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• Microwave power output measurement is made with 

the microwave oven supplied at its rated voltage and 

operated at its maximum microwave power setting with 

a load of (1000±5) g of potable water. 

• The water is contained in a cylindrical borosilicate glass 

vessel having a maximum material thickness of 3 mm 

and an outside diameter of approximately 190mm. 

• The oven and the empty vessel are at ambient 

temperature prior to the start of the test. 

• The initial temperature (T1) of the water is (10±2)°C It 

is measured immediately before the water is added to 

the vessel. After addition of the water to the vessel, 

the load is immediately placed on the center of the 

turntable which is in the lowest position and the 

microwave power switched on. 

• The time T for the temperature of the water to rise by a 

value . T of (10±2)°K is measured, where T is the time 

in seconds and .T is the temperature rise. The initial 

and final water temperatures are selected so that the 

maximum difference between the final water 

temperature and the ambient temperature is

 

2°K. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

• The microwave power output P in watts is 

calculated 

approximately from the following formula : 

P=4187 x (△T)/T 

is measured while the microwave generator is 

operating at full power. Magnetron filament heat-up 

time is not included. (about 3 sec) 

• The water is stirred to equalize temperature 

throughout 

the vessel, prior to measuring the final water 

temperature. 

• Stirring devices and measuring instruments are 

selected in order to minimize addition or removal of 

heat. 

 

 

Before overhauling a microwave oven, you should judge the breakdown and the cause correctly, then you can repair it with 
corresponding ways. The overhauling must be proceed in order, any hasty conclusion is not recommendable, otherwise 
overworking would be done when repair. The microwave oven may occur compound breakdown due to all kinds of different 
reasons, thus, when overhaul, they all should be taken into consideration. Special attention must be given to the microwave 
leakage and the electric insulation when examine because they may do harmful to the repairing staff. 
Ⅰ

      How to examine a microwave oven with breakdown? A better means which demonstrated in practical operating are through 
inspecting and listening. On the basis of large amounts of perceptual knowledge, you can judge and analysis the breakdown 
quickly and correctly. 
1. Inspection. 

Inspect whether the oven shape is disordered and where is the disordered position, if any. It is normal if the outer case 
disordered a little, but abnormal if the oven, the door disordered, the door hook broken, the door crooked, or there are too 
much looseness between the door and the oven after the door is closed. 

2. Listening. 

Listening to the voice of the oven operating and the noise of the fan after it conducted. Minor “wen wen” noise, cycling “kala” 
noise and “shishi” noise should be consider as normal. But it is abnormal if the following noise occurred: 

(1)  Sound “wenwen” noise. 
(2)  Long time “shishi” noise. 
(3)  Strike voice like “pipa pipa”. 

Ⅱ

1.  Examine the microwave insulating resistance. 

Measure the insulating resistance with a multi meter or a mega ohmmeter. The value should not less than 2 mega ohm. 
Otherwise, part examination should be taken at once. Such as checking whether the motor, the thermal cutout, the 
transformer or the capacitor are electricity leaking. 

2.  Examination of the resistance value of the microwave oven. 

Close the door, set the time (the oven is at operating condition but the power plug haven’t been plugged in), measure the two 
feet(L-N)of the power plug with Rx1 grade of a multimeter, the resistance value should be about 22 ohm. 
If open circuit occurred, then you must check whether the

 

10 A fuse is broken、the primary winding of the transformer is open 

circuit、the thermal cutout is open circuit or not, you must check whether the interlock device is put through or all the plugs 

are connected well. If short circuit occurred, you should check whether the primary winding of the power transformer is short-
circuited or part short-circuited. 

3.  Examination of microwave leakage 

Measure the microwave leakage with a microwave leakage Measure. Place a graduate of 275ml water at the middle of the 
glass tray of the oven (FIG.5-1). Close the door, power set high, time set to 3 minutes, press the starting button to operate the 
oven. After rectified the microwave leakage measure, measure around the door crack, those hole position of the window and 
the air vent at four sides of the oven with the probe of the measure. When measure, the moving speed of the probe should 
not exceed 2.5cm per second, and the measuring direction should be the same with the outing direction of the microwave 
leakage (FIG.5-2). 
When measuring, the ultimate value of microwave leakage of all the measured position should not exceed 1.0 mili watt/cm

2

, 

or should be considered as abnormal. 

 

 

 

 

         

                                                                                                 

 

 

 

 

 

 

 

 

 

 

275ml

 

FIG.5-1 

Glass tray 

FIG.5-2 

 

 

 

 

4.  Examine when the oven at operating, but the food can’t be heated 

Pull out the power plug, take off the outer case, discharge the capacitor, measure the resistance value of the primary winding and 

the secondary winding of the transformer with a multi meter

 

(FIG.5-3 and FIG.5-4). The resistance value of the primary winding 

should be about 2.2 ohm, the secondary winding should be about 130 ohm, otherwise, it indicates the transformer has broken, 

and should be replaced by a new one. 

