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R-959(SL)M-AA

6 – 3

Damper position is set automatically by damper motor, damper switch
and motor cam. These components are operated by a signal that
judges if microwave cooking or convection cooking operation is
selected by the CPU unit.

Damper is in the open position, because a portion of cooling air is
channelled through the cavity to remove steam and vapours given off
from the heating foods. It is then exhausted at the top of the oven cav-
ity into a condensation compartment.

Damper is in the closed position, so that no hot air will be allowed to
leak out the oven cavity.

1. When power supply cord is plugged in or when the control unit

resumes after energy save mode finishes:

1) When power supply cord is plugged in, a signal is sensed in the

control unit, and operates shut-off relay (RY8).

2) Contacts of shut-off relay (RY8) close, the damper motor is

energized, opening the damper door.

3) When the damper is moved to the open position by the damper

cam, damper switch SW4 is closed (ON position).

4) The signal of damper switch is re-sensed in the control unit and

shut-off relay (RY8) is turned off.

5) The rated voltage to the damper motor is stopped and the motor

turns off.

2. When oven is microwave cooking: Damper is in the open position

3. When oven is convection cooking:

1) Damper motor is energized right after the oven is started.

2) When damper is in the closed position (damper switch is OFF),

its signal is sensed by the control unit, and shut-off relay (RY8)
is de-energized.

3) The damper is held in the closed position during the convection

cooking operation.

4) At the end of the convection cooking, when the fan motor stops,

shut-off relay (RY8) is energized, and the damper is returned to
the open position.

NOTE: If the damper door is not in the proper position, closed during

convection, grill or dual, or open during microwave, the control
unit will stop oven operation after 1 minute.

4. Operation of damper is shown below.

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d

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M

 

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o

C

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p

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o

c

 

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v

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w

o

r

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M

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k

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C
Grill; during backed up with convection
         heating element

Closed

Grill; after convection heating element
         backed up has stopped

Open

Dual (Microwave and Convection)

Closed

Dual (Microwave and Grill) Open

Open

Fire sensing condition Closed

Closed

R-959(SL)M-AA

7 – 1

R959(SL)MA

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 noise filter (fuse F8A).

R-959(SL)M-AA

7 – 2

Fuse

F8A

blows

when

the

door

is

opened.

Home

fuse

blows

when

power

cord

is

plugged

into

wall

outlet.

Fuse

20A

blows

when

power

cord

is

plugged

into

wall

outlet.

Nothing

appears

in

display

when

power

cord

is

plugged

into

w

all

outlet

and

the

door

is

opened

and

closed.

Display

does

not

operate

properly

when

STOP/CLEAR

key

is

touched.

Oven

lamp

does

not

light

when

door

is

opened.

(Display

operates.)

Oven

does

not

start

when

the

START

key

is

touched.

(Display

operates.)

Oven

lamp

does

not

light

and

turntable

m

otor

does

not

operate.

Fan

m

otor

does

not

operate.

(Oven

lamp

lights.)

Convection

fan

m

otor

does

not

operate.

(Fan

motor

operates.)

Turntable

m

otor

does

not

operate.

(Oven

lamp

lights.)

Oven

or

any

electrical

parts

(except

fan

m

otor)

does

not

stop

when

cooking

time

is

0

or

S

TOP/CLEAR

key

is

touched.

Oven

stops

after

4

minutes

and

15

seconds

since

S

TART

key

is

touched.

(Except

Microwave

m

ode

and

Dual

cook

mode)

Display

operates

properly

but

all

electrical

parts

do

not

operate.

Oven

goes

into

cook

cycle

but

shuts

down

before

end

of

cooking

cycle.

After

cooking,

the

temperature

of

oven

cavity

is

higher

than

120ºC

but

fan

m

otor

does

not

operate.

The

oven

stops

1

m

inute

after

starting.

Oven

seems

to

be

operating

but

little

or

no

heat

is

produced

in

oven

load.

(Microwave

power

control

is

set

at

100%)

Oven

does

not

seem

to

be

operating

properly

during

variable

cooking

condition

except

100%

cooking

condition.

Oven

goes

into

cook

cycle

but

shuts

down

before

end

of

cooking

cycle.

Convection

cooking

mode

does

not

heat.

Oven

seems

to

be

operating

but

the

temperature

in

the

oven

cavity

is

lower

or

higher

than

preset

one.

Grill

heating

element

does

not

operate.

Oven

goes

into

cook

cycle

but

shuts

down

before

end

of

cooking

cycle.

Oven

seems

to

be

operating

but

little

or

no

heat

is

produced

in

oven

load.

