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R-734 - 13

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 60705, 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.

The formula is as follows;

P x t / 4.187 = V x 

∆

 T+ 0.55 x mc (T2-T0)

P (W) = 4.187 x V x 

∆

T / t + 0.55 x mc (T2-T0)/t

Our condition for water load is as follows:

Room temperature (T0) ... around 20

°

C

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

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

Initial temperature (T1) ...................... 10

±

1

°

C

Heating time .................... 47+3=50 sec.

Mass of container (mc) ......................... 330 g

T2 ............................ Final Temperature

P = 90 x 

∆

T + 0.55 x mc (T2-T0)/47

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

±

1)

°

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˚C.

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.

R-734 - 14

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

°

C

Temperature after (47 + 3) = 50 sec ......................................................................  T2 = 21

°

C

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

∆

T1 = 10C

Measured output power
The equation is “P = 90 x 

∆

T” ...................................................... P = 90 x 10

°

C = 900 Watts

±

CAUTION: 1

°

C CORRESPONDS TO 90 WATTS. REPEAT MEASUREMENT IF THE POWER IS

INSUFFICIENT.

Heat up for 50 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.

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 probably 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 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 

Ω

b. Secondary winding .......................... approximately 130 

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.

1000g

1000g

1000g

D2

D1

ASYMMETRIC
RECTIFIER

HIGH VOLTAGE 
RECTIFIER

A

B

C

R-734 - 15

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: Thermal Cut-out Test

Temperature of "ON"

Temperature of "OFF"

Indication of ohmmeter

        Parts Name

 condition  (closed circuit).

condition (open circuit).

(When room temperature

(˚C)

(˚C)

is approx. 20˚C.)

Thermal cut-out TC1 125˚C

This is not resetable

Above 125˚C

Closed circuit

Thermal cut-out TC2 145˚C

Below 115˚C.

Above 145˚C

Closed circuit

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 10M

Ω

resistance.

F. When the internal wire is opened in the high voltage 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.

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 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, BE-
CAUSE OTHERWISE AN INFINITE RESISTANCE MIGHT BE SHOWN IN BOTH DIREC-
TIONS.

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

An open circuit thermal cut-out (MG) TC1 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) TC2 indicates that the oven cavity has cought fire, this may be
due to over heating produced by omproper setting of the cooking time or failure of the control panel.

CARRY OUT 4R CHECKS.

R-734 - 16

CARRY OUT 3D CHECKS.

If the special fuse F1 20A 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.

1. If the fuse F2 F8A  is blown when the door is opened, check the monitored latch switch and monitor

switch.

2. If the fuse F2 F8A is blown by incorrect door switching replace the defective switch(es) and the  fuse

F2 F8A.

3. If the fuse F2 F8A is blown, there could be shorts in the asymmetric rectifier or there is a ground in

wire harness. A short in the asymmetric rectifier may be occurred due to short or ground in H.V.
rectifier, magnetron, high voltage transformer or H.V. wire. 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 motor. Using an ohmmeter, check the resistance between the two
terminals as described in the table below.

L (min)

Cx 

±

 20%

Cy 

±

 20%

1.0mH

0.22

µ

F

4700pF

Between N and L

Approx. 680 k

Ω

Between terminal N and WHITE

Short circuit

Between terminal L and RED

Short circuit

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

CARRY OUT 4R CHECKS.

CARRY OUT 3D CHECKS.

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

Table: Resistance of Motor

Motors

Resistance

Fan motor

Approximately

290 

Ω

Turntable motor

Approximately 14.6 k

Ω

If incorrect readings are obtained, replace the motor.

CARRY OUT 4R CHECKS.

RED

WHT

L

L

R1

L

F2: F8A

R2

N

Cx

Cy

F1: 20A

R1: 10M ohm 

±

 20%

R2: 680k ohm 

±

 20%