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R-874(W)M
R-874(SL)M
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SERVICE MANUAL
S70199R874M//
MICROWAVE OVEN WITH
GRILL AND CONVECTION
MODELS R-874(W)M
R-874(SL)M
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 .................................................................................................... 2
GENERAL INFORMATION ....................................................................................................................... 3
SERVICING/WARNING ........................................................................................................................... 3
PRODUCT SPECIFICATIONS ................................................................................................................. 4
APPEARANCE VIEW .............................................................................................................................. 5
OPERATION SEQUENCE ........................................................................................................................ 6
FUNCTION OF IMPORTANT COMPONENTS ....................................................................................... 9
TROUBLESHOOTING GUIDE ............................................................................................................... 11
TEST PROCEDURES ........................................................................................................................... 12
TOUCH CONTROL PANEL ASSEMBLY .............................................................................................. 18
COMPONENT REPLACEMENT AND ADJUSTMENT PROCEDURE .................................................. 24
MICROWAVE MEASUREMENT ........................................................................................................... 30
SCHEMATIC DIAGRAMS ...................................................................................................................... 31
PICTORIAL DIAGRAM .......................................................................................................................... 36
POWER UNIT CIRCUIT ......................................................................................................................... 37
CPU UNIT CIRCUIT ............................................................................................................................... 38
PRINTED WIRING BOARD .................................................................................................................... 39
PARTS LIST .......................................................................................................................................... 40
EXPLODED DIAGRAM OF OVEN PARTS ............................................................................................ 43
CONTROL PANEL/DOOR PARTS ......................................................................................................... 44
MISCELLANEOUS/PACKING & ACCESSORIES .................................................................................. 45
SHARP CORPORATION
R-874M - 1
R-874 S/MANUAL 1 11/7/01, 10:59 am
SERVICE MANUAL
SERVICING
PRODUCT SPECIFICATIONS
MICROWAVE OVEN WITH
GRILL AND CONVECTION
R-874(W)M/ R-874(SL)M
GENERAL INFORMATION
GENERAL IMPORTANT INFORMATION
This Manual has been prepared to provide Sharp Corp. Service
engineers with Operation and Service Information. APPEARANCE VIEW
It is recommended that service engineers carefully study the entire text
of this manual, so they will be qualified to render satisfactory customer
service.
OPERATING SEQUENCE
WARNING
MICROWAVE RADIATION
Personnel should not be exposed to the microwave energy FUNCTION OF IMPORTANT
COMPONENTS
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 TROUBLESHOOTING GUIDE
microwave energy absorbing load attached. Never look into an AND TEST PROCEDURE
open waveguide or antenna while the device is energized.
WARNING
TOUCH CONTROL PANEL
Never operate the oven until the following points are ensured.
(A) The door is tightly closed. COMPONENT REPLACEMENT
(B) The door brackets and hinges are not defective. AND ADJUSTMENT PROCEDURE
(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.
MICROWAVE MEASUREMENT
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. WIRING DIAGRAM
Removal of the outer wrap gives access to potentials above 250V.
All the parts marked "" on parts list may cause undue microwave
exposure, by themselves, or when they are damaged, loosened or PARTS LIST
removed.
Never operate the Grill and/ or Bottom heater with the oven
outer cabinet removed. (Because air flow is eliminated, and the
excess heat generated on adjacent components). It can cause
permanent damage or a fire.
SHARP CORPORATION
OSAKA, JAPAN
R-874M - 2
R-874 S/MANUAL 2 11/7/01, 10:59 am
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 harness.
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.
