Midea MM720CFA-PM0C, Service Manual

Page: 13 / 21

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
5-5
FIG5-6 FIG5-5
FIG.5-8