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R658WM

6 – 1

R657(W)

Service Manual

 

The door can be opened by pulling the door.

1. When  the  oven  door  is  closed,  the  contacts  (COM-NO)  must  be

closed.

2. When the oven door is opened, the contacts (COM-NO) must be

opened.

1. When the oven door is closed, the contacts (COM - NC) must be

opened.

2. When the oven door is opened, the contacts (COM - NC) must be

closed.

3. If  the  oven  door  is  opened  and  the  contacts  (COM  -  NO)  of  the

monitored  latch  switch,  the  relay  RY1  and  the  relay  RY3  fail  to

open,  fuse  F8A  blows  simultaneously  with  closing  the  contacts

(COM - NC) of the monitor switch.

CAUTION: BEFORE REPLACING NOISE FILTER (A BLOWN FUSE

F8A)  TEST  THE  MONITORED  LATCH  SWITCH,  MONI-

TOR SWITCH, RELAY RY1 AND THE  RELAY RY3 FOR

PROPER  OPERATION.  (REFER  TO  CHAPTER  “TEST

PROCEDURE”.)

1. When  the  oven  door  is  closed,  the  contacts  (COM-NO)  must  be

closed.

2. When the oven door is opened, the contacts (COM-NO) must be

opened.

If  the  wire  harness  or  electrical  components  are  short-circuited,  this

fuse 20A blows to prevent an electric shock or fire hazard.

1. The fuse F8A blows when the contact (COM-NO) of the monitored

latch switch, the relay RY1 and relay RY3 remain closed with the

oven door open and when the monitor switch closes.

2. The fuse F8A also blows when the H.V. rectifier, H.V. wire harness,

H.V.  capacitor,  magnetron  or  secondary  winding  of  high  voltage

transformer is shorted.

3. If the wire harness or electrical components are shortcircuited, the

fuse F8A blows to prevent an electric shock or fire hazard.

The thermal cut-out located on the top of the oven cavity is designed

to prevent damage to the oven if the foods in the oven catch fire due to

over heating produced by improper setting of cook time or failure of

control unit. Under normal operation, the oven thermal cut-out remains

closed.  However,  when  abnormally  high  temperatures  are  reached

within  the  oven  cavity,  the  oven  thermal  cut-out  will  open  at  145°C,

causing the oven to shut down. The defective thermal cut-out must be

replaced with new one.

This thermal cut-out protects  the  magnetron against  overheat.  If  the

temperature  goes  up  higher  than  145°C  because  the  fan  motor  is

interrupted or the ventilation openings are blocked, the thermal cut-out

will open and line voltage to the high voltage transformer will cut off

and operation of the magnetron will be stopped. The defective thermal

cut-out must be replaced with a new one.

The turntable motor drives the turntable roller assembly to rotate the

turntable.

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 flows through the oven cavity to remove

steam and vapors given off from the heating foods. It is then vented

through the exhaust air vents at the rear of oven cavity.

The  noise  filter  assembly  prevents  radio  frequency  interference  that

might flow back in the power circuit.

R6

58WM

7 – 1

R657(W)

Service Manual

 

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.

If the oven becomes inoperative because of a blown fuse F8A. Check the monitored latch switch, monitor switch, relay RY1 and relay RY3 before
replacing the noise filter (fuse F8A).

CONDITION

PROBLEM

A

B

C

E

D

E

F

F

G

Home fuse blows when power cord is
plugged into wall outlet.

"88:88"does not operate in display
when power cord is plugged into wall
outlet.

Display does not operate properly
when STOP pad is touched.

Oven lamp does not light, when door
is opened. (Display operates).

Oven does not start when START pad

is touched.(Display operates).

Fan motor does not operate (Oven
lamp lights).

Turntable motor assembly does not
operate (Oven lamp lights).

Oven or electrical parts does not stop
when cooking time is "0" or STOP
pad is touched.

Display operates properly but all
electrical parts do 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 level is set at
HIGH).

Oven does not seem to be operating
properly

during

variable

cooking

condition(Oven operates properly at
HIGH).

Grill heating elelment does not heat.
(oven seems to be operating).

Fuse F8A blows.

OFF

CONDITION

MICROWAVE

COOKING

CONDITION

GRILL

COOKING

CONDITION

TEST PROCEDURE

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

MAGNETRON

HIGH

VOLTAGE

TRANSFORMER

H.V.

RECTIFIER

ASSY

H.V.HARNESS

HIGH

VOLTAGE

CAPACITOR

MONITORED

LATCH

SWITCH

STOP

SWITCH

MONITOR

SWITCH

THERMAL

CUT--OUT

(MG)

THERMAL

CUT-OUT

(OVEN)

FUSE

F8A

(NOISE

FILTER)

FUSE

20A

NOISE

FILTER

TURNTABLE

MOTOR

FAN

MOTOR

CONTROL

UNIT

KEY

UNIT

(MEMBRANE

SWITCH)

RELAY

RY1,2,3,4

GRILL

HEATING

ELEMENT

FOIL

PATTERN

ON

P.W.B.

