Sharp R-770AM Service Manual ▷ View online
4
770AM -
MICROWAVE COOKING CONDITION
100% (HIGH) COOKING
70% (MEDIUM HIGH), 50% (MEDIUM),
30% (MEDIUM LOW), 10% (LOW COOKING)
The following levels of microwave power are given.
SETTING
100%
32 sec. ON
Approx. 100%
70%
24 sec. ON
8 sec. OFF
Approx. 70%
50%
18 sec. ON
14 sec. OFF
Approx. 50%
30%
12 sec. ON
20 sec. OFF
Approx. 30%
10%
6 sec. ON
26 sec. OFF
Approx. 10%
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.
to the percentage of microwave power, because
approx. 3 seconds are needed for heating up the
magnetron filament.
DUAL COOKING CONDITION
Power level
ON time
OFF time
100%
48 sec.
0 sec.
70%
36 sec.
12 sec
50%
26 sec.
22 sec.
30%
16 sec.
32 sec
10%
8 sec.
40 sec
Power level 0% can not be programmed in Dual Cooking
condition.
condition.
LIMITATIONS OF POWER OUTPUT IN MANUAL
OPERATION
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.
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.
Specified cooking
Limited power
Time base
Cooking mode
time (minutes)
output (%)
(seconds)
Microwave
20
70
32
Top grill
30
50
48
Bottom heater
45
50
48
Top grill and Bottom
15 (Top)
50
48
heater or Oven cooking
15 (Bottom)
50
48
Micro. + Top
20 (Micro.)
70
48
grill heater
15 (Grill)
50
48
Microwave +
20 (Micro.)
70
48
bottom heater
15 (Heater)
50
48
NOTE:
1. In case of Automatic operations, the limitations of
1. In case of Automatic operations, the limitations of
power output are not carried out.
2. In case that the stop key is pressed or the oven door is
opened during cooking, the limitations of power output
are carried out after the total cooking time beyond the
specified cooking time.
are 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 carried
out after the total cooking time beyond the specified
cooking time.
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.
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.
power output are carried out.
OPERATION SEQUENCE
D
U
A
L
5
770AM -
DOOR OPEN MECHANISM
DOOR
LATCH HOOK
MONITOR SWITCH
PRIMARY
LATCH SWITCH
LATCH SWITCH
STOP SWITCH
OPEN LEVER
LATCH
HEADS
HEADS
Figure D-1. Door Open Mechanism
FIRE SENSING FEATURE
This model incorporates a sensing feature which will stop
the oven's operation if there is a fire in the oven cavity
during microwave cooking.
This accomplished by the LSI repeatedly measures the
voltage across the temperature measurement circuit (ther-
mistor) during it's 32-seconds time base comparing the
obtained voltage measurements. If the most recent volt-
age measured is 300mV grater than the previous voltage
measured, the LSI judges it as a fire in the oven cavity and
switches off the relays to the high voltage transformer and
fan motor. The LSI also stops counting down. Please refer
to the following section for a more detailed description.
the oven's operation if there is a fire in the oven cavity
during microwave cooking.
This accomplished by the LSI repeatedly measures the
voltage across the temperature measurement circuit (ther-
mistor) during it's 32-seconds time base comparing the
obtained voltage measurements. If the most recent volt-
age measured is 300mV grater than the previous voltage
measured, the LSI judges it as a fire in the oven cavity and
switches off the relays to the high voltage transformer and
fan motor. The LSI also stops counting down. Please refer
to the following section for a more detailed description.
Operation
Please refer to the timing diagrams below.
1. The thermistor operates within a 32-seconds time base
and it is energized for three (3) seconds and off for 29
seconds. Two (2) seconds after the thermistor is
energized, the voltage across the temperature meas-
urement circuit is sampled by the LSI.
seconds. Two (2) seconds after the thermistor is
energized, the voltage across the temperature meas-
urement circuit is sampled by the LSI.
2. The above procedure is repeated. If the difference
between the first voltage measured (in step 1) and the
voltage measured when the procedure is repeated
(step 2) is greater than 300mV the LSI makes the
judgment that there is a fire in the oven cavity and will
switch off the relays to the high voltage transformer and
fan motor. The LSI also stops counting down.
voltage measured when the procedure is repeated
(step 2) is greater than 300mV the LSI makes the
judgment that there is a fire in the oven cavity and will
switch off the relays to the high voltage transformer and
fan motor. The LSI also stops counting down.