GENERAL INFORMATION
WARNING
THIS APPLIANCE MUST BE EARTHED
IMPORTANT
THE WIRES IN THIS MAINS LEAD ARE COLOURED IN ACCORDANCE WITH THE FOLLOWING CODE:
GREEN-AND-YELLOW : EARTH
BLUE : NEUTRAL
BROWN : LIVE
R-874M - 3
R-874 S/MANUAL 3 11/7/01, 10:59 am
PRODUCT DESCRIPTION
SPECIFICATION
ITEM DESCRIPTION
Power Requirements 230 - 240 Volts / 50 Hertz / Single phase, 3 wire earthed
Power Consumption Microwave cooking 1 .45kW Approx. 6.5 A
Convection cooking 1.53 kW Approx. 6.4 A
Top Grill mode ...................... 1.05 kW Approx. 4.2 A
Grill cooking Bottom Grill mode ................. 0.55 kW Approx. 2.3 A
Top and Bottom mode ......... 1.55 kW Approx. 6.3 A
Micro and Top Grill ................. 2.4 kW Approx. 10.4 A
Micro and Bottom Grill .......... 1.95 kW Approx. 8.5 A
Dual cooking Micro and Convection 2.4 kW Approx. 10.4A
Power Output 900 W nominal of RF microwave energy (measured by method of IEC 60705)
Operating frequency 2450 MHz
Top Grill Grill heating element Power Output 1.0 kW
Bottom heating element Power Output 0.5 kW
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 325 mm
Control Complement Touch Control System
Clock (1:00 - 12:59) / Timer (0 - 99 minutes 90 sec.)
Microwave Power for Variable Cooking
Repetition Rate;
100 (HIGH) ....................... Full power throughout the cooking time
70 (MEDIUM HIGH) ......................... approx. 70% of FULL Power
50 (MEDIUM) ................................... approx. 50% of FULL Power
30 (MEDIUM LOW) .......................... approx. 30% of FULL Power
10 (LOW) ......................................... approx. 10% of FULL Power
Convection temperature control range:
250°C, 230°C, 220°C, 200°C, 180°C, 160°C, 130°C, 100°C, 70°C and 40°C
DISPLAY indicators
TIME keys DUAL COVECTION/GRILL key
INSTANT COOK keys LESS/MORE keys
EXPRESS COOK keys WEIGHT keys
EXPRESS DEFROST keys KITCHEN TIMER/CLOCK SET key
MEAL IN ONE keys INFO key
GRILL key WEIGHT CONVERSION key
MICROWAVE POWER LEVEL key STOP/CLEAR key
0
CONVECTION Ckey START/AUTO MINUTE key
Net Weight Approx. 20 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-874M - 4
R-874 S/MANUAL 4 11/7/01, 10:59 am
APPEARANCE VIEW
OVEN
3 2
10
12 11
1
6 5
8
9 7 10 4
13 15
14
1. Control panel
2. Oven lamp 9. Door seals and sealing surfaces
3. Grill heating element (Top Grill) 10.Ventilation openings
4. Door opening button 11.Outer cabinet
5. Waveguide cover 12.Power supply cord
6. Oven cavity 13.Turntable
7. Turntable motor shaft 14.Low rack
8. Grill heating element 15.High rack
(Bottom Grill)
CONTROL PANEL
COOK indicator showing oven in operation.
DEFROST indicator.
TOP GRILL indicator.
AUTO indicator, showing auto cooking on progress.
BOTTOM GRILL indicator
DIGITAL DISPLAY.
CONVECTION indicator.
LESS/MORE indicators.
MICROWAVE indicator. INFO indicator for interactive display.
WEIGHT indicators. % percentage microwave power level indicator.
TIME keys.
INSTANT COOK keys.
EXPRESS COOK keys.
EXPRESS DEFROST keys.
MEAL IN ONE keys GRILL key.
Press to select Grill
MICROWAVE POWER LEVEL key. DUAL CONVECTION/GRILL key.
0
CONVECTION C key.
LESS/MORE keys.
WEIGHT keys.
KITCHEN TIMER/CLOCK SETkey.
INFO key.
WEIGHT CONVERSION key
STOP/CLEAR key. START/AUTO MINUTE key.
