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® SERVICE MANUAL
S1111118740M/
MICROWAVE OVEN WITH
GRILL AND CONVECTION
MODEL
R-8740M
R-8740M
In interests of user-safety the oven should be restored to its original
condition and only parts identical to those specified should be
used.
TABLE OF CONTENTS
Page
CAUTION, MICROWAVE RADIATION .................................................................................................... 3
GENERAL INFORMATION ....................................................................................................................... 3
SERVICING .............................................................................................................................................. 4
PRODUCT SPECIFICATIONS ................................................................................................................. 5
APPEARANCE VIEW .............................................................................................................................. 6
OPERATION SEQUENCE ........................................................................................................................ 7
FUNCTION OF IMPORTANT COMPONENTS ....................................................................................... 9
TROUBLESHOOTING GUIDE ............................................................................................................... 12
TEST PROCEDURE .............................................................................................................................. 14
CONTROL PANEL ASSEMBLY ............................................................................................................ 21
COMPONENT REPLACEMENT AND ADJUSTMENT PROCEDURE ................................................... 28
MICROWAVE MEASUREMENT ........................................................................................................... 33
TEST DATA AT A GLANCE/WIRING/REWIRING .................................................................................. 34
SCHEMATIC DIAGRAMS ....................................................................................................................... 35
PICTORIAL DIAGRAM .......................................................................................................................... 39
CONTROL PANEL CIRCUIT DIAGRAM ................................................................................................ 40
PRINTED WIRING BOARD DIAGRAM .................................................................................................. 41
PARTS LIST ........................................................................................................................................... 42
EXPLODED DIAGRAM OF OVEN PARTS ............................................................................................ 45
CONTROL PANEL/DOOR PARTS ......................................................................................................... 46
MISCELLANEOUS/PACKING & ACCESSORIES .................................................................................. 47
SHARP CORPORATION
R-8740M - 1
SERVICING
WARNING TO SERVICE PERSONNEL
Microwave ovens contain circuitry capable of producing very high voltage and current, contact with following parts will
result in electrocution.
High voltage capacitor, High voltage transformer, Magnetron, High voltage rectifier assembly, High voltage wires.
REMEMBER TO CHECK 3D REMEMBER TO CHECK 4R
1) Disconnect the supply. 1) Reconnect all leads removed from components during
2) Door opened, and wedged open. testing.
3) Discharge high voltage capacitor. 2) Replace the outer case (cabinet).
3) Reconnect the supply.
WARNING: AGAINST THE CHARGE OF THE HIGH-
4) Run the oven. Check all functions.
VOLTAGE CAPACITOR
The high-voltage capacitor remains charged about 60 sec-
Microwave ovens should not be run empty. To test for the
onds after the oven has been switched off. Wait for 60
presence of microwave energy within a cavity, place a cup of
seconds and then short-circuit the connection of the high-
cold water on the oven turntable, close the door and set the
voltage capacitor (that is, of the connecting lead of the high-
power to HIGH and set the microwave timer for two (2) minutes.
voltage rectifier) against the chassis with the use of an
When the two minutes has elapsed (timer at zero) carefully
insulated screwdriver.
check that the water is now hot. If the water remains cold carry
Sharp recommend that wherever possible fault-finding is out 3D checks and re-examine the connections to the compo-
carried out with the supply disconnected. It may in, some nent being tested.
cases, be necessary to connect the supply after the outer
case has been removed, in this event carry out 3D checks and When all service work is completed, and the oven is fully
then disconnect the leads to the primary of the High voltage assembled, the microwave power output should be checked and
transformer. Ensure that these leads remain isolated from a microwave leakage test should be carried out.
other components and the oven chassis. (Use insulation tape
if necessary.) When the testing is completed carry out 3D
checks and reconnect the leads to the primary of the High
voltage transformer.
R-8740M - 2
SERVICE MANUAL
®
GRILL AND CONVECTION MICROWAVE OVEN
R-8740M
GENERAL IMPORTANT INFORMATION
This Manual has been prepared to provide Sharp Corp. Service engineers with Operation and Service Information.
It is recommended that service engineers carefully study the entire text of this manual, so they will be qualified to
render satisfactory customer service.