CAUTION:
BEFORE REPLACING A BLOWN FUSE
F8A TEST THE MONITORED LATCH
SWITCH, MONITOR SWITCH AND
MONITOR RESISTOR FOR PROPER
OPERATION.
F8A TEST THE MONITORED LATCH
SWITCH, MONITOR SWITCH AND
MONITOR RESISTOR FOR PROPER
OPERATION.
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 proce-
dure letter which will be found in the “Test Procedure”
section.
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 proce-
dure letter which will be found in the “Test Procedure”
section.
TROUBLESHOOTING GUIDE
IMPORTANT: If the oven becomes inoperative because
of a blown fuse F8A in the primary latch
switch - monitor switch circuit, check the
primary latch switch, monitor switch and
monitor resistor before replacing the fuse
F8A.
switch - monitor switch circuit, check the
primary latch switch, monitor switch and
monitor resistor before replacing the fuse
F8A.
3. Once the fire sensor feature has shut the unit down, the
programmed cooking cycle may be resumed by press-
ing the "START" pad or the unit may be reset by
pressing the "CLEAR" pad.
ing the "START" pad or the unit may be reset by
pressing the "CLEAR" pad.
THERMISTOR
SENSING
VOLTAGE
ON
OFF
ON
OFF
0
29
64 (sec.)
3 sec.
Sensing the voltage across temperature mesure circuit.
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 or dual
cooking (Microwave and Convection) mode, the oven
is turned off.
when the oven is started in convection, grill or dual
cooking (Microwave and Convection) 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.
15 seconds because this condition is same as above.
MONITORED
FUNCTION OF IMPORTANT COMPONENTS
6
770AM -
MICROWAVE OUTPUT POWER (IEC-705)
The power output of this oven is rated using the method specified by IEC-705. Full details of how to carry out this
procedure can be found in the Sharp Technical Training notes which is available from Sharp Parts Centre (part
number SERV-LITMW01).
procedure can be found in the Sharp Technical Training notes which is available from Sharp Parts Centre (part
number SERV-LITMW01).
Using this procedure, the heating time to raise 1000g of water by 10
°
C is approximately 50 seconds.
The IEC-705 procedure must be carried out using laboratory-type procedures and equipment. These requirements
make the procedure unsuitable for routing performance checks. An indication of the power being produced by the
oven can however be obtained using the procedure given below.
make the procedure unsuitable for routing performance checks. An indication of the power being produced by the
oven can however be obtained using the procedure given below.
Alternative simplified method:
1. Place 2 litres of cold water (between 12
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 the water temperature as T2.
5. Calculate the output power using the following formula:
4. When the 2 minutes is completed, remove the container and stir the water. Note the water 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 10% of the power rating stated on the rating label.
MICROWAVE LEAKAGE TEST
This oven should be tested for microwave leakage on completion of any repair or adjustment, following the
procedure described in the Sharp Technical Training notes (part number SERV-LITMW01). The maximum leakage
permitted in BS EN 60335-2-25 is 50W/m
procedure described in the Sharp Technical Training notes (part number SERV-LITMW01). The maximum leakage
permitted in BS EN 60335-2-25 is 50W/m
2
(equivalent to 5mW/cm
2
), however it is not normal to detect any
significant leakage, therefore, any leakage which is detected should be investigated.
It is essential that only leakage detectors with current calibration traceable to the National Physical Laboratories are
used.
Suitable leakage detectors :
CELTEC A100
APOLLO X1
APOLLO X1
OUTPUT POWER TEST PROCEDURE
7
770AM -
TEST PROCEDURES
PROCEDURE
LETTER
A
TOUCH CONTROL PANEL ASSEMBLY TEST
COMPONENT TEST
The touch control panel consists of circuits including semiconductors such as LSI, ICs, etc. Therefore,
unlike conventional microwave ovens, proper maintenance can not be performed with only a
voltmeter and ohmmeter.
In this service manual, the touch control panel assembly is divided into two units, Control Unit and Key
Unit, and troubleshooting by replacement is described according to the symptoms indicated.