R-874M - 5
R-874 S/MANUAL 5 11/7/01, 10:59 am
OPERATION SEQUENCE
OFF CONDITION the monitored latch switch and monitor switch are made
Closing the door activates the monitored latch switch and open. The circuit to the fan motor is cut off when the relay
the stop switch. RY6 is made open. The circuit to the turntable motor is cut
off when the contacts (COM-NO) of the monitored latch
IMPORTANT: switch are made open. The relay RY2 and RY6 are made
When the oven door is closed, the contacts COM-NC open when the door is opened. The oven lamp remains on
of the monitor switch must be open. When the micro- even if the oven door is opened after the cooking cycle has
wave oven is plugged in a wall outlet (230 - 240V / been interrupted, because the relay RY1 stays closed.
50Hz), the line voltage is supplied to the noise filter. Shown in the display is remaining time.
Figure O-1 on page 30 6. MONITOR SWITCH CIRCUIT
1. The control unit is not energized. The display shows The monitor switch is mechanically controlled by the
nothing (Fig. O-1 (a)). oven door, and monitors the operation of the moni-
2. Open the door. The contacts (COM-NC) of the moni- tored latch switch.
tored latch switch are closed and the control unit is 6-1. When the oven door is opened during or after the
energized. Then contacts of relays RY1 and RY5 are cycle of a cooking program, the monitored latch
closed, and the oven lamp will light and the display will switch and stop switch must open their contacts
show "ENERGY SAVE MODE" (Fig. O-1(b)). (COM-NO) first. And the contacts (COM-NC) of the
3. Close the door. The contacts (COM-NC) of the moni- monitored latch switch are made closed. After that the
tored latch switch are opened and the contacts of relay contacts (COM-NC) of the monitor switch can be
RY1 are opened and the oven lamp will be turned off. closed and the contacts (COM-NO) of monitor switch
The display will show " . 0". (Fig. O-1(c)). are made open.
NOTE: Energy save mode 6-2. When the oven door is closed, the contacts (COM-
1. If the oven has not been used for more than 2 minutes, NC) of the monitor switch SW3 must be opened and
the contacts of the relay RY5 will be opened and the the contacts (COM-NO) of monitor switch must be
control unit will be not energized. Open and close the closed. After that the contacts (COM-NO) of the
door, the control unit will resume. monitored latch switch and the stop switch are made
2. If the clock is set, this energy save mode does not work. closed. And the contacts (COM-NC) of the monitored
latch switch are made open.
MICROWAVE COOKING CONDITION 6-3. When the oven door is opened and the contacts
HIGH COOKING (COM-NO) of the monitored latch switch remain
Enter a desired cooking time by touching the TIME keys closed, the fuse F8A will blow. Because the relay RY1
and start the oven by touching START key. and monitor switch are closed and a short circuit is
caused.
Function sequence Figure O-2 on page 31
CONNECTED COMPONENTS RELAY MEDIUM HIGH, MEDIUM, MEDIUM LOW, LOW
Oven lamp, Turntable motor RY1 COOKING
High voltage transformer RY2 When the microwave oven is preset for variable cooking
Fan motor RY6 power, the line voltage is supplied to the high voltage
1. The line voltage is supplied to the primary winding of transformer intermittently within a 32-second time base
the high voltage transformer. The voltage is converted through the relay contact which is coupled with the cur-
to about 3.3 volts A.C. output on the filament winding rent-limiting relay RY2. The following levels of microwave
and high voltage of approximately 2000 volts A.C. on power are given.