WARNING
MICROWAVE RADIATION
Personnel should not be exposed to the microwave energy
which may radiate from the magnetron or other microwave
generating devices if it is improperly used or connected. All
input and output microwave connections, waveguides, flanges
and gaskets must be secured. Never operate the device
without a microwave energy absorbing load attached. Never
look into an open waveguide or antenna while the device is
energized.
WARNING
Never operate the oven until the following points are ensured.
(A) The door is tightly closed.
(B) The door and oven hinges are not defective.
(C) The door packing is not damaged.
(D) The door is not deformed or warped.
(E) There is not any other visible damage with the oven.
Servicing and repair work must be carried out only by trained
Service Engineers.
All the parts marked "*" on parts list are used at voltages more
than 250V.
Removal of the outer wrap gives access to potentials above
250V.
All the parts marked "" on the parts list may cause undue
microwave exposure, by themselves, or when they are dam-
aged, loosened or removed.
SHARP CORPORATION
OSAKA, JAPAN
R-8740M - 3
SERVICING
WARNING TO SERVICE PERSONNEL
Microwave ovens contain circuitry capable of producing very high voltage and current, contact with following parts will
result in electrocution.
High voltage capacitor, High voltage transformer, Magnetron, High voltage rectifier assembly, High voltage wires.
REMEMBER TO CHECK 3D REMEMBER TO CHECK 4R
1) Disconnect the supply. 1) Reconnect all leads removed from components during
2) Door opened, and wedged open. testing.
3) Discharge high voltage capacitor. 2) Replace the outer case (cabinet).
3) Reconnect the supply.
WARNING: AGAINST THE CHARGE OF THE HIGH-
4) Run the oven. Check all functions.
VOLTAGE CAPACITOR
The high-voltage capacitor remains charged about 60 sec-
Microwave ovens should not be run empty. To test for the
onds after the oven has been switched off. Wait for 60
presence of microwave energy within a cavity, place a cup of
seconds and then short-circuit the connection of the high-
cold water on the oven turntable, close the door and set the
voltage capacitor (that is, of the connecting lead of the high-
power to HIGH and set the microwave timer for two (2) minutes.
voltage rectifier) against the chassis with the use of an
When the two minutes has elapsed (timer at zero) carefully
insulated screwdriver.
check that the water is now hot. If the water remains cold carry
Sharp recommend that wherever possible fault-finding is out 3D checks and re-examine the connections to the compo-
carried out with the supply disconnected. It may in, some nent being tested.
cases, be necessary to connect the supply after the outer
case has been removed, in this event carry out 3D checks and When all service work is completed, and the oven is fully
then disconnect the leads to the primary of the High voltage assembled, the microwave power output should be checked and
transformer. Ensure that these leads remain isolated from a microwave leakage test should be carried out.
other components and the oven chassis. (Use insulation tape
if necessary.) When the testing is completed carry out 3D
checks and reconnect the leads to the primary of the High
voltage transformer.
CAUTION/WARNING
CAUTION WARNING
MICROWAVE RADIATION
Servicing and repair work must be carried out only by
trained service engineers.
Service engineers should not be exposed to the
microwave energy which may radiate from the magnetron All the parts marked "*" on parts list are used at
or other microwave generating devices if it is improperly voltages more than 250V.
used or connected. All input and output microwave Removal of the outer wrap gives access to potentials
connections, waveguides, flanges and gaskets must be above 250V.
secured. Never operate the device without a microwave All the parts marked "" on parts list may cause undue
energy absorbing load attached. Never look into an open microwave exposure, by themselves, or when they
waveguide or antenna while the device is energized. are damaged, loosened or removed.
