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Model
R-8740 (serv.man17)
Pages
48
Size
1.46 MB
Type
PDF
Document
Service Manual
Brand
Device
Home Appliance / Microwave Oven / Complete
File
r-8740-sm17.pdf
Date

Sharp R-8740 (serv.man17) Service Manual ▷ View online

R-8740M  - 6
APPEARANCE VIEW
OVEN
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:
4. NUMBER keys for time and memory
programming
5. START key
6. STOP key
7. SOUND key
8. BOTTOM GRILL key
9. TOP GRILL key
10. CONVECTION key
To change the convection temperature
11. MICROWAVE key
To change the microwave power setting
12. MANUAL key
13. MEMORY key
14. TIME REDUCE key
14
13
12
11
10
9
8
7
6
1
2
3
4
5
7
10
4
1
8
6
5
3
2
10
12
11
9. Door seals and sealing surfaces
10. Ventilation openings
11. Outer cabinet
12. Power supply cord
13. Turntable
14. Handle
1. Control panel
2. Oven lamp
3. Grill heating element (Top Grill)
4. Door opening button
5. Waveguide cover
6. Oven cavity
7. Turntable motor shaft
8. Grill heating element
(Bottom Grill)
14
13
R-8740M  - 7
 OPERATION SEQUENCE
OFF CONDITION
Closing the oven door activates all door interlock switches
(Monitored latch switch and stop switch.
IMPORTANT:
When the oven door is closed, the contacts COM-NC of the
monitor switch SW2 must be open and the contacts (COM-
NO) must be closed. When the microwave oven is plugged
in a wall outlet (230V / 50Hz), 230 volts A.C. is supplied to
the point A9 + A11 in the control unit.
Figure O-1 on page 35
1. The display flashes ".0"
2. To set any programmes or set the clock, you must first
"put the oven in MANUAL MODE .  Press the MANUAL
key for 6-10 seconds. (oven will 'beep').  Press the
MANUAL key once more.  The oven is now in manual
mode (this will be indicated in the display window, above
the         symbol).
NOTE: When the oven door is opened, the oven lamp
comes on at this time.
MICROWAVE COOKING CONDITION
HIGH COOKING
Enter a desired cooking time with the number keys, press
the " MICROWAVE " 
  key, then press the START key 
.
Function sequence Figure O-2 on page 35
CONNECTED COMPONENTS
RELAY
Oven lamp, Turntable motor
RY1
High voltage transformer
RY2
Fan motor
RY5
1. 230 volts A.C. is supplied to the primary winding of the
high voltage transformer. The voltage is converted to
about 3.3 volts A.C. output on the filament winding and
high voltage of approximately 2000 volts A.C. on the
secondary winding.
2. The filament winding voltage (3.3 volts) heats the
magnetron filament and the high voltage (2000 volts) is
sent to the voltage doubling circuit, where it is doubled to
negative voltage of approximately 4000 volts D.C..
3. The 2450 MHz microwave energy produced in the
magnetron generates a wave length of 12.24 cm. This
energy is channelled through the waveguide (transport
channel) into the oven cavity, where the food is placed to
be cooked.
4. When the cooking time is up, a signal tone is heard and
the relays RY1 + RY2 + RY5 go back to their home
position. The circuits to the oven lamp, high voltage
transformer, fan motor and turntable motor are cut off.
5. When the oven door is opened during a cooking cycle,
the switches come to the following condition.
Switch
Contact
          Condition
During
Oven Door
Cooking
Open(No cooking)
Monitored latch Switch COM-NO
Closed
Opened
Monitor Switch
COM-NO
Closed
Opened
Monitor Switch
COM-NC
Opened
Closed
Stop switch
COM-NO
Closed
Opened
The circuit to the high voltage transformer, is cut off when
the contact of relay RY2 contacts COM-NO of the monitored
latch switch, and monitor switch SW2 are made open. The
circuit to the fan motor is cut off when the relay RY5 is made
open. The circuit to the turntable motor is cut off when the
contacts COM-NO of the monitored latch switch are made
open. The relays RY2 and RY5 are made open when the
door is opened. The oven lamp remains on even if the oven
door is opened after the cooking cycle has been interrupted,
because the relay RY1  stay closed. Shown in the display is
remaining time.
6. MONITOR SWITCH CIRCUIT
The monitor switch SW2 is mechanically controlled by
the oven door, and monitors the operation of the
monitored latch switch SW1.
6-1. When the oven door is opened during or after the cycle
of a cooking program, the monitored latch switch SW1
and stop switch SW3 must open their contacts (COM-
NO) first. After that the contacts (COM-NC) of the
monitor switch SW2 can be closed and the contacts
(COM-NO) of monitor switch SW2 are made open.