32 sec. ON
the secondary winding. 100%
900W (HIGH)
2. The filament winding voltage (3.3 volts) heats the
magnetron filament and the high voltage (2000 volts) is 24 sec. ON 8 sec. OFF
630W MEDIUM Approx. 70%
sent to the voltage doubling circuit, where it is doubled HIGH
to negative voltage of approximately 4000 volts D.C.. 18 sec. ON 14 sec. OFF
3. The 2450 MHz microwave energy produced in the 450W (MEDIUM) Approx. 50%
magnetron generates a wavelength of 12.24 cm. This 12 sec. ON 20 sec. OFF
energy is channelled through the waveguide (transport 270W MEDIUM Approx. 30%
LOW
channel) into the oven cavity, where the food is placed 6 sec. ON 26 sec. OFF
to be cooked. 90W (LOW) Approx. 10%
4. When the cooking time is up, a signal tone is heard and
the relays RY1 + RY2 + RY6 go back to their home Note: The On/Off time ratio does not exactly correspond
position. The circuits to the oven lamp, high voltage to the percentage of microwave power, because
transformer, fan motor and turntable motor are cut off. approx. 3 seconds are needed for heating up the
5. When the oven door is opened during a cooking cycle, magnetron filament.
the switches come to the following condition.
Switch Contact Condition GRILL COOKING CONDITION
During Oven Door TOP GRILL (Figure O-3a)
Cooking Open(No cooking) In this condition the food is cooked by the top grill heating
COM-NO Closed Opened element . Programme the desired cooking time by touch-
Monitored latch switch
COM-NC Opened Closed ing the TIME keys and touh the GRILL key once. When the
Stop switch COM-NO Closed Opened STARTkey is touched, the following operations occur:
COM-NO Closed Opened 1. The numbers on the digital readout start the count
Monitor Switch
COM-NC Opened Closed down to zero.
The circuit to the high voltage transformer is cut off when 2. The oven lamp, cooling fan motor and turntable motor
the contacts of relay RY2, and the contacts (COM-NO) of are energized.
3. The relay RY3 is energized and the main supply
R-874M - 6
R-874 S/MANUAL 6 11/7/01, 10:59 am
OPERATION SEQUENCE
voltage is applied to the top grill heating element . temperature by touching the CONVECTION TEMPERA-
4. Now, the food is cooked by the top grill heating ele- TURE key . When the START key is touched, the following
ment. operations occur:
1. The numbers on the digital readout start the count
BOTTOM GRILL (Figure O-3b) down to zero.
In this condition the food is cooked by bottom grill heating 2. The oven lamp, cooling fan motor, turntable motor and
element energy. Programme the desired cooking time by convection motor are energized.
touching the TIME keys and touh the GRILL key twice.
When the STARTkey is touched, the following operations NOTE: When the convection temperature 40°C is se-
occur: lected, the convection motor is not energized.
1. The numbers on the digital readout start the count 3. The relays RY3 and RY4 are energized (if the cavity
down to zero. temperature is lower than selected temperature) and
2. The oven lamp, cooling fan motor and turntable motor the main supply voltage is applied to the top and
are energized. bottom heating elements to return to the selected
cooking temperature.
3. The relay RY4 is energized and the main supply 4. Upon completion of the cooking time, the audible sig-
voltage is applied to the bottom grill heating element . nal will sound, and the oven lamp, turntable motor,
4. Now, the food is cooked by the bottom grill heating cooling fan motor, convection motor, top and bottom
element . heating elements are de-energized. At the end of con-
vection cycle, if the cavity air temperature rise above
TOP AND BOTTOM GRILL (Figure O-3c)
120°C, the circuit to the relay RY6 will be maintained
In this condition the food is cooked by top and bottom grill
(by the thermistor circuit) to continue operation of the
heating elements energy. Programme the desired cook-
cooling fan motor until temperature drops below 105°C,
ing time by touching the TIME keys and touh the GRILL
at that time the relay RY6 will be de-energized, turning
key three times. When the STARTkey is touched, the
off the fan motor.
following operations occur:
1. The numbers on the digital readout start the count DUAL COOKING CONDITION
down to zero. MICROWAVE AND CONVECTION (Figure O-5a)
2. The oven lamp, cooling fan motor and turntable motor Programme the desired cooking time by touching the
are energized. TIME keys. Touch the DUAL CONVECTION/GRILL key
3. The relay RY4 is energized and the main supply once. Select the microwave power level by touching the
voltage is applied to the bottom grill heating element. MICROWAVE POWER LEVEL key. And select the con-
4. The relay RY3 is energized and the main supply vection temperature by touching the CONVECTION TEM-
voltage is applied to the top grill heating element. PERATURE key.