WARNING
THIS APPLIANCE MUST BE EARTHED. THE WIRES IN THIS MAINS LEAD ARE COLOURED IN
ACCORDANCE WITH THE FOLLOWING CODE:
GREEN-AND-YELLOW : EARTH BLUE : NEUTRAL BROWN : LIVE
R-8740M - 4
PRODUCT DESCRIPTION
SPECIFICATION
ITEM DESCRIPTION
Power Requirements 230-240V Volts / 50 Hertz / Single phase, 3 wire earthed
Power Consumption Microwave cooking 1.45kW Approx. 6.3A
Dual cooking
(Microwave and Top Grill) 2.4kW Approx. 10.3A
(Microwave and Bottom Grill) 1.95kW Approx. 8.2A
(Top and Bottom Grill) 1.5kW Approx. 6.3A
Convection cooking 1.525kW Approx. 6.4A
Bottom Grill cooking 550kW Approx. 2.3A
Top Grill cooking 1000kW Approx. 4.2A
Power Output 900 watts nominal of RF microwave energy (measured by method of IEC
60705) Operating fequency 2450 MHz
Top Grill heating element Power Output 1000W
Bottom Grill heating element Power Output 500W
Case Dimensions Width 520 mm Height 309 mm (including foot) Depth 488 mm
Cooking Cavity Dimensions Width 349 mm Height 207 mm Depth 357 mm
Turntable diameter 325mm
Control Complement
1. DIGITAL DISPLAY
2. INDICATORS:
These indicators will light corresponding with
below symbols
: Time reduce
: Memory
: Manual
: Percentage (for microwave
power level)
: Lock (showing oven lock is set)
: Cook (showing oven in operation)
3. COOKING MODE indicator lights.
These indicator lights will light during
each cooking mode.
OPERATION KEYS:
10. CONVECTION key
4. NUMBER keys for time and memory To change the convection
programming temperature
5. START key 11. MICROWAVE key
6. STOP key To change the microwave power
7. SOUND key setting
8. BOTTOM GRILL key 12. MANUAL key
9. TOP GRILL key 13. MEMORY key
14. TIME REDUCE key
Net Weight Approx. 19 kg
As part of our policy of continuous improvement, we reserve the right to
alter design and specifications without notice
Note:
Temperature measurements taken whilst the oven is in convection mode will differ from the displayed level.
This is due to the grill elements turning on and off in order to regulate the oven temperature.
This will not affect the cooking results as long as the operation manual and cook book are followed correctly.
R-8740M - 5
APPEARANCE VIEW
OVEN
3 2 11
12 10
1
6 5
8
9 7 10 4
1. Control panel 9. Door seals and sealing surfaces
2. Oven lamp 10. Ventilation openings
3. Grill heating element (Top Grill) 11. Outer cabinet
4. Door opening button 12. Power supply cord
5. Waveguide cover 13
13. Turntable
6. Oven cavity 14. Handle
7. Turntable motor shaft 14
8. Grill heating element
(Bottom Grill)
1
1. DIGITAL DISPLAY
2 2. INDICATORS:
These indicators will light corresponding with
below symbols
3
: Time reduce
14 : Memory
: Manual
13
: Percentage (for microwave
power level)
12
: Lock (showing oven lock is set)
: Cook (showing oven in operation)
11
4 3. COOKING MODE indicator lights.
These indicator lights will light during
10 each cooking mode.
OPERATION KEYS:
9 4. NUMBER keys for time and memory
programming
5. START key
6. STOP key
8 7. SOUND key
8. BOTTOM GRILL key
9. TOP GRILL key
7
10. CONVECTION key
To change the convection temperature
11. MICROWAVE key
To change the microwave power setting
6 5 12. MANUAL key
13. MEMORY key
14. TIME REDUCE key
R-8740M - 6
OPERATION SEQUENCE
OFF CONDITION circuit to the fan motor is cut off when the relay RY5 is made
Closing the oven door activates all door interlock switches open. The circuit to the turntable motor is cut off when the
(Monitored latch switch and stop switch. contacts COM-NO of the monitored latch switch are made
open. The relays RY2 and RY5 are made open when the
IMPORTANT:
door is opened. The oven lamp remains on even if the oven
When the oven door is closed, the contacts COM-NC of the
door is opened after the cooking cycle has been interrupted,
monitor switch SW2 must be open and the contacts (COM-
because the relay RY1 stay closed. Shown in the display is
NO) must be closed. When the microwave oven is plugged
remaining time.
in a wall outlet (230V / 50Hz), 230 volts A.C. is supplied to
6. MONITOR SWITCH CIRCUIT
the point A9 + A11 in the control unit.
The monitor switch SW2 is mechanically controlled by
Figure O-1 on page 35 the oven door, and monitors the operation of the
1. The display flashes ".0" monitored latch switch SW1.
2. To set any programmes or set the clock, you must first 6-1. When the oven door is opened during or after the cycle
"put the oven in MANUAL MODE . Press the MANUAL of a cooking program, the monitored latch switch SW1
key for 6-10 seconds. (oven will 'beep'). Press the and stop switch SW3 must open their contacts (COM-
MANUAL key once more. The oven is now in manual NO) first. After that the contacts (COM-NC) of the
mode (this will be indicated in the display window, above monitor switch SW2 can be closed and the contacts
the symbol). (COM-NO) of monitor switch SW2 are made open.