6-2. When the oven door is closed. The contacts (COM-NC)
of the monitor switch SW2 must be opened and the
contacts (COM-NO) of monitor switch SW2 must be
closed.  After that the contacts of the monitored latch
switch SW1 and the stop switch SW3 are made closed.
6-3. When the oven door is opened and the contacts of the
monitored latch switch SW1 remain closed, the fuse F2
F8A will blow. Because the relay RY1 and monitor
switch SW2 are closed  and a short circuit is caused.
CONVECTION COOKING CONDITION
PREHEATING CONDITION (Figure O-4)
Program desired convection temperature by touching
"MANUAL", then the CONVECTION key. When the START key
is touched, the following operations occur:
1. The coil of shut-off relays RY1, RY5 and RY7 are energized,
the oven lamp, cooling fan motor, turntable motor and
convection motor are turned on.
2. The coil of shut-off relays RY3  and RY4 are energized by
the control unit and the main supply voltage is added to the
MEDIUM HIGH, MEDIUM, MEDIUM LOW, LOW
COOKING
When the microwave oven is preset for variable cooking
power, the 230 volts A.C. power is supplied to the high voltage
transformer intermittently within a 32-second time base through
the relay contact which is coupled with the current-limiting relay
RY2. The following levels of microwave power are given.
Note: The On/Off time ratio does not exactly correspond to the
percentage of microwave power, because approx. 3
seconds  are needed for heating up the magnetron
filament.
32 sec. ON
24 sec. ON
18 sec. ON
12 sec. ON
6 sec. ON
8 sec. OFF
14 sec. OFF
20 sec. OFF
26 sec. OFF
HIGH
MEDIUM HIGH
MEDIUM
MEDIUM LOW
LOW
Approx. 70% = 630 Watts
100%
Approx. 50% = 450 Watts
Approx. 30% = 270 Watts
Approx. 10% = 90 Watts
 = 900 Watts
R-8740M  - 8
top and bottom heater.
3. When the oven temperature reaches the selected preheat
temperature, the following operations occur;
3-1. The shut-off relays RY3 and RY4 are de-energized by the
control unit temperature circuit and thermistor, opening
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
maintain the selected preheat temperature. The oven will
shut-down completely after 15 minutes.
CONVECTION COOKING CONDITION (Figure O-4)
When the preheat temperature is reached, a beep signal  will
sound indicating that the holding temperature has been reached
in the oven cavity. Open the door and place the food to be cooked
in the oven. Program desired cooking time  then touch the
CONVECTION key repeatedly to set the temperature.  When
the START key is touched, the following operations occur:
1. The numbers of the digital readout start the count down to
zero.
2. The oven lamp, cooling fan motor, turntable motor and
convection motor are energized.
3. Relays RY3 and RY4 are energized (if the cavity temperature
is lower than the selected temperature) and the main supply
voltage is applied to the top and bottom heaters to return to
the selected cooking temperature.
4. Upon completion of the cooking time, the audible signal will
sound, and oven lamp, turntable motor, cooling fan motor
and convection motor are de-energized. At the end of
convection cycle, if the cavity air temperature is above
104˚C, the circuit to RY5 will be maintained (by the thermistor
circuit) to continue operation of the cooling fan motor until
temperature drops below 104˚C, at that time the relay will
be de-energized, turning off the fan motor .
GRILL COOKING CONDITION
TOP GRILL (Figure O-3a)
In this condition the food is cooked by grill heating element
energy. Programme the desired cooking time and top grill mode
by turning the mode select knob to           " TOP GRILL" and the
second rotary knob to set the time. 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 and turntable motor are
energized.
3. The relay RY3 is energized and the main supply voltage is
applied to the top grill heater.
4. Now the food is cooked by the top grill heater.
GRILL COOKING CONDITION
BOTTOM GRILL (Figure O-3b)
In this condition the food is cooked by bottom grill heating
element energy.  Program the desired cooking time and bottom
grill mode by turning the mode select knob to
 
  "BOTTOM  GRILL" and the second rotary knob to set
the time.  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 and turntable motor are
energized.
3. The relay RY4 is energized and the main supply voltage is
applied to the bottom grill heater.
4. Now the food is cooked by the bottom grill heater.
GRILL COOKING CONDITION
TOP AND BOTTOM GRILL (Figure O-3c)
In this condition the food is cooked by the top and bottom grill
heating element energy.  Program the desired cooking time
then set top+bottom grill mode by touching  
  "TOP GRILL"
and   
  "BOTTOM GRILL"keys.  (led's will light on the
display panel).
  "TOP & BOTTOM GRILL" .  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 and turntable motor are
energized.
