R-222STM - Sharp Home Appliance Service Manual (repair manual). Page 12

 

 

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

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or should be considered as abnormal. 

 

 

 

 

         

                                                                                                 

 

 

 

 

 

 

 

 

 

 

275ml

 

FIG.5-1 

Glass tray 

FIG.5-2