Sunday, December 20, 2015

MEDIA EM820CFD-P00E MICROWAVE OVEN Circuit description and trouble shooting


As the door is closed, the contact of MONITOR SWITCH opens. This switch creates the short circuit to blow fuse during operation under abnormal condition. (ie, should the contacts of primary and secondary switch fail to open the circuit)
# The latches are secured by latch board. The oven light turns on while the oven is in operation
# The contacts of the timer switch changes to ON position when the timer knob is rotated. (While power control knob is set )
# The contacts of primary and secondary switch close the circuit.
# Input Volts AC. is applied to the high voltage transformer through V.P.C.
# Turntable rotates.
# The fan motor rotates and cools the magnetron by blowing the air (coming from the intake holes on the baseplate) over the magnetron.
# The air is also directed into the oven to exhaust the vapor in the oven through the upper plate.
# Timer starts rotating.
# 3.2 volts AC is generated from the filament winding of the high voltage transformer. This 3.2 volts is applied to the magnetron to heat the magnetron filament through two noise-preventing choke coils. A high voltage of approximately 2100 volts AC is generated in the secondary of the high voltage transformer which is increased by the action of the diode and charging of the high voltage capacitor. The negative DC voltage is then applied to the filament of the magnetron
# Both primary switch and secondary switch open to cut off the primary winding voltage to the high voltage transformer to stop microwave oscillation.
# Timer, turntable and fan motor stop.
# As the door is opened, if the contact of primary switch fail to open, the fuse opens due to the large current surge caused by the monitor switch activation which in turn stops magnetron,oscillation.
# 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.
3 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 2K.
3 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.
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”.
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 shortcircuited 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.). 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.).
When measuring, the ultimate value of microwave leakage of all the measured position should not exceed 1.0 mili watt/cm2, or should be considered as abnormal.
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/cm2 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/cm2. Generally, it should be controlled below 0.8 mill watt/cm2 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.
2. Means of repair when the oven can heat, but the turntable glass can’t move
Firstly, check whether the turntable holder is placed correctly. If it is correct, then pull out the power plug and take down the turntable combination, measure the resistance value of the turntable motor with Rx1k grade of a multi meter If it is opencircuited, it indicates the turntable motor has broken, and should be replaced by a new, same model one. If the resistance value is between 15-22k,it indicates the turntable motor is normal, then check the connecting shaft weave. If the plastic that the shaft insert in has broken, a new shaft weave should replace it.
3. Repair when the oven can heat, but the lamp is not on.
Pull out the power plug, take down the outer case and discharge the capacitor. Pull out the two terminal plugs of the lamp, measure the two plugs of the lamp with the Rx100 grade of a multi meter. If it is open-circuited, it indicates the lamp has broken, and should be replaced by a same model one.
4. Means of repair when the oven stop working after several minutes operating
The phenomenon indicated the thermal cutout is playing its protective role, and you should check whether the fan is working in normal. Turn off the oven, pull out the power plug, take down the outer case, discharge the capacitor, then turn the fan with hand to see whether it is moving flexibly. If not, it indicates that the oil bearing of the fan motor has run off the oil, and should take down the fan combination to repair the motor. Loosen the two screws which fix the bearing out the shaft and the bearing, and rinse them with kerosene (ATTENTION: The bearing can only be wiped with a silk which moistened with kerosene rather than be washed in the kerosene because there are felt on it. If the felt are soaked with kerosene, then the engine oil can not be sucked up.).After the bearing being cleaned, the felt should be refueled fully with engine oil(for when the oven is operating, the engine oil empty into the oil bearing slowly).Fix the bearing cover with two screws, turn the fan around till it can move flexibly. Than install them to the oven, and plug in the two terminal plugs. If the fan can move flexibly, then the winding of the fan motor should be examined. Measure the winding with Rx100 grade of a multi meter, if it is open-circuited, it indicates the winding of the fan motor has broken, and should be replaced by a new, same model one.