5. Now, the food is cooked by the top and bottom grill NOTE: The microwave power level can be selected from
heating elements. 90W, 270W and 450W.
When the STARTkey is touched, the following operations
CONVECTION COOKING CONDITION occur:
PRE-HEATING (Figure O-4) 1. The numbers on the digital readout start the count
Programme the desired convection temperature by touch- down to zero.
ing CONVECTION TEMPERATURE key. When the 2. The oven lamp, cooling fan motor, turntable motor and
STARTkey is touched, the following operations occur: convection motor are energized.
1. The coil shut-off relays RY1 and RY6 are energized,
the oven lamp, cooling fan motor and turntable motor 3. The relay RY3 will be energized and the main supply
are turned on. voltage is applied to the top grill heatier.
2. The coil shut-off relays RY3 RY 7 and RY4 are 4. The relay RY4 is energized (if the cavity temperature is
energized by control unit and the main supply voltage lower than selected temperature) and the main supply
is added to the top and bottom heating elements and voltage is applied to the bottom heating elementer.
covection motor. 5. The relay RY2 is energized and the microwave energy
3. When the oven temperature reaches the selected pre- is generated by magnetron.
heat temperature, the following operations occur: 6. Now, the food is cooked by microwave and convection
3-1. The coil shut-off relays RY3 and RY4 are energized energy simultaneously.
by control unit temperature circuit and thermistor,
opening the circuit to the top and bottom grill heating MICROWAVE AND TOP GRILL (Figure O-5b)
elements and convection motor. Programme the desired cooking time by touching the
3-2. The oven will continue to function for 30 minutes, TIME keys. Touch the DUAL CONVECTION/GRILLL key
turning the top and bottom heating elements on and twice. Select the microwave power level by touching the
off, as needed to maintain the selected pre-heat MICROWAVE POWER LEVEL key. When the STARTkey
temperature. The ovenwill shut-down completely af- is touched, the following operations occur:
ter 30 minutes. 1. The numbers on the digital readout start the count
NOTE: The fan motor will be turned on and off at down to zero.
preheating. And the convection motor will be turned 2. The oven lamp, cooling fan motor and turntable motor
off at preheating. See "Power setting" on page 9. are energized.
3. The relay RY3 is energized and the main supply
CONVECTION COOKING (Figure O-4) voltage is applied to the top grill heating element.
When the pre-heat temperature is reached, a beep signal
will sound indicating that the holding the temperature has 4. The relay RY2 is energized and the microwave energy
been reached in the oven cavity. Open the door and place is generated by magnetron.
the food to cooked in the oven. Program desired cooking 5. Now, the food is cooked by microwave and top grill
time by touching the TIME keys and program convection
R-874M - 7
R-874 S/MANUAL 7 11/7/01, 10:59 am
OPERATION SEQUENCE
simultaneously. POWER OUTPUT ON TIME OFF TIME
MICROWAVE AND BOTTOM GRILL (Figure O-5c) 100% 48 sec. 0 sec.
90% 44 sec. 4 sec.
Programme the desired cooking time by touching the
TIME keys. Touch the DUAL CONVECTION/GRILL three 80% 40 sec. 8 sec.
times. Select the microwave power level by touching the 70% 36 sec. 12 sec.
MICROWAVE POWER LEVEL key. When the STARTkey 60% 32 sec. 16 sec.
is touched, the following operations occur:
50% 26 sec. 22 sec.
1. The numbers on the digital readout start the count
down to zero. 40% 22 sec. 26 sec.
2. The oven lamp, cooling fan motor and turntable motor 30% 16 sec. 32 sec.
are energized. 20% 12 sec. 36 sec.
3. The relay RY4 is energized and the main supply 10% 8 sec. 40 sec.
voltage is applied to the bottom grill heating element.