1. Key Unit Note : Check key unit ribbon connection before replacement.
unlike conventional microwave ovens, proper maintenance can not be performed with only a
voltmeter and ohmmeter.
In this service manual, the touch control panel assembly is divided into two units, Control Unit and Key
Unit, and troubleshooting by replacement is described according to the symptoms indicated.
1. Key Unit Note : Check key unit ribbon connection before replacement.
The following symptoms indicate a defective key unit. Replace the key unit.
a) When touching the pads, a certain pad produces no signal at all.
b) When touching a number pad, two figures or more are displayed.
c) When touching the pads, sometimes a pad produces no signal.
b) When touching a number pad, two figures or more are displayed.
c) When touching the pads, sometimes a pad produces no signal.
2. Control Panel
The following symptoms indicate a defective control unit. Before replacing the control unit.
perform the key unit test (Procedure N) to determine if control unit is faulty.
perform the key unit test (Procedure N) to determine if control unit is faulty.
2-1 In connection with pads
a) When touching the pads, a certain group of pads do not produce a signal.
b) When touching the pads, no pads produce a signal.
b) When touching the pads, no pads produce a signal.
2-2 In connection with indicators
a) At a certain digit, all or some segments do not light up.
b) At a certain digit, brightness is low.
c) Only one indicator does not light up.
d) The corresponding segments of all digits do not light up; or they continue to light up.
e) Wrong figure appears.
f) A certain group of indicators do not light up.
g) The figure of all digits flicker.
b) At a certain digit, brightness is low.
c) Only one indicator does not light up.
d) The corresponding segments of all digits do not light up; or they continue to light up.
e) Wrong figure appears.
f) A certain group of indicators do not light up.
g) The figure of all digits flicker.
2-3 Other possible troubles caused by defective control unit.
a) Buzzer does not sound or continues to sound.
b) Clock does not operate properly.
c) Cooking is not possible.
d) Proper temperature measurement is not obtainned.
b) Clock does not operate properly.
c) Cooking is not possible.
d) Proper temperature measurement is not obtainned.
CARRY OUT 4R CHECKS.
B
KEY UNIT TEST
If the display fails to clear when the STOP/CLEAR pad is depressed, first verify the flat ribbon
cable is marking good contact, verify that the door sensing switch (stop switch) operates properly;
that is the contacts are closed when the door is closed and open when the door is open. If the
door sensing switch (stop switch) is good, disconnecct the flat ribbon cable that connects the key
unit to the control unit and make sure the door sensing switch is closed (either close the door or
short the door sensing switch connecter). Use the Key unit matrix indicated on the control panel
schematic and place a jumper wire between the pins that correspond to the STOP/CLEAR pad
marking momentary contact. If the control unit responds by clearing with a beep the key unit is
faulty and must be replaced. If the control unit does not respond, it is a faulty and must be
replaced. If a specific pad does not respond, the above method may be used (after clearing the
control unit) to determine if the control unit or key pad is at fault.
cable is marking good contact, verify that the door sensing switch (stop switch) operates properly;
that is the contacts are closed when the door is closed and open when the door is open. If the
door sensing switch (stop switch) is good, disconnecct the flat ribbon cable that connects the key
unit to the control unit and make sure the door sensing switch is closed (either close the door or
short the door sensing switch connecter). Use the Key unit matrix indicated on the control panel
schematic and place a jumper wire between the pins that correspond to the STOP/CLEAR pad
marking momentary contact. If the control unit responds by clearing with a beep the key unit is
faulty and must be replaced. If the control unit does not respond, it is a faulty and must be
replaced. If a specific pad does not respond, the above method may be used (after clearing the
control unit) to determine if the control unit or key pad is at fault.
G11
G7
G5,9
G2
G4,12
G10
G6
G3,8
G1
KEY UNIT
PIZZA
GRILL
DUAL
GRILL
AUTO
COOK
OVEN
(C˚)
STOP/
CLEAR
AUTO
START/
CLOCK
START/
AUTO
MINUTE
CONVENI
-ENCE
FOODS
AUTO
DEFROST
MICROW.
POWER
LEVEL
TIMER
Kg / Lb
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