Thursday, December 31, 2015

MIDEA MM720 700W ( 230V 50Hz) MICROWAVE OVEN Test mode and troubleshooting

MIDEA MM720

MICROWAVE LEAKAGE TEST
CAUTIONS
# Be sure to check microwave leakage prior to servicing the oven if the oven is operative prior to servicing.
# The service personnel should inform the manufacture importer, or assembler of any certified oven unit found to have a microwave emission level in excess of 5 mW/cm2 and should repair any unit found to have excessive emission levels at no cost to the owner and should ascertain the cause of the excessive leakage. The service personnel should instruct the owner not to use the unit until the oven has been brought into compliance.
# If the oven operates with the door open, the service personnel should:
- Tell the user not to operate the oven.
- Contact the manufacturer.
# The service personnel should check all surface and vent openings for microwave leakage.
# Check for microwave leakage after every servicing. The power density of the microwave radiation leakage emitted by the microwave oven should not exceed 5 mW/cm2. Always start measuring of an unknown field to assure safety for operating personnel from radiation leakage.
MEASURING MICROWAVE ENERGY LEAKAGE
# Pour 275±15 cc of 20±5°C(68±9°F) water in a beaker which is graduated to 600 cc, and place the beaker on the center of the turntable.
# Set the energy leakage monitor to 2,450 MHz and use it following the manufacturer's recommended test procedure to assure correct result.
# When measuring the leakage, always use the 2- inch (5 cm) spacer supplied with the probe.
# Operate the oven at its maximum output.
# Measure the microwave radiation using and electromagnetic radiation monitor by holding the probe perpendicular to the surface being measured Move probe along shaded area Probe scanning speed: Less than 2.5 cm/sec
MEASUREMENT WITH OUTER CASE REMOVED
# When you replace the magnetron, measure for microwave energy leakage before the outer case is installed and after all necessary components are replaced or adjusted. Special care should be taken in measuring the following parts.
- Around the magnetron
- The waveguide
MEASUREMENT WITH A FULLY ASSEMBLED OVEN
# After all components, including the outer case, are fully assembled, measure for microwave energy leakage around the door viewing window, the exhaust opening, and air inlet openings.
# Microwave energy leakage must not exceed the values prescribed below.
NOTE: Leakage with the outer case removed less than 3 mW/cm2. Leakage for a fully assembled oven (Before the latch switch (primary) is interrupted) with the door in a slightly opened position-less than 1 mW/cm2
NOTES WHEN MEASURING
# Do not exceed meter full scale deflection.
3 The test probe must be removed no faster than 1 inch/sec (2.5 cm/sec) along the shaded area, otherwise a false reading may result.
# The test probe must be held with the grip portion of the handle.
A false reading may result if the operator's hand is between the handle and the probe.
# When testing near a corner of the door, keep the probe perpendicular to the surface making sure the probe horizontally along the oven surface, this may possibly cause probe damage.
MEASUREMENT OF MICROWAVE POWER OUTPUT
# 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.
REPAIRING METHOD OF SEVERAL BREAKDOWN
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.
COMMON BREAKDOWN AND MEANS OF REPAIRING
PHENOMENON
CAUSE
REPAIRING MEANS
1.When starting the
oven, the lamp is not on,
the turntable tray can’t
rotate and the food can’t
be heated






1. fuse broken
2. The primary and secondary winding of the
transformer are short-circuited.
3. The earthing or the polarity of the capacitor is
punctured.
4. The pilot switch can’t cut off.
5. The interlock switch hasn’t closed.
6. The power plug and the socket are not in good
connection.
7. The door hook broken.

1. Change a new fuse.
2. Change a new
transformer.
3. Change a new capacitor.
4. Change a new pilot
switch.
5. Change a new interlock
switch.
6. Adjust the connection or
replace it by a new one.
7. Change a new book.
2.When starting the
oven, the lamp is on, the
turntable rotating, the fan
cycling but the food can’t
be heated.



1. The primary and secondary winding, the
filament of the transformer are open-circuited.
2. The magnetron filament is open-circuited, the
magnetic steel of the magnetron broken or the
magnetron is air leaking.
3. Time and power distributor broken..
4. The plugs of the magnetron of the capacitor
loosed.

1. Change a new
transformer.
2. Change the magnetron.
3. Change the time power
distributor or the micro
switch.
4. Fix them.

3.The food can be
heated, but the lamp is
not on.

1. The lamp broken.
2. The plug fall off.


1. Change a new lamp
2. Insert the plug again


4.The food can be
heated but the turntable
tray is not rotating.


1. The turntable motor broken
2. The plug fall off
3. Connecting shaft weave broken


1. Change the turntable
motor
2. Insert the plug securely
3. Change the weave

5.The oven can heat
within 2-3 minutes, but
can not heat from the
fourth minutes.



1. The winding of the fan motor in open-circuited.
2. The fan falls off
3. The plug of the fan motor falls off
4. The turntable shaft is griped with the motor
bearing.
5. The cooling vent blocked

1. Change the fan motor
2. Change the fan
3. Insert the plug
4. Overhauling them
5. Repairing it


6.When starting the
oven, it can’t heat, and
with “wenwen” noise


The high voltage diode was punctured




Change a new diode



7.The oven can heat,but
with sound “shishi” noise

8.Large amount of
microwave leakage







The iron core of the transformer loosed


1. The door deformed
2. The door metal net cracked
3. The gap of the door crack is too large
4. The welding point of the oven fall off
5. The screws which fix the magnetron loosed
6. The wave guide connection oxidized
7. The magnetron copper filament washer is too
thin cause the wave guide opening not in good
earth.
Change a new transformer

1. Mend the door
2. Change the door
3. Adjust the gap
4. Change the oven
5. Tighten the screws
6. Scrape the oxidized and
tighten the screws
7. Thick the copper filament
washer


9.The door can’t open





1. After long time using, the wear and the rust
eaten enlarged the gap of the door shaft and
the shaft hole, thus cause the door crooked.
2. The door hook broken

1. Adjust the hinge to rectify
the position of the door.





10.The door release
button fall off

Wore aged after long time operating

2. Change the hook
Overhaul it or renew it


11.Electricity leaking
The earthing insulation resistances of all the
motors or the transformer are less than 2 mega
ohm.
Test where is the leaking
place, then repair it or
change those damaged
components.