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R-671M  -  9

This thermal cut-out protects the high voltage transformer
against overheating. 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 voltages to the high voltage transformer will
be cut off and the operation of the magnetron will be
stopped. The thermal cut-out will cut back in at 115˚C.

The door can be opened by pushing the open button on the
control panel. When the button is pushed, the open lever
pushes the lower latch head on the door upward. The latch
head is linked with the lower latch head, so now, the door
can be opened.

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 primary latch
switch - monitor switch , check the primary
latch switch and  monitor switch before
replacing the fuse F1 (F8A).

LATCH HOOK

STOP SWITCH
SW2

MONITORED
LATCH SWITCH
SW1

MONITOR SWITCH
SW3

LATCH
HEADS

OPEN LEVER

DOOR OPEN BUTTON

DOOR

FIG D-1. Door Open Mechanism

The thermal cut-out protects the oven against over heating
during grill cooking, also the thermal cut-out  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 the cooking time or failure of control unit. Under
the normal operation, the oven thermal cut-out remains
closed. However, when abnormally high temperature are
reached within the oven cavity, oven thermal cut-out will
open at 150˚C causing the oven to shut down. The thermal
cut-out will cut back in at 130˚C.

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 is
channelled through the oven cavity to remove steam and
vapours given off from heating food. It is then exhausted
through the exhausting air vents of the oven cavity.

The noise filter assembly prevents radio frequency
interference that might flow back in the power circuit.

The grill heating element is located on the top of the oven
cavity assembly. The grill heating element sends out heat
to grill foods.

The bottom heating element is located on the floor of the
oven cavity assembly. The bottom heating element sends
out heat to grill foods.

R-671M  -  10

CONDITION

PROBLEM

MAGNETRON

HIGH VOLTAGE  TRANSFORMER

H.V. RECTIFIER

HIGH VOLTAGE CAPACITOR

PRIMARY LATCH SWITCH

MONITOR SWITCH

STOP SWITCH

THERMAL CUT-OUT 145

˚C (HVC) 

THERMAL CUT-OUT 150

˚C (OVEN)

FUSE 15A 

FUSE F8A 

NOISE FILTER

FAN MOTOR

TURNTABLE MOTOR

GRILL HEATING ELEMENT (TOP)

BOTTOM HEATING ELEMENT

POWER SUPPLY CORD

OVEN LAMP OR SOCKET

SHORTED WIRE HARNESS

OPENED WIRE HARNESS

MIS-ADJUSTMENT OF SWITCHES

TOUCH CONTROL PANEL

KEY UNIT

RELAY RY1

RELAY RY2

RELAY RY3

RELAY RY4

RELAY RY5

RELAY RY6

FOIL PATTERN ON P.W.B.

LIMITATION OF OPERATION

A B C D E E E F F G H I

J J K K

L M N N N N N N O

Fuse 15A blows when power 
cord is plugged into wall outlet.

 

Fuse F8A blows when the door 
is opened.

 

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

 

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

 

Nothing appear in display when 
power cord is plugged into wall 
outlet and open and close the 
door.

 

Display does not operate 
properly when STOP key is 
pressed.

 

Oven does not start when the 
START key is pressed. (Display 
operates.)

 

Oven lamp does not light. 
(Turntable motor operates.)

 

Fan motor does not operate. 
(Oven lamp lights.)

 

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

 

Oven or electrical parts does 
not stop when cooking time is 0 
or STOP key is pressed.

  

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.

 

Oven does not operating 
properly during variable cooking 
condition except 100% cooking 
condition.

 

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

 

Grill heating element (top) 
doesn't operate.

 

Bottom heating element doesn't 
operate.

 

Oven seems to be operating but 
little or no heat is produced in 
oven load. (Microwave power 
does not seem to be generated 
properly.)

 

Grill heating element (top) 
doesn't operate.

 

Bottom heating element doesn't 
operate.

OFF

CONDITION

COOKING

CONDITION

(COMMON MODE)

MICROWAVE

COOKING

CONDITION

GRILL

COOKING

CONDITION

DUAL

COOKING

CONDITION

TEST PROCEDURE

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

R-671M  -  11

CARRY OUT 3D CHECKS.

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 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 following test procedure should be carried out with the microwave oven in a fully assembled
condition (outer case fitted). Microwave output power from the magnetron can be measured by way
of IEC 705, i.e. it is measured by how much power the water load can absorb. To measure the
microwave output power in the microwave oven, the relation of calorie and watt is used. When P(W)
heating works for t(second), approximately P x t/4.187 calorie is generated. On the other hand, if the
temperature of the water with V(ml) rises 

∆

T (

°

C) during this microwave heating period, the calorie

of the water is V x 

∆

T.

Measuring condition:
1. Container

The water container must be a cylindrical borosilicate glass vessel having a maximum material
thickness of 3 mm and an outside diameter of approximately 190 mm.

2. Temperature of the oven and vessel

The oven and the empty vessel are at ambient temperature prior to the start the test.

3. Temperature of the water. The initial temperature of the water is (10

±

2)

°

C.

4. Select the initial and final water temperature so that the maximum difference between the final

water temperature and the ambient temperature is 5K.

5. Select stirring devices and measuring instruments in order to minimize addition or removal of heat.
6. The graduation of the thermometer must be scaled by 0.1

°

C at minimum and be an accurate

thermometer.

7. The water load must be (1000

±

5) g.

8. “t” is measured while the microwave generator is operating at full power. Magnetron filament heat-

up time is not included.

NOTE: The operation time of the microwave oven is “t + 3” sec. (3 sec. is magnetron filament heat-up time.)

Therefore total heating time = 55 sec.

Measuring method:
1. Measure the initial temperature of the water before the water is added to the vessel.

(Example: The initial temperature T1 = 11

°

C)

2. Add the 1 litre water to the vessel.
3. Place the load on the centre of the shelf.
4. Operate the microwave oven at HIGH for the temperature of the water rises by a value 

∆

 T of

(10 

±

 2) K.

5. Stir the water to equalize temperature throughout the vessel.
6. Measure the final water temperature. (Example: The final temperature T2 = 21

°

C)

7. Calculate the microwave power output P in watts from above formula.

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

The formula is as follows;

P x t / 4.187 = V x 

∆

 T

P (W) = 4.187 x V x 

∆

T / t

Our condition for water load is as follows:

Room temperature ........... around 20

°

C

Power supply Voltage .............. Rated voltage

Water load .................................. 1000 g

Initial temperature .............................. 10

±

2

°

C

Heating time .............................. 52 sec.
P = 80 x 

∆

T

R-671M  -  12

Initial temperature ..................................................................................................  T1 = 11

°

C

Temperature after (52 + 3) = 55 sec ......................................................................  T2 = 21

°

C

Temperature difference Cold-Warm ......................................................................

∆

T1 = 10C

Measured output power
The equation is “P = 80 x 

∆

T” ...................................................... P = 80 x 10

°

C = 800 Watts

±

CAUTION: 1

°

C CORRESPONDS TO 80 WATTS. REPEAT MEASUREMENT IF THE POWER IS

INSUFFICIENT.

1000g

1000g

1000g

Heat up for 55 sec

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.

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.
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 2.6 

Ω

b. Secondary winding .......................... approximately 143 

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.
CARRY OUT 4R CHECKS.