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6

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 T6.3A in the monitored
latch switch  - monitor switch  circuit, check
the monitored latch switch and monitor
switch  before replacing the fuse T6.3A.

MAGNETRON

HIGH VOLTAGE TRANSFORMER

H.V. RECTIFIER ASSEMBLY

HIGH VOLTAGE WIRE

HIGH VOLTAGE CAPACITOR

MONITORED  LATCH SWITCH

2ND. INTERLOCK RELAY CONTROL SWITCH

MONITOR SWITCH

THERMAL CUT-OUT 125˚C (OVEN)

THERMAL CUT-OUT 95˚C (MG)

FAN MOTOR

TURNTABLE MOTOR

FUSE T6.3A

CONTROL PANEL

TACT SWITCH

RELAY 

RY1,

RELAY 

RY2,

FOIL PATERN ON P.W.B.

POWER SUPPLY CORD

SHORTED WIRE HARNESS

OPENED WIRE HARNESS

OVEN LAMP

WALL OUTLET

MISADJUSTMENT SWITCH

HOME FUSE OR BREAKER

BLOCKED COOLING FAN

BLOCKED VENTILATION

NOISE FILTER

H.V. FUSE

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

FUSE T6.3A blows when
power supply cord is plugged
into wall outlet.

Oven does not start when the
timer is set.

Oven lamp does not light (Other
function is normal.)

Fan motor does not operate.
(Other function is normal.)

Turntable motor does not op-
erate. (Other function is nor-
mal.)

Oven or any electrical parts
does not stop when cooking
time is 0.

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 seems to be
operating properly when
MEDIUM HIGH, MEDIUM,
MEDIUM LOW or LOW is set.
(Oven operates properly at
HIGH.)

Oven goes into cook cycle but
shuts down before end of cook-
ing cycle.

Oven lamp, turntable motor
and fan motor do not operate.

The microwave power level can
not be changed.

OFF
CONDITION

PROBLEM

CONDITION

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

TEST PROCEDURE

ON
CONDITION

CK = Check / RE = Replace

A B C

CK

D E E E F   F   G  G   H   K   L M M N

RECKCKRE CKCKCK CK CK

I J

7

CARRY OUT 3D CHECK.

Isolate the magnetron from high voltage circuit by removing all leads connected to filament terminal.

To test for an open circuit filament use an ohmmeter to make a continuity test between the magnetron
filament terminals, the meter should show a reading of less than 1 ohm.

To test for short circuit filament to anode condition, connect ohmmeter between one of the filament
terminals and the case of the magnetron (ground). This test should be indicated an infinite resistance.
If a low or zero resistance reading is obtained then the magnetron should be replaced.

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 power. If the true output power is required, then the IEC60705
test method must be used.

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 48 seconds on full power.

4. When the 1 minute and 48 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.

±

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

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

NEVER TOUCH ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL
WHILE THE OVEN IS IN OPERATION.

8

Table: Terminal Connection of Switch

Plunger Operation

COM to NO

COM to NC

Released

Open circuit

Short circuit

Depressed

Short circuit

Open circuit

COM; Common terminal,
NO;

Normally open terminal

NC;

Normally close terminal

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

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.

CARRY OUT 3D CHECKS.
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 .......................... 2.8 ohms approximately
b. Secondary winding .................... 189 ohms approximately
c. Filament winding .....................................  less than 1 ohm

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

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.

A. Isolate the high voltage capacitor from the circuit.
B. Continuity check must be carried out with measuring instrument which is set to the highest

resistance range.

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

Ω after

it has been charged.

D. A short-circuited capacitor shows continuity all the time.
E. An open capacitor constantly shows a resistance about 10 M

Ω because of its internal 10MΩ

resistance.

F. When the internal wire is opened in the high voltage capacitor, the capacitor shows an infinite

resistance.

G. 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.

9

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.

CARRY OUT 4R CHECKS.

Table: Thermal Cut-out Test

Temperature of "ON"

Temperature of "OFF"

Indication of ohmmeter (When room

Parts Name

condition (closed circuit).

condition (open circuit).  temperature is approx. 20˚C.)

Thermal cut-out 95˚C (MG.)

 Below 75˚C

Above 95˚C

Closed circuit.

Thermal cut-out 125˚C (OVEN)

This is not resetable type. Above 125˚C

Closed circuit

If incorrect readings are obtained, replace the thermal cut-out.

An open circuit thermal cut-out (MG) indicates that the magnetron has overheated, this may be due
to resistricted ventilation, cooling fan failure or a fault condition within the magnetron or HV circuit.

An open circuit thermal cut-out (OVEN) indicates that the food in the oven cavity may catch fire, this may
be due to overheating produced by improper setting of the cooking time or failure of the control panel.

CARRY OUT 3D CHECKS.

Disconnect the leads from the motor. Using an ohmmeter, check the resistance between the two
terminals as described in the table below.

Table: Resistance of Motor

Motors

Resistance

Fan motor

Approximately 400 

Ω

Turntable motor

Approximately 12 k

Ω

If incorrect readings are obtained, replace the motor.
CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

If the fuse T6.3A is blown when the door is opened, check the monitored latch switch and monitor
switch.
If the fuse T6.3A is blown by incorrect door switching, replace the defective switch(s) and the fuse
T6.3A.
If the fuse T6.3A is blown, there is a shorts or grounds in electrical parts or wire harness.
Check them and replace the defective parts or repair the wire harness.
CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

Disconnect the leads from the terminals of noise filter.
Using an ohmmeter, check between the terminals as described in
the following table.

Between N and L

Open circuit

Between terminal N and WHITE

Short circuit

Between terminal L and RED

Short circuit

If incorrect readings are obtained, replace the noise filter unit.

CARRY OUT 4R CHECKS.

FUSE
T6.3A

N

L

LINE CROSS CAPACITOR

0.068

µ/AC250V 

NOISE SUPPRESSION COIL 

LINE BYPASS
CAPACITOR
0.0022

µ/AC250V

LINE BYPASS
CAPACITOR
0.0022

µ/AC250V

NOISE FILTER

WHT

RED