NOTE: When the oven door is opened, the oven lamp 6-2. When the oven door is closed. The contacts (COM-NC)
comes on at this time. of the monitor switch SW2 must be opened and the
contacts (COM-NO) of monitor switch SW2 must be
MICROWAVE COOKING CONDITION closed. After that the contacts of the monitored latch
HIGH COOKING switch SW1 and the stop switch SW3 are made closed.
Enter a desired cooking time with the number keys, press 6-3. When the oven door is opened and the contacts of the
the " MICROWAVE " key, then press the START key . monitored latch switch SW1 remain closed, the fuse F2
F8A will blow. Because the relay RY1 and monitor
Function sequence Figure O-2 on page 35 switch SW2 are closed and a short circuit is caused.
CONNECTED COMPONENTS RELAY
Oven lamp, Turntable motor RY1
High voltage transformer RY2 MEDIUM HIGH, MEDIUM, MEDIUM LOW, LOW
Fan motor RY5 COOKING
When the microwave oven is preset for variable cooking
1. 230 volts A.C. is supplied to the primary winding of the power, the 230 volts A.C. power is supplied to the high voltage
high voltage transformer. The voltage is converted to transformer intermittently within a 32-second time base through
about 3.3 volts A.C. output on the filament winding and the relay contact which is coupled with the current-limiting relay
high voltage of approximately 2000 volts A.C. on the RY2. The following levels of microwave power are given.
secondary winding. 32 sec. ON
2. The filament winding voltage (3.3 volts) heats the HIGH
100% = 900 Watts
magnetron filament and the high voltage (2000 volts) is 24 sec. ON 8 sec. OFF
sent to the voltage doubling circuit, where it is doubled to MEDIUM HIGH Approx. 70% = 630 Watts
negative voltage of approximately 4000 volts D.C.. 18 sec. ON 14 sec. OFF
3. The 2450 MHz microwave energy produced in the MEDIUM Approx. 50% = 450 Watts
magnetron generates a wave length of 12.24 cm. This 12 sec. ON 20 sec. OFF
energy is channelled through the waveguide (transport MEDIUM LOW Approx. 30% = 270 Watts
channel) into the oven cavity, where the food is placed to 6 sec. ON 26 sec. OFF
be cooked. LOW Approx. 10% = 90 Watts
4. When the cooking time is up, a signal tone is heard and
the relays RY1 + RY2 + RY5 go back to their home
Note: The On/Off time ratio does not exactly correspond to the
position. The circuits to the oven lamp, high voltage
percentage of microwave power, because approx. 3
transformer, fan motor and turntable motor are cut off.
seconds are needed for heating up the magnetron
5. When the oven door is opened during a cooking cycle,
filament.
the switches come to the following condition.
Switch Contact Condition CONVECTION COOKING CONDITION
During Oven Door PREHEATING CONDITION (Figure O-4)
Cooking Open(No cooking) Program desired convection temperature by touching
Monitored latch Switch COM-NO Closed Opened "MANUAL", then the CONVECTION key. When the START key
Monitor Switch COM-NO Closed Opened is touched, the following operations occur:
Monitor Switch COM-NC Opened Closed 1. The coil of shut-off relays RY1, RY5 and RY7 are energized,
Stop switch COM-NO Closed Opened the oven lamp, cooling fan motor, turntable motor and
convection motor are turned on.
The circuit to the high voltage transformer, is cut off when 2. The coil of shut-off relays RY3 and RY4 are energized by
the contact of relay RY2 contacts COM-NO of the monitored the control unit and the main supply voltage is added to the
latch switch, and monitor switch SW2 are made open. The
R-8740M - 7
OPERATION SEQUENCE
top and bottom heater. 2. The oven lamp, cooling fan motor and turntable motor are
3. When the oven temperature reaches the selected preheat energized.
temperature, the following operations occur; 3. The relay RY4 is energized and the main supply voltage is
3-1. The shut-off relays RY3 and RY4 are de-energized by the applied to the bottom grill heater.
control unit temperature circuit and thermistor, opening 4. Now the food is cooked by the bottom grill heater.
the circuit to the top and bottom heating elements.
