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R-234 - 9

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.

14 sec. OFF

8 sec. OFF

32 sec. ON

24 sec. ON

MEDIUM HIGH

HIGH

MEDIUM

MEDIUM LOW

LOW

18 sec. ON

6 sec. ON

12 sec. ON

26 sec. OFF

20 sec. OFF

Approx. 10% = 80 Watts

Approx. 30% = 240 Watts

Approx. 50% = 400 Watts

6. MONITOR SWITCH CIRCUIT

The monitor switch (SW2) is mechanically controlled by
oven door, and monitors the operation of the monitored
latch switch (SW1)

6-1

When the oven door is opened during or after the cycle
of a cooking program, the monitored latch switch
(SW1) must open its contacts first. After that the
contacts (COM - NC) of the monitor switch (SW2) can
be closed.

6-2

When the oven door is closed, the contacts
(COM - NC) of the monitor switch (SW2) must be
opened first. The contacts (COM - NO) of the monitored
latch switch (SW1) and stop switch (SW3) are closed
after.

6-3

When the oven door is opened and the contacts of the
monitored latch switch (SW1) remain closed, the fuse
F1 will blow, because the monitor switch is closed and
a short circuit is caused.

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the power transformer
intermittently within a 26-second time base through the vari
contact. The following levels of microwave power are given.
SETTING

Closing the door activates the door interlock switch (moni-
tored latch switch).

IMPORTANT
When the oven door is closed, the monitor switch contacts
COM - NC must be open.
When the microwave oven is pluged in a wall outlet (220-
230V 50Hz), the noise filter is energised.

NOTE: When the oven door is opened, the oven lamp

comes on at this time.

Enter a desired cooking time with the touching TIME pad
and start the oven with touching START pad.

1. The line voltage is supplied to the primary winding of the

power transformer. The voltage is converted to about
3.3 volts A.C. output on the filament winding and high
voltage of approximately 2000 volts A.C. on the
secondary winding.

2. The filament winding voltage (3.3 volts) heats the

magnetron filament and the high voltage (2000 volts) is
sent to the voltage doubling circuit, where it is doubled
to negative voltage of approximately 4000 volts D.C..

3. The 2450 MHz microwave energy produced in the

magnetron generates a wave length of 12.24 cm. This
energy is channeled through the waveguide (transport
channel) into the oven cavity, where the food is placed
to be cooked.

4. When the cooking time is up, a single tone is heard and

the relays RY1 + RY2 go back to their home position.
The circuits to the oven lamp, power transformer, fan
motor and turntable motor are cut off.

5. When the door is opened during a cook cycle, the

switches come to the following condition.

CONDITION

DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Monitor switch

COM-NC

Open

Closed

Monitored latch switch

COM-NO

Closed

Open

The circuits to the power transformer, fan motor and
turntable motor are cut off when the monitored latch
switch is opened.
The oven lamp remains on even if the oven door is
opened after the cooking cycle has been interrupted,
because the timer switch remains closed.

Oven lamp, Fan motor, Turntable motor

RY1

Power transformer

RY2

Approx. 70% = 560 Watts

100%          = 800 Watts

R-234 - 10

1. The fuse F1 also blows when the H.V. rectifer, H.V. wire

harness, H.V. capacitor, magnetron or secondary
winding of power transformer is shorted.

2. If the wire harness or electrical components are short-

circuited, the fuse F1 blows to prevent an electric shock
of fire hazard.

°

°

The thermostat protects the magnetron against overheat.
If this temperature goes up higher than 150

°

C because the

fan motor is interrupted, the ventilation openings are blocked,
the thermostat  will open and line voltages to the power
transformer will cycle and the operation of the magnetron
will also  cycle.

°

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. How-
ever, when abnormally high temperatures are reached
within the oven cavity, the oven thermal cut-out will open at
125

°

C, causing the oven to shut down. The defective

thermal cut-out (TC2) must be replaced with 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 prevents the radio frequency interference
that might flow back in the power circuit.

DOOR OPEN MECHANISM
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 (SW1) fail to open, fuse
F8A blows simultaneously with closing the contacts
(COM - NC) of the monitor switch (SW2).

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.

CAUTION: BEFORE REPLACING A BLOWN FUSE (F1)

TEST THE MONITORED LATCH SWITCH
(SW1) AND MONITOR SWITCH (SW2) FOR
PROPER OPERATION. (REFER TO
CHAPTER "TEST PROCEDURE".)

Latch hook

SW3: Stop switch

Door

SW2: Monitor switch

SW1:Monitored latch switch

Latch heads

R-234 - 11

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 F. Check the monitored latch
switch and monitor switch before replacing
the F8A fuse.

CONDITION

PROBLEM

A

B

C

D

E

E E

F G

H

I

I

J

K

L

L

M

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

Fuse F1  blows when power cord 
is plugged into wall outlet. (Timer 
off)

Oven does not operate at all.

Timer is not set but when door 
closed, oven starts operation.

Oven does not start when timer 
knob is turned, door closed.

Oven lamp does not light, when in 
operation. 

Fan motor does not operate. 

Turntable motor assembly does 
not operate. 

Oven or electrical parts does not 
stop when timer knob is at "0". 

Oven seems to be operating but 
little or no heat is produced in oven 
load. (Microwave power level is set 
at 800W.)

Oven does not operate properly during 
the variable cooking condition except 
800w cooking condition.

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

Oven stops as soon as the oven is 
started.

Fan motor, oven lamp and 
turntable motor do not operate at 
the same time.

OFF

CONDITION

COOKING

CONDITION

TEST PROCEDURE

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

MAGNETRON

HIGH VOLTAGE  TRANSFORMER

H.V. RECTIFIER

HIGH VOLTAGE CAPACITOR

MONITORED LATCH SWITCH

STOP SWITCH

MONITOR SWITCH

THERMAL CUT-OUT 125

0

C (OVEN)

FUSE F1

NOISE FILTER

FAN MOTOR

TURNTABLE MOTOR

POWER SUPPLY CORD

OVEN LAMP OR SOCKET

SHORTED WIRE HARNESS

OPENED WIRE HARNESS

MIS-ADJUSTMENT SWITCHES

NO POWER AT WALL OUTLET

HVT THERMOSTAT

CONTROL UNIT

TACT SWITCH

RELAY RY1

RELAY RY2

FOIL PATTERN ON P.W.B.

R-234 - 12

CARRY OUT 3D CHECK

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

condition (outer case fitted). Microwave output power from the magnetron can be measured by way of
IEC 60705.  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.

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 + 3 = 55 sec.

P = 80 x 

∆

T

The following test procedure should be carried out with the microwave oven in a fully assembled

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

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 of 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 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 seconds are needed for magnetron

filament heat-up time.

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.