3. The relay RY4 is energized and the main supply voltage is
applied to the bottom grill heater.
4. The relay RY3 is energized and the main supply voltage is
applied to the top grill heater.
5. Now the food is cooked by the top+bottom grill heater.
DUAL COOKING CONDITION
MICROWAVE AND CONVECTION (Figure O-5a)
Program the desired cooking time and then Dual cook mode
by touching the CONVECTION  
  and MICROWAVE keys.
(led's will light on the display panel).  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.
3. Relay RY4 is energized (if the cavity temperature is lower
than the selected temperature) and the main supply voltage
is applied to the bottom heater.
4. Relay RY2 is energized and the microwave energy is
generated by the magnetron.
5. During operation relay RY2 will de-energize and relay RY3
will be energized and the main supply voltage is applied to
the top grill heater.  This will continue to cycle throughout
this mode.
6. Now the food is cooked by microwave and convection
energy simultaneously.
DUAL COOKING CONDITION
MICROWAVE AND TOP GRILL (Figure O-5b)
Program the desired cooking time and then Dual cook mode
by touching the "TOP GRILL"  
  and MICROWAVE keys
(led's will light on display panel).  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 and turntable motor are
energized.
3. Relay RY3 is energized and the main supply voltage is
applied to the top heater.
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.
 OPERATION SEQUENCE
R-8740M  - 9
FUNCTION OF IMPORTANT COMPONENTS
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 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 SW1 AND STOP
SWITCH  SW3
1. When the oven door is closed, the contacts COM-NO of
each switch must be closed.
2. When the oven door is opened, the contacts COM-NO of
each switch must be opened.
MONITOR SWITCH  SW2
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 F2 F8A when the contacts of the moni-
tored latch switch SW1 fail to open when the door is opened.
LATCH
HEADS
LATCH HOOK
MONITORED LATCH
SWITCH
STOP SWITCH
OPEN LEVER
DOOR OPEN BUTTON
MONITOR 
SWITCH
FUSE F1 20A 250V
If the wire harness or electrical components are short-circuited,
this fuse F1 20A 250V blows to prevent an electric shock or fire
hazard.
FUSE F2 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 monitored latch switch SW1
remains closed with the oven door open and when the
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
TC TRANSFORMER
TC transformer converts A.C. line voltage into low voltage to
drive the control unit.
THERMAL CUT-OUT TC1 150˚C (OVEN)
The thermal cut out protects the oven against overheat during
grill cooking, convection cooking or dual (combination) cook-
ing. 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 the all electrical parts.
When the oven cools itself down to the operating temperature
of 130˚C, the contacts of the thermal cut-out will close again.
THERMAL CUT-OUT TC2 125˚C  (MG)
This thermal cut-out protects the magnetron against overheat.
If the temperature goes up higher than 125˚C because the fan
motor is interrupted or the ventilation openings are blocked,
the thermal cut-out TC2 will open and line voltage to the high
voltage transformer T will cut off and operation of the magnetron
MG will be stopped. The defective thermal cut-out must be
replaced with a new one.
DUAL COOKING CONDITION
MICROWAVE AND BOTTOM GRILL (Figure O-5c)
Program the desired cooking time and then touch the
"BOTTOM GRILL"   
   and MICROWAVE keys (led's will
light on the display panel). 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 and turntable motor are
energized.
3. Relay RY4 is energized and the main supply voltage is
applied to the bottom heater.
4. Relay RY2 is energized and the microwave energy is
generated by the magnetron.
5. Now the food is cooked by microwave and bottom grill
simultaneously.
AUTO COOK
When the number keys are pressed (not in "MANUAL" 
mode). the oven chooses cooking mode (Microwave, Convec-
tion, Grill or Dual) and sets cooking time automatically accord-
ing to the pre-programmed information in IC-1 (LSI).
Function
1. When the door is opened, the contacts (COM-NC) of
monitor switch SW2 close (to the ON condition) due to their
being normally closed and contacts (COM-NO) open. At
this time the monitored latch SW1 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
SW2 contacts (COM-NC) are opened and contacts (COM-
NO) closed  and then the monitored latch switch and stop
switch contacts close.(On opening the door, each of these
switches operate inversely.)
3. If the door is opened and the monitored latch switch  SW1
contacts fail to open, the fuse F2 (F8A) blows immediately
after closing of the monitor switch (COM-NC) contacts.
CAUTION: BEFORE REPLACING A BLOWN FUSE F2 F8A
TEST THE MONITORED LATCH  SWITCH,
MONITOR SWITCH  FOR PROPER OPERA-
TION.
(REFER TO CHAPTER “TEST PROCEDURE”).
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