3-2. The oven will continue to function for 15 minutes, turning
the convection heating elements on and off, as needed to GRILL COOKING CONDITION
maintain the selected preheat temperature. The oven will TOP AND BOTTOM GRILL (Figure O-3c)
shut-down completely after 15 minutes. In this condition the food is cooked by the top and bottom grill
heating element energy. Program the desired cooking time
CONVECTION COOKING CONDITION (Figure O-4) then set top+bottom grill mode by touching "TOP GRILL"
When the preheat temperature is reached, a beep signal will and "BOTTOM GRILL"keys. (led's will light on the
sound indicating that the holding temperature has been reached display panel). "TOP & BOTTOM GRILL" . When the
in the oven cavity. Open the door and place the food to be cooked START key is touched, the following operations occur:
in the oven. Program desired cooking time then touch the 1. The numbers on the digital readout start the count down to
CONVECTION key repeatedly to set the temperature. When zero.
the START key is touched, the following operations occur: 2. The oven lamp, cooling fan motor and turntable motor are
1. The numbers of the digital readout start the count down to energized.
zero. 3. The relay RY4 is energized and the main supply voltage is
2. The oven lamp, cooling fan motor, turntable motor and applied to the bottom grill heater.
convection motor are energized. 4. The relay RY3 is energized and the main supply voltage is
3. Relays RY3 and RY4 are energized (if the cavity temperature applied to the top grill heater.
is lower than the selected temperature) and the main supply 5. Now the food is cooked by the top+bottom grill heater.
voltage is applied to the top and bottom heaters to return to
the selected cooking temperature. DUAL COOKING CONDITION
4. Upon completion of the cooking time, the audible signal will MICROWAVE AND CONVECTION (Figure O-5a)
sound, and oven lamp, turntable motor, cooling fan motor Program the desired cooking time and then Dual cook mode
and convection motor are de-energized. At the end of by touching the CONVECTION and MICROWAVE keys.
convection cycle, if the cavity air temperature is above (led's will light on the display panel). When the START key is
104°C, the circuit to RY5 will be maintained (by the thermistor touched, the following operations occur:
circuit) to continue operation of the cooling fan motor until 1. The numbers on the digital readout start the count down to
temperature drops below 104°C, at that time the relay will zero.
be de-energized, turning off the fan motor . 2. The oven lamp, cooling fan motor, turntable motor and
convection motor are energized.
GRILL COOKING CONDITION 3. Relay RY4 is energized (if the cavity temperature is lower
TOP GRILL (Figure O-3a) than the selected temperature) and the main supply voltage
In this condition the food is cooked by grill heating element is applied to the bottom heater.
energy. Programme the desired cooking time and top grill mode 4. Relay RY2 is energized and the microwave energy is
by turning the mode select knob to " TOP GRILL" and the generated by the magnetron.
second rotary knob to set the time. When the START key is 5. During operation relay RY2 will de-energize and relay RY3
touched, the following operations occur: will be energized and the main supply voltage is applied to
1. The numbers on the digital readout start the count down to the top grill heater. This will continue to cycle throughout
zero. this mode.
2. The oven lamp, cooling fan motor and turntable motor are 6. Now the food is cooked by microwave and convection
energized. energy simultaneously.
3. The relay RY3 is energized and the main supply voltage is
applied to the top grill heater. DUAL COOKING CONDITION
4. Now the food is cooked by the top grill heater. MICROWAVE AND TOP GRILL (Figure O-5b)
Program the desired cooking time and then Dual cook mode
GRILL COOKING CONDITION by touching the "TOP GRILL" and MICROWAVE keys
BOTTOM GRILL (Figure O-3b) (led's will light on display panel). When the START key is
In this condition the food is cooked by bottom grill heating touched, the following operations occur:
element energy. Program the desired cooking time and bottom 1. The numbers on the digital readout start the count down to
grill mode by turning the mode select knob to zero.
"BOTTOM GRILL" and the second rotary knob to set 2. The oven lamp, cooling fan motor and turntable motor are
the time. When the START key is touched, the following energized.
operations occur: 3. Relay RY3 is energized and the main supply voltage is
1. The numbers on the digital readout start the count down to applied to the top heater.
zero. 4. Relay RY2 is energized and the microwave energy is
generated by the magnetron.