4. The relay RY2 is energized and the microwave energy AUTOMATIC COOKING
is generated by magnetron.
5. Now, the food is cooked by microwave and bottom grill PIZZA
simultaneously. BREAKFAST
INSTANT COOK
POWER SETTING AUTO COOK
AUTO DEFROST
Following table shows each power setting of the top Above functions are automatic cooking. They automati-
heating element, bottom heating element, magnetron, fan cally work out the correct cooking mode and cooking time
motor and convection motor on Convection cooking mode, and/or cooking temperature. They will cook according to
Dual (Micro + Convec.) mode and Convection preheating the special cooking sequence.
mode. CONV. COOKING DUAL/CONVEC. COOKING CONVE
TOP BOTTOM TOP BOTTOM MAGNETRON FAN CTION
HEATER HEATER HEATER HEATER (MICRO) MOTOR MOTOR
LIMITATIONS OF POWER OUTPUT IN MANUAL
OPERATION
250°C 70% 70% 70% 70% 10, 30, 50% 100% 100%
230°C 60% 70% 50% 70% 10, 30, 50% 100% 100% After the same cooking mode is carried out for more than
220°C 40% 70% 40% 70% 10, 30, 50% 100% 100%
the specified cooking time, the power output is automati-
cally reduced by turning the control relays on and off
200°C 40% 60% 40% 60% 10, 30, 50% 100% 100%
intermittently, as shown in the table below. This is to
180°C 40% 50% 40% 50% 10, 30, 50% 100% 100% protect the oven door against temperature rising.
160°C 20% 40% 20% 40% 10, 30, 50% 100% 100%
130°C 20% 30% 20% 30% 10, 30, 50% 100% 100% Specified cooking Limited power Time base
100°C 20% 20% 20% 20% 10, 30, 50% 100% 100% Cooking mode time (minutes) output (%) (seconds)
70°C 10% 20% 10% 20% 10, 30, 50% 100% 100% Microwave (100%) 20 70 32
40°C 10% 10% 10% 10% 10, 30, 50% 100% 0% Top grill 30 50 48
CONVECTION PREHEATING Bottom grill 45 50 48
TOP HEATER BOTTOM HEATER FAN MOTOR CONV. MOTOR Top grill and 15 (Top) 50 48
250°C 100% 100% 60% 0%
Bottom grill 15(Bottom) 50 48
230°C 100% 100% 60% 0% Micro. (100%) 20 (Micro.) 70 48
D
220°C 100% 100% 60% 0% U + Top grill 15 (Grill) 50 48
A
200°C 100% 100% 60% 0% L Micro. (100%) 20 (Micro.) 70 48
180°C 100% 100% 60% 0% + Bottom grill 15 (Heater) 50 48
160°C 100% 100% 60% 0%
NOTE:
130°C 100% 100% 60% 0%
1. In case of Automatic operations, the limitations of
100°C 100% 100% 60% 0% power output are not carried out.
70°C 100% 100% 60% 0% 2. In case that the stop key is touched or the oven door
40°C 100% 100% 60% 0% is opened during cooking, the limitations of power
output are not carried out after the total cooking time
beyond the specified cooking time.
ON/OFF TIME RATIO 3. In case of the two or more same cooking modes are
In grill cooking, convection cooking or dual cooking, the carried out, the limitations of power output are not
top heater, bottom heater or magnetron operate whithin a carried out after the total cooking time beyond the
48 second time base. The following table is the ON / OFF specified cooking time.
time ratio at each power output of the top heaters, bottom 4. In case of the two or more different cooking modes are
heater or magnetron. 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
R-874M - 8
R-874 S/MANUAL 8 11/7/01, 10:59 am
FUNCTION OF IMPORTANT COMPONENTS
DOOR OPEN MECHANISM circuited, this fuse 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 FUSE F2 F8A 250V
lever pushes the lower latch head on the door upward.
The upper latch head is linked with the lower latch head, 1. If the wire harness or electrical components are short-
so now, the door can be opened. circuited, this fuse blows to prevent an electric shock or
fire hazard.