If the transformer is normal, then the high voltage capacitor should be checked. Pull out the connecting plug of the capacitor, 

and measure it with Rx1 grade of a multi meter, the two rod of the multi meter connect the two polarity of the capacitor. When 

they just connected, the reading of the multi meter should be zero, then enlarge to nine mega ohm slowly. Change the rod to 

different polarity, the reading repeat from zero to 9 mega ohm (FIG.5-5),it means the capacitor is normal. If the indicator of the 

multi meter can’t point out from zero to 9  mega ohm, it indicates the high voltage capacitor has broken, and should be 

replaced by a new one. 

 

 

 

 

 

 

 

 

 

If it is normal between the two pole of the capacitor, then the insulation between the capacitor pole and the cabinet (FIG.5-6) 

should be checked. 

If the resistance value between the capacitor pole and the cabinet is “∞”,the capacitor is normal. Then check the earth of the 

magnetron’s two filament to see whether they are short-circuited (FIG.5-7).If they are short-circuited and the filament strikes 

the shell of the magnetron, it indicates the magnetron has broken, and should be replaced by a new, same model one. 

If there is no problem with the magnetron, check the high voltage diode then. Measure the diode with Rx10k grade of a multi 

meter, the “-” rod end of the multi meter connect the cathode of diode, the “+ “ rod end of the multi meter connect the anode of 

the diode(FIG.5-8).The multi meter reading should be about 150 thousand ohm. Then change the rod to different 

electrode(FIG.5-9), the reading should be “∞”.If the reading is very small, and near to short circuit, it indicates the high 

voltage diode has been punctured, and should be replaced by a new one. 

If high voltage diode is also normal, then test the pilot switch(FIG.5-10). Pull out the two plugs of the switch, Measure it with 

the Rx1 grade of a multi meter, the two rod connect the plug of the switch, the resistance value should be “∞”.Then press 

down the pilot switch with a screwdriver, if the reading of the multi meter pointed to zero, it indicates the pilot switch has 

broken, and should replace it with a new, same model one.

FIG.5-7 

FIG5-6 

 

FIG5-5 

 

FIG.5-8 

 

 

Ⅲ

1.  Repair when there occurred large amounts microwave leakage. There are many factors, which may cause microwave leaking. 

Following mentioned may be the main cause of microwave leakage: 

(1)  The door deformed, the hinge loosed or damaged that caused the door can not close tightly. 

(2)  The door pressing cover or the embed piece damaged or come off. 

(3)  Obvious damage or uneven of the oven. 

(4)  There are filth between the door and the oven. 

(5)  The door and the oven are serious loosed after the door closed. 

(6)  The crack of the door shielding net cover. 

Before repairing, check whether the above listed point are existed, if not, you can start the microwave oven. Place a 

graduate of about 275ml water at the middle of the glass tray, close the door, time set at 3 minutes, power at high, 

makes the oven operating in normal. Rectify the microwave leakage measure, measure the amount of the microwave 

leakage around the oven with its probe. If there are places which the leakage exceed the standard requirement, then 

repair them accordingly. If the leakage amount exceed 1.0 mW/cm

2

 at the left door crack, then pull out the power plug, 

take down the outer case, adjust the screws of the hinge to less the gap between the door and the oven. Then measure 

again, the leakage amount should less than 1.0 mW/cm

2

. Generally, it should be controlled below 0.8 mill watt/cm

2

 with 

some allowance. 

If the leakage occurred at the right door crack, adjust the screws that fix the interlock holder and the hook. If the leakage 

is on the larger side at the right-above of the oven, then adjust the upper screw. Loosen out the screw, push the door 

close to the oven to hook the door hook with the plastic parts, then    tighten the screw again. If the leakage is larger at 

the right-below, then adjust the lower screw. Loosen the screw, push the door close to the oven to hook the door hook 

with the switch holder tightly, then tighten the screw again, and open and close the door repeatedly to check whether the 

door can operate flexibly, whether the hook and the switch are in their normal position. If it is not in position, then adjust 

the door hook and the switch holder the loose between the door and the oven, then measure the leakage with 

microwave leakage measure again. 

If there still exist microwave leakage, measure near the magnetron with the probe of the microwave leakage measure. If 

the leakage is larger, the oven should be turned off and check whether the 2 or 4 screws which fix the magnetron have 

been loosed, if loosed, twist them tightly with socket wrench. If the screws are fixedly, then the magnetron should be 

take down to check the copper filament weaved washer of the magnetron has been placed well or whether the wave 

guide housing coupling has been oxidized or have lacquer on it. If do have, scrape the oxidized layer or the lacquer off, 

when fix the magnetron, the copper filament weaved washer must be placed well, the screws must be twist tightly. Then 

turn on the oven and measure again until it complies with the requirement. If the microwave leakage is larger at those 

hole position of the window board, the oven should be turned off to inspect whether there are crack among them. If 

several holes formed a crack, it would enlarge the microwave leakage. If that is the case, it indicates the door has 

broken, and should be replaced with a new door. 

 

 

 

 

FIG.5-9 

FIG5.10