5. Now the food is cooked by microwave and top grill
simultaneously.
R-8740M - 8
FUNCTION OF IMPORTANT COMPONENTS
DUAL COOKING CONDITION Function
MICROWAVE AND BOTTOM GRILL (Figure O-5c) 1. When the door is opened, the contacts (COM-NC) of
Program the desired cooking time and then touch the monitor switch SW2 close (to the ON condition) due to their
"BOTTOM GRILL" and MICROWAVE keys (led's will being normally closed and contacts (COM-NO) open. At
light on the display panel). When the START key is touched, this time the monitored latch SW1 is in the OFF condition
the following operations occur: (contacts open) due to their being normally open contact
1. The numbers on the digital readout start the count down to switches.
zero. 2. As the door goes to a closed position, the monitor switch
2. The oven lamp, cooling fan motor and turntable motor are SW2 contacts (COM-NC) are opened and contacts (COM-
energized. NO) closed and then the monitored latch switch and stop
3. Relay RY4 is energized and the main supply voltage is switch contacts close.(On opening the door, each of these
applied to the bottom heater. switches operate inversely.)
4. Relay RY2 is energized and the microwave energy is 3. If the door is opened and the monitored latch switch SW1
generated by the magnetron. contacts fail to open, the fuse F2 (F8A) blows immediately
5. Now the food is cooked by microwave and bottom grill after closing of the monitor switch (COM-NC) contacts.
simultaneously. CAUTION: BEFORE REPLACING A BLOWN FUSE F2 F8A
TEST THE MONITORED LATCH SWITCH,
AUTO COOK MONITOR SWITCH FOR PROPER OPERA-
When the number keys are pressed (not in "MANUAL" TION.
mode). the oven chooses cooking mode (Microwave, Convec- (REFER TO CHAPTER "TEST PROCEDURE").
tion, Grill or Dual) and sets cooking time automatically accord-
ing to the pre-programmed information in IC-1 (LSI). FUSE F1 20A 250V
If the wire harness or electrical components are short-circuited,
DOOR OPEN MECHANISM this fuse F1 20A 250V blows to prevent an electric shock or fire
The door can be opened by pushing the open button on the hazard.
control panel. When the open button is pushed, the open lever FUSE F2 F8A 250V
pushes lower latch head on the door upward. The upper latch
1. If the wire harness or electrical components are short-
head is linked with the lower latch head, so now, the door can
circuited, this fuse blows to prevent an electric shock or fire
be opened.
MONITOR hazard.
LATCH HOOK
SWITCH 2. The fuse also blows when monitored latch switch SW1
remains closed with the oven door open and when the
MONITORED LATCH
SWITCH monitor switch SW2 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 transformer is shorted
LATCH
HEADS TC TRANSFORMER
STOP SWITCH
TC transformer converts A.C. line voltage into low voltage to
drive the control unit.
THERMAL CUT-OUT TC1 150°C (OVEN)
OPEN LEVER The thermal cut out protects the oven against overheat during
grill cooking, convection cooking or dual (combination) cook-
DOOR OPEN BUTTON
ing. If the temperature rises above 150°C because the fan
Figure D-1. Door Open Mechanism motor is interrupted, the air inlet duct is blocked or the
ventilation openings are obstructed, the thermal cut-out opens
MONITORED LATCH SWITCH SW1 AND STOP and switches off the all electrical parts.
SWITCH SW3 When the oven cools itself down to the operating temperature
1. When the oven door is closed, the contacts COM-NO of of 130°C, the contacts of the thermal cut-out will close again.
each switch must be closed.
2. When the oven door is opened, the contacts COM-NO of
each switch must be opened. THERMAL CUT-OUT TC2 125°C (MG)
This thermal cut-out protects the magnetron against overheat.
MONITOR SWITCH SW2
If the temperature goes up higher than 125°C because the fan
The monitor switch is activated (the contacts opened) by the
motor is interrupted or the ventilation openings are blocked,
upper latch head on the door while the door is closed. The
the thermal cut-out TC2 will open and line voltage to the high
switch is intended to render the oven inoperative by means
voltage transformer T will cut off and operation of the magnetron
of blowing the fuse F2 F8A when the contacts of the moni-
MG will be stopped. The defective thermal cut-out must be
tored latch switch SW1 fail to open when the door is opened.
replaced with a new one.
