)
20 (Micro.)
70
48
+ Top grill
15 (Grill)
50
48
Micro. (100%)
20 (Micro.)
70
48
+ Bottom grill
15 (Heater)
50
48
CONV. COOKING
DUAL/CONVEC. COOKING
CONVE
TOP
BOTTOM
TOP
BOTTOM
MAGNETRON
FAN
CTION
HEATER
HEATER
HEATER
HEATER
(MICRO)
MOTOR
MOTOR
250˚C
70%
70%
70%
70%
10, 30, 50%
100%
100%
230˚C
60%
70%
50%
70%
10, 30, 50%
100%
100%
220˚C
40%
70%
40%
70%
10, 30, 50%
100%
100%
200˚C
40%
60%
40%
60%
10, 30, 50%
100%
100%
180˚C
40%
50%
40%
50%
10, 30, 50%
100%
100%
160˚C
20%
40%
20%
40%
10, 30, 50%
100%
100%
130˚C
20%
30%
20%
30%
10, 30, 50%
100%
100%
100˚C
20%
20%
20%
20%
10, 30, 50%
100%
100%
70˚C
10%
20%
10%
20%
10, 30, 50%
100%
100%
40˚C
10%
10%
10%
10%
10, 30, 50%
100%
0%
CONVECTION PREHEATING
TOP HEATER
BOTTOM HEATER
FAN MOTOR
CONV. MOTOR
250˚C
100%
100%
60%
0%
230˚C
100%
100%
60%
0%
220˚C
100%
100%
60%
0%
200˚C
100%
100%
60%
0%
180˚C
100%
100%
60%
0%
160˚C
100%
100%
60%
0%
130˚C
100%
100%
60%
0%
100˚C
100%
100%
60%
0%
70˚C
100%
100%
60%
0%
40˚C
100%
100%
60%
0%
ON/OFF TIME RATIO
In grill cooking, convection cooking or dual cooking, the
top heater, bottom heater or magnetron operate whithin a
48 second time base. The following table is the ON / OFF
OPERATION SEQUENCE
R-872M - 8
DOOR OPEN MECHANISM
The door can be opened by pushing the open button on the
control panel. When the open button is pushed, the open
lever pushes the lower latch head on the door upward.
The upper latch head is linked with the  lower latch head,
so now, the door can be opened.
Figure D-1. Door Open Mechanism
MONITORED LATCH SWITCH
1. When the oven door is closed, the contacts (COM-NO)
of the switch must be closed. And the contacts (COM-
NC) must be opened.
2. When the oven door is opened, the contacts (COM-
NO) of the switch must be opened. And the contacts
(COM-NC) must be closed.
STOP SWITCH
1. When the oven door is closed, the contacts (COM-NO)
of the switch must be closed.
2. When the oven door is opened, the contacts (COM-
NO) of switch must be opened.
MONITOR SWITCH
The monitor switch is activated (the contacts opened) by the
upper latch head on the door while the door is closed. The
switch is intended to render the oven inoperative by means of
blowing the fuse F8A when the contacts of the monitored
latch switch fail to open when the door is opened.
Function
1. When the door is opened, the contacts (COM-NC) of
monitor switch  close (to the ON condition) due to their
being normally closed and contacts (COM-NO) open.
At this time the contacts (COM-NO) of monitored latch
switch is in the OFF condition (contacts open) due to
their being normally open contact switches.
2. As the door goes to a closed position, the monitor
switch contacts (COM-NC) are opened and contacts
(COM-NO) closed and then contacts (COM-NO) of
monitored latch switch and stop switch are closed.(On
opening the door, each of these switches operate
inversely.)
3. If the door is opened and the monitored latch switch
contacts  (COM-NO) fail to open, the fuse F8A blows
immediately after closing of the monitor switch (COM-
NC) contacts.
CAUTION: BEFORE REPLACING A BLOWN FUSE F8A,
TEST THE MONITORED LATCH SWITCH
AND  MONITOR SWITCH FOR PROPER
OPERATION.  (REFER TO CHAPTER “TEST
PROCEDURE”).
FUSE 15A 250V
If the wire harness or electrical components are short-
circuited, this fuse blows to prevent an electric shock or fire
hazard.
FUSE F8A 250V
1. If the wire harness or electrical components are short-
circuited, this fuse blows to prevent an electric shock or
fire hazard.
2. The fuse also blows when the monitored latch switch
remains closed with the oven door open and when the
monitor switch contact (COM-NC) closes.
3. The fuse also blows when the asymmetric rectifier,
H.V. rectifier,.H.V. wire harness, H.V. capacitor,
magnetron or secondary winding of high voltage trans-
former is shorted.
TC TRANSFORMER
T/C transformer converts A.C. line voltage into low voltage
to drive the control unit.
THERMAL CUT-OUT 125˚C  (MG)
This thermal cut-out protects the magnetron against over-
heat. If the temperature goes up higher than 125˚C be-
cause the fan motor is interrupted or the ventilation open-
ings are blocked, the thermal cut-out will open and line
voltage to the high voltage transformer will cut off and
operation of the magnetron will be stopped. The defec-
tive thermal cut-out must be replaced with a new one.
THERMAL CUT-OUT 150˚C (OVEN)
This thermal cut-out protects the oven against the over-
heat during grill cooking, convection cooking or dual
(combination) cooking. If the temperature rises above
150˚C because the fan motor is interrupted, the air inlet
duct is blocked or the ventilation openings are obstructed,
the thermal cut-out opens and switches off all the electrical
parts. When the cools itself down to the operating tem-
perature of 130˚C, the contacts of the thermal cut-out will
close again.
ASYMMETRIC RECTIFIER
The asymmetric rectifier is solid state device that prevents
current flow is both directions. And it prevents the tem-
perature rise of the high voltage transformer by blowing
the fuseF8A when the high voltage rectifier is shorted.
The rated peak reverse voltage of D1 of the asymmetric
rectifier is 6 KV. The rated peak reverse voltage of D2 of
the asymmetric rectifier is 1.7 KV. D1 and D2 of the
asymmetric rectifier or high voltage rectifier are shorted
when the each peak reverse voltage goes beyond the
each rated peak reverse voltage. (The process of the
blowing the fuseF8A.)
1. The high voltage rectifier is shorted by some fault when
FUNCTION OF IMPORTANT COMPONENTS
D2
D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE 
RECTIFIER
microwave cooking or dual cooking.
2. The peak reverse voltage of D2 of the rectifier goes
beyond the rated peak reverse voltage 1.7 KV in the
voltage doubler circuit.
3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-
age winding of the high voltage transformer.
5. The large electric currents beyond 8A flow through the
primary winding of the high voltage transformer.
6. The fuse F8A blows by the large electric currents.
7. The power supplying to the high voltage transformer is
cut off.
DOOR
LATCH HOOK
MONITOR SWITCH
MONITORED
LATCH SWITCH
 STOP SWITCH
OPEN LEVER
LATCH
HEADS
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.
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