Monday, January 11, 2016

DAEWOO Microwave Oven KOC-1M0K9S Power output measurement – Radiation test – High voltage transformer test – magnetron test – Schematic diagram - Microwave oven repair and service


Microwave output power can be checked by indirectly measuring the temperature rise of a certain amount of water exposed to the microwave as directed below.
1. Microwave power output measurement is made with the microwave oven supplied at rated voltage and operated at its maximum microwave power setting with a load of 1000 +/- 5cc of potable water.
2. The water is contained in a cylindrical borosilicate glass vessel having a maximum material thickness of 3 mm and an outside diameter of approximately 190 mm.
3. The oven and the empty vessel are at ambient temperature prior to the start of the test. The initial temperature of the water is 10 +/- 2 Degree C (50 +/- 3.6 Degree F). If 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 shelf, which is in the lowest normal position.
4. Microwave power is switched on.
5. Heating time should be exactly A seconds.
Heating time is measured while the microwave generator is operating at full power. The filament heatup time for magnetron is not included.
6. The initial and final temperature of water is selected so that the maximum difference between the ambient and final water temperature is 5K.
7. The microwave power output P in watts is calculated from the following formula:
P = 4187 X Delta T/t
# Delta T is difference between initial and ending temperature.
# t is the heating time.
The power measured should be B W +/- 10.0 %.
1. Water load should be measured exactly to 1 liters.