R-8740M - 9
FUNCTION OF IMPORTANT COMPONENTS
ASYMMETRIC RECTIFIER CONVECTION COOKING SYSTEM
The asymmetric rectifier is solid state device that prevents This oven is designed with a hot air heating system where food
current flow is both directions. And it prevents the tempera- is heated by forced circulation of the hot air produced by the
ture rise of the power transformer by blowing the fuse F2 F8A grill heaters.
when the high voltage rectifier is shorted. The air heated by the grill heaters is circulated through the
convection passage provided on the outer casing of the oven
D2 D1 cavity by means of the convection fan which is driven by the
convection motor. It is then circulated inside the oven through
ASYMMETRIC the vent holes provided on the back side of the oven. Next, the
RECTIFIER
HIGH VOLTAGE
hot air heats the food on the turntable and leaves the oven
RECTIFIER cavity through the vent in the oven cavity rear wall.
In this way, the hot air circulates inside the oven cavity to raise
The rated peak reverse voltage of D1 of the asymmetric its temperature and, at the same time, comes into contact with
rectifier is 6 KV The rated peak reverse voltage of D2 of the the food being cooked.
asymmetric rectifier is 1.7 KV. D1 and D2 of the asymmetric When the temperature inside the oven cavity reaches the
rectifier or high voltage rectifier are shorted when the each selected temperature, the heating elements are
peak reverse voltage goes beyond the each rated peak de-energized. When the temperature inside the oven cavity
reverse voltage. (The process of the blowing the fuse F2 drops below the selected temperature, the heating elements
F8A.) are energized again. In this way, the inside of the oven cavity
1. The high voltage rectifier is shorted by any causes when is maintained at approximately the selected temperature.
microwave cooking or dual cooking. When the convection time reaches "0", the heating elements
2. The peak reverse voltage of D2 of the rectifier goes are de-energized and the convection fan stops operating and
beyond the rated peak reverse voltage 1.7 KV in the the oven shuts off. At the high temperature (more than 104°C),
voltage doubler circuit. the fan motor remains rotating. Automatically the fan motor will
3. D2 of the rectifier is shorted. be shut down at low temperature (less than 104°C).
4. The large electric currents flow through the high voltage Door
Grill Heating
Oven Cavity Elements
winding of the high voltage transformer.
Convection
5. The large electric currents beyond 8A flow through the Passage
primary winding of the high voltage transformer.
6. The fuse F2 F8A blows by the large electric currents.
7. The power supplying to the high voltage transformer is cut
Auxiliary
off. Fan
NOISE FILTER
Convection
The noise filter assembly prevents radio frequency interfer- Motor
ence that might flow back in the power circuit.
Convection
Fan
TURNTABLE MOTOR TTM
Turntable Turntable
The turntable motor drives the roller stay to rotate the turnta- Motor Roller Stay
Insulator
ble.
Bottom
Heater
FAN MOTOR FM Element
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 channeled Figure D-2. Convection Cooking System
through the oven cavity to remove steam and vapours given
off from the heating foods. It is then exhausted through the
exhausting air vents at the oven cavity.
CONVECTION MOTOR CM
The convection motor drives the convection fan and provide
the heated air
TOP GRILL HEATING ELEMENT GH
The grill heating element is provided to brown the food and is
located on the top of the oven cavity.
BOTTOM GRILL HEATING ELEMENT BH
The grill heating element is provided to brown the food and is
located at the base of the oven cavity
R-8740M - 10
FUNCTION OF IMPORTANT COMPONENTS
SENSING FIRE OPERATION
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 measure-
ment circuit intermittently within a 32-second time base after
since the oven is started in microwave cooking condition.
The oven will stop its operation when the difference of the
voltages is more than 0.69 volts in microwave cooking
condition.
1. Within a 32-second base, first the thermistor is energized
for 3 seconds. After 2 seconds since the thermistor is
energized, the voltage across the temperature
measurement circuit is measured. And after 21 seconds
since the thermistor is cut off, the convection motor
operates for 10 seconds.
2. The oven carries out the procedure above again. If the
second voltage is 0.69V higher than the first voltage, LSI
judges it is a fire in the oven cavity and stops the oven.