POWER

SUPPLY

CORD

SHORTED

WIRE

HARNESS

OPENED

WIRE

HARNESS

OVEN

LAMP

BLOCKED

VENTILATION

OPENINGS

WRONG

OPERATION

MIS-ADJUSTMENT

OF

SWITCHES

HOME

FUSE

OR

BREAKER

NO

POWER

AT

WALL

OUTLET

BLOCKED

COOLING

FAN

H I

J

J

K

L

M N O

COOKING

CONDITION

(COMMON)

E

R65

8WM

8 – 1

R657(W)

Service Manual

 

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).

The IEC60705 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

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 44 seconds on full power.

4. When the 1 minute and 44 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 80.

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 at 20

°C:

                  a.Primary winding -------------------- approximately 2.64 

Ω

                  b.Secondary winding ---------------- approximately 196 

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 

R65

8WM

8 – 2

3. Also, the high voltage transformer has the thermal cut-out in the primary coil. The thermal cut-out will open when the temperature of the thermal

cut-out in the primary coil reaches approximately 160

°C. The thermal cut-out resets automatically at 140°C. If an ohmmeter indicates an open cir-

cuit under normal condition, replace the high voltage transformer because the primary coil (thermal cut-out) has opened. An open primary coil
(thermal cut-out) indicates overheating of the high voltage transformer. Check for restricted air flow to the high voltage transformer, especially the
ventilation opening.

4. CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

Isolate the high voltage rectifier assembly from the HV circuit. The high voltage rectifier can be tested using an ohmmeter set to its highest range.
Connect the ohmmeter across the terminal B+C of the high voltage rectifier and note the reading obtained. Reverse the meter leads and note this
second reading. The normal resistance is infinite in one direction and more than 100 k

Ω in the other direction.

CARRY OUT 4R CHECKS.

ASYMMETRIC RECTIFIER TEST

CARRY OUT 3D CHECKS.

Isolate the high voltage rectifier assembly from the HV circuit. The asymmetric rectifier can be
tested using an ohmmeter set to its highest range across the terminals A+B of the asymmetric
rectifier and note the reading obtained. Reverse the meter leads and note this second reading.
If an open circuit is indicated in both directions then the asymmetric rectifier is good. If the
asymmetric rectifier is shorted in either direction, then the asymmetric rectifier is faulty and must be replaced with high voltage rectifier. When the
asymmetric rectifier is defective, check whether magnetron, high voltage rectifier, high voltage wire or filament winding of the high voltage transformer
is shorted.

CARRY OUT 4R CHECKS.

NOTE: FOR MEASUREMENT OF THE RESISTANCE OF THE RECTIFIER, THE BATTERIES OF THE MEASURING INSTRUMENT MUST HAVE

A VOLTAGE AT LEAST 6 VOLTS, BECAUSE OTHERWISE AN INFINITE RESISTANCE MIGHT BE SHOWN IN BOTH DIRECTIONS.

CARRY OUT 3D CHECKS.

1. Isolate the high voltage capacitor from the circuit.

2. Continuity check must be carried out with measuring instrument which is set to the highest resistance range.

3. A normal capacitor shows continuity for a short time (kick) and then a resistance of about 10M

Ω after it has been charged.

4. A short-circuited capacitor shows continuity all the time.

5. An open capacitor constantly shows a resistance about 10 M

Ω because of its internal 10MΩ resistance.

6. When the internal wire is opened in the high voltage capacitor shows an infinite resistance.

7. The resistance across all the terminals and the chassis must be infinite when the capacitor is normal.

If incorrect reading are obtained, the high voltage capacitor must be replaced.

CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

Isolate the switch to be tested and using an ohmmeter check between the terminals as described in the following table.

If incorrect readings are obtained, make the necessary switch adjustment or replace the switch.

CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

Disconnect the leads from the terminals of the thermal cut-out. Then using an ohmmeter, make a continuity test across the two terminals as described
in the below.

Table: Terminal Connection of Switch

Plunger Operation

Common terminal to Normally open terminal

Common terminal to Normally close terminal

Released

Open circuit

Short circuit 

Depressed

Short circuit 

Open circuit.

Table: Thermal Cut-out Test

Parts Name

Temperature of “ON” condition 

(closed circuit).

Temperature of “OFF” condition 

(open circuit).

Indication of ohmmeter (When room 
temperature is approx. 20

°C.)

Thermal cut-out 145

°C (OVEN)

This is not resetable type.

Above 145

°C

Closed circuit

Thermal cut-out 145

°C (MG)

This is not resetable type.

Above 145

°C

Closed circuit

D2

D1

ASYMMETRIC
RECTIFIER

HIGH VOLTAGE
RECTIFIER