(Microwave

power

does

not

seem

to

be

generated

properly)

Oven

seems

to

be

operating

but

the

temperature

in

the

oven

cavity

is

lower

or

higher

than

preset

one.

Convection

heating

elements

do

not

heat.

Grill

heating

element

does

not

heat.

OFF

CONDITION

COOKING

CONDITION

(COMMON

M

ODE)

MICROWAVE

COOKING

CONDITION

GRILL

C

OOKING

CONDITION

CONVECTION

COOKING

CONDITION

COMBI.

COOKING

COMBI.

COOKING

COMBI.

COOKING

CONDITION

(COMMON

MODE)

CONDITION

(MICRO./CONV.)

CONDITION

(MICRO./GRILL)

CONDITION

PROBLEM

BLOCKED COOLING FAN
BLOCKED CONVECTION FAN
NO POWER AT WALL OUTLET
HOME FUSE OR BREAKER
MIS-ADJUSTMENT OF SWITCHES
BLOCKED VENTILATION OPENINGS
OPENED WIRE HARNESS
SHORTED WIRE HARNESS
OVEN LAMP OR SOCKET
POWER SUPPLY CORD
FOIL PATTERN ON P.W.B.

P

O
O
O
O
O
O
O
O
N
M

L
L

K

J

I

H
H
H
H
G
G
G

F
E
E
E
E

D

C
B
A

RELAY RY8
RELAY RY7
RELAY RY6
RELAY RY5
RELAY RY4
RELAY RY3
RELAY RY2
RELAY RY1
KEY UNIT
TOUCH CONTROL PANEL
TC TRANSFORMER
CONVECTION HEATING ELEMENT
GRILL HEATING ELEMENT
NOISE FILTER (FUSE F8A)
FUSE 20A
NOISE FILTER
DAMPER MOTOR
CONVECTION FAN MOTOR
FAN MOTOR
TURNTABLE MOTOR
THERMAL CUT-OUT 170ºC (CONV.)
THERMAL CUT-OUT 170ºC (GRILL)
THERMAL CUT-OUT 125ºC
THERMISTOR
DAMPER SWITCH
MONITOR SWITCH
STOP SWITCH
MONITORED LATCH SWITCH
HIGH VOLTAGE CAPACITOR
H.V. HARNESS
H.V. RECTIFIER ASSEMBLY
HIGH VOLTAGE TRANSFORMER
MAGNETRON

TEST

PROCEDURE

POSSIBLE

C

AUSE

AND

DEFECTIVE

PARTS

R-959(SL)M-AA

8 – 1

R959(SL)MA

1. CARRY OUT 3D CHECKS.

2. To test for an open filament, isolate the magnetron from the high voltage circuit. A continuity check across the magnetron filament leads should

indicate less than 1 ohm.

3. To test for a shorted magnetron, connect the ohmmeter leads between the magnetron filament leads and chassis ground. This test should indicate

an infinite resistance. If there is little or no resistance the magnetron is grounded and must be replaced.

4. Reconnect all leads removed from components during testing.

5. Reinstall the outer case (cabinet).

6. Reconnect the power supply cord after the outer case is installed.

7. Run the oven and check all functions.

The following test procedure should be carried out with the microwave oven in a fully assembled condition (outer case fitted).

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).

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

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 1 minute and 40 seconds on full power.

4. When the 1 minute and 40 seconds 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 90.

NOTE: The result from this test should be within the allowance of 2000cc alternative method. (

±15%).

This oven should be tested for microwave leakage on completion of any repair or adjustment, following the procedure described in the Sharp Techni-
cal Training notes (part number SERV-LITMW01). The maximum leakage permitted in BS EN 60335-2-25 is 50W/m

2

 (equivalent to 5W/m

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 National Physical Laboratories are used.

Suitable leakage detectors: CELTEC A100

                                            APOLLO X1

1. CARRY OUT 3D CHECKS.

2. Disconnect the leads to the primary winding of the high voltage transformer. Disconnect the filament and secondary winding connections from the

rest of the HV circuitry. Using an ohmmeter, set on a low range, it is possible to check the continuity of all three winding. The following readings
should be obtained:

                  a.Primary winding -------------------- approximately 1.9 

Ω

                  b.Secondary winding --------------- approximately 123 

Ω

                  c.Filament winding --------------------------- less than 1 

Ω

If the readings obtained are not stated as above, then the high voltage transformer is probably faulty and should be replaced.

WARNING: High voltages and large currents are present at the secondary winding and filament winding of the power transformer. It is