3. When sensor cooking, the sensing fire operation is not
carried out until the oven senses the steam from food.
Because food cannot be cooked well by rotating the
convection fan at that time. After sensing the steam, the
sensing fire operation is started.
4. When LSI judges it is fire in the oven cavity, LSI will switch
off the relays to the power transformer, fan motor and
convection motor and LSI stops counting down. And then
the damper is closed so that the fresh air does not come
into the oven cavity.
20 30 32 64 (sec.)
02
10 SEC.
CONVECTION
MOTOR
ON
OFF
. 3 sec.
ON
THERMISTOR
OFF
ON
SENSING
VOLTAGE OFF
Sensing the voltage across temperature measurement circuit.
.
R-8740M - 11
TROUBLESHOOTING GUIDE
TEST PROCEDURE A B C D E E E G G H H H I
HIGH VOLTAGE TRANSFORMER
THERMAL CUT-OUT 150°C TC1
THERMAL CUT-OUT 125°C TC2
MONITORED LATCH SWITCH
HIGH VOLTAGE CAPACITOR
H.V. RECTIFIER ASSEMBLY
CONVECTION FAN MOTOR
POSSIBLE CAUSE
AND
TURNTABLE MOTOR
DEFECTIVE PARTS
MONITOR SWITCH
H.V. HARNESS
STOP SWITCH
NOISE FILTER
MAGNETRON
FAM NOTOR
PROBLEM
Fuse F2 F8A blows when the door is opened.
Home fuse blows when power cord is plugged into wall outlet.
Fuse F1 15A blows when power cord is plugged into wall
outlet.
OFF
CONDITION ":0" does not appear in display when power cord is plugged
into wall outlet.
Display does not operate properly when STOP/CLEAR key is
touched.
Oven lamp does not light when door is opened. (Display
operates.)
COOKING Oven does not start when the START key is touched. (Display
CONDITION operates.)
(COMMON MODE)
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 about 4 minutes since START key is touched.
(Except Microwave and Dual Cook modes)
Display operates properly but all electrical parts do not oper-
ate.
The oven stops 1 minute after starting.
Oven goes into cook cycle but shuts down before end of
cooking cycle.
After cooking, the temperature of oven cavity is higher than
104°C but the fan motor does not operate.
MICROWAVE Oven seems to be operating but little or no heat is produced in
COOKING oven load. (Microwave power control is set at HIGH)
CONDITION
Oven does not seem to be operating properly during variable
cooking condition. (Oven operates properly at HIGH)
Oven goes into cook cycle but shuts down before end of
cooking cycle.
CONVECTION Convection cooking mode does not heat.
COOKING
CONDITION Oven seems to be operate when the temperature in the oven
cavity is lower or higher than preset one.
GRILL Grill heating element does not heat.
COOKING
CONDITION Convection heating does not stop when the temperature of
oven cavity is higher than 210°C or it stop to heat when the
temperature of oven cavity is lower than 210°C
Oven goes into cook cycle but shuts down before end of
DUAL COOKING cooking cycle.
CONDITION
(COMMON MODE) Oven seems to be operating but little or no heat is produced in
oven load. (Microwave power does not seem to be generated
properly)
DUAL COOKING
Oven seems to be operating but the temperature of oven cavity
CONDITION is lower or higher than preset one.
(MICRO./CONV.)
Convection heating element does not heat.
DUAL COOKING Grill heating does not heat.
CONDITION
(MICRO./GRILL)
R-8740M - 12
J
FUSE 20A
FUSE F2 F8A
GRILL HEATING ELEMENT (TOP)
GRILL HEATING ELEMENT (BOTTOM)
THERMISTOR
K M M N
POWER SUPPLY CORD
SHORTED WIRE HARNESS
OPENED WIRE HARNESS
OVEN LAMP OR SOCKET
TC TRANSFORMER
O
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
R-8740M - 13
P Q R
KEY UNIT
TROUBLESHOOTING GUIDE
TEST PROCEDURES
PROCEDURE
LETTER COMPONENT TEST
A MAGNETRON TEST
NEVER TOUCH ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL WHILE
THE OVEN IS IN OPERATION.
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 IEC-60705 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 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/m2 (equivalent to 5W/m2), 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
R-8740M - 14
TEST PROCEDURES
PROCEDURE
LETTER