DOOR LATCH HOOK 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.
MONITOR SWITCH
LATCH
3. The fuse also blows when the asymmetric rectifier,
HEADS MONITORED H.V. rectifier,.H.V. wire harness, H.V. capacitor,
LATCH SWITCH 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.
STOP SWITCH
THERMAL CUT-OUT 125°C (MG)
OPEN LEVER
This thermal cut-out protects the magnetron against over-
Figure D-1. Door Open Mechanism heat. If the temperature goes up higher than 125°C be-
MONITORED LATCH SWITCH cause the fan motor is interrupted or the ventilation open-
ings are blocked, the thermal cut-out will open and line
1. When the oven door is closed, the contacts (COM-NO) voltage to the high voltage transformer will cut off and
of the switch must be closed. And the contacts (COM- operation of the magnetron will be stopped. The defec-
NC) must be opened. tive thermal cut-out must be replaced with a new one.
2. When the oven door is opened, the contacts (COM-
NO) of the switch must be opened. And the contacts THERMAL CUT-OUT 150°C (OVEN)
(COM-NC) must be closed.
This thermal cut-out protects the oven against the over-
STOP SWITCH heat during grill cooking, convection cooking or dual
(combination) cooking. If the temperature rises above
1. When the oven door is closed, the contacts (COM-NO) 150°C because the fan motor is interrupted, the air inlet
of the switch must be closed. duct is blocked or the ventilation openings are obstructed,
2. When the oven door is opened, the contacts (COM- the thermal cut-out opens and switches off all the electrical
NO) of switch must be opened. parts. When the cools itself down to the operating tem-
perature of 130°C, the contacts of the thermal cut-out will
MONITOR SWITCH close again.
The monitor switch is activated (the contacts opened) by the
upper latch head on the door while the door is closed. The ASYMMETRIC RECTIFIER
switch is intended to render the oven inoperative by means of The asymmetric rectifier is a solid state device that pre-
blowing the fuse F8A when the contacts of the monitored vents current flow in both directions. It prevents the
latch switch fail to open when the door is opened. overheating of the high voltage transformer by blowing the
Function fuseF8A when the high voltage rectifier is shorted.
1. When the door is opened, the contacts (COM-NC) of
monitor switch close (to the ON condition) due to their The rated peak reverse voltage of D1 of the asymmetric
being normally closed and contacts (COM-NO) open. rectifier is 6 KV. The rated peak reverse voltage of D2 of
At this time the contacts (COM-NO) of monitored latch the asymmetric rectifier is 1.7 KV. D1 and D2 of the
switch is in the OFF condition (contacts open) due to asymmetric rectifier or high voltage rectifier are shorted
their being normally open contact switches. when the each peak reverse voltage goes beyond the
2. As the door goes to a closed position, the monitor each rated peak reverse voltage. (The process of the
switch contacts (COM-NC) are opened and contacts blowing the fuse F8A.)
(COM-NO) closed and then contacts (COM-NO) of
monitored latch switch and stop switch are closed.(On
D2 D1
opening the door, each of these switches operate
inversely.)
3. If the door is opened and the monitored latch switch ASYMMETRIC
RECTIFIER
contacts (COM-NO) fail to open, the fuse F8A blows HIGH VOLTAGE
immediately after closing of the monitor switch (COM- RECTIFIER
NC) contacts. 1. The high voltage rectifier is shorted by some fault when
CAUTION: BEFORE REPLACING A BLOWN FUSE F8A, microwave cooking or dual cooking.
TEST THE MONITORED LATCH SWITCH 2. The peak reverse voltage of D2 of the rectifier goes
AND MONITOR SWITCH FOR PROPER beyond the rated peak reverse voltage 1.7 KV in the
OPERATION. (REFER TO CHAPTER "TEST voltage doubler circuit.
PROCEDURE"). 3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-
FUSE F1 20A 250V age winding of the high voltage transformer.
If the wire harness or electrical components are short- 5. The large electric currents beyond 8A flow through the
R-874M - 9
R-874 S/MANUAL 9 11/7/01, 10:59 am
FUNCTION OF IMPORTANT COMPONENTS
primary winding of the high voltage transformer. Door Oven Cavity Grill Heating
6. The fuse F8A blows by the large electric currents. Element Convection
7. The power supplying to the high voltage transformer is Passage
cut off.
NOISE FILTER
The noise filter assembly prevents radio frequency inter-
Cooling
ference that might flow back in the power circuit. Fan
TURNTABLE MOTOR
The turntable motor rotates the turntable. Convection
Motor
FAN MOTOR
Convection
The fan motor drives a blade which draws external cool air. Fan
Turntable Turntable
This cool air is directed through the air vanes surrounding Motor Insulation Bottom Grill
the magnetron and cools the magnetron. This air is chan-
Heating Element
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. Figure D-2. Convection Cooking System
CONVECTION MOTOR FIRE SENSING FEATURE
The convection motor drives the convection fan and The oven will stop its operation when there is a fire in the
provide the heated air. oven cavity in microwave cooking condition.
LSI measures the voltage across the temperature meas-
TOP GRILL HEATING ELEMENT urement circuit intermittently within 32-seconds time base
since the oven is started in microwave cooking condition.
The grill heating element is provided to brown the food and The oven will stop its operation when the difference of the
is located on the top of the oven cavity. voltage is more than 0.781 volts in microwave cooking
condition.
BOTTOM GRILL HEATING ELEMENT
1. Within a 32-seconds base, the thermistor is energized
The grill heating element is provided to brown the food and for 2 seconds. At that time, the voltage across the
is located at the base of the oven cavity. temperature measurement circuit is measured.
CONVECTION COOKING SYSTEM 2. The oven carries out the procedure above again. If the
second voltage is 0.781V higher than first voltage, LSI
This oven is designed with a hot air heating system where judges it is a fire in the oven cavity and stop the oven.
food is heated by forced circulation of the hot air produced
by the grill heaters. The air heated by the grill heating 3. When LSI judges it is a fire in the oven cavity, LSI will
elements is circulated through the convection passage switch off the relays to high voltage transformer and fan
provided on the outer casing of the oven cavity by means motor and LSI stops counting down.
of the convection fan which is driven by the convection 30 32 (sec.)
motor. It is then enters the inside of the oven through the THERMISTOR ON
vent holes provided on the back side of the oven. Next, the
OFF
hot air heats the food on the turntable and leaves the oven
cavity through the vent in the oven cavity rear wall. In this Sensing
ON
way, the hot air circulates inside the oven cavity to raise its Voltage OFF
temperature and, at the same time, comes into contact
with the food being cooked. When the temperature inside Sensing the voltage across temperature measurement circuit.
the oven cavity reaches the selected temperature, the OPEN JUDGE BY THERMISTOR
heating elements are de-energized. When the tempera-
ture inside the oven cavity drops below the selected 1. If the temperature of the thermistor does not rise to
temperature, the heating elements are energized again. In more than 40°C after 4 minutes and 15 seconds from
this way, the inside of the oven cavity is maintained at when the oven is started in convection, grill (top and
approximately the selected temperature. When the con- bottom grills) or dual cooking mode, the oven is turned
vection time reaches "0", the heating elements are de- off.
energized and the convection fan stops operating and the 2. When the thermistor or the wire harness to the thermis-
oven shuts off. At that time if the cavity air temperature has tor is opened, the oven is turned off after 4 minutes and
rised above120°C, the fan motor remains rotating. Auto- 15 seconds because this condition is same as above.
matically the fan motor will be shut down at low tempera-
ture (less than 105°C).
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-874M - 10
R-874 S/MANUAL 10 11/7/01, 10:59 am
R-874 S/MANUAL
TEST PROCEDURE 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
KEY UNIT
RELAY RY1
RELAY RY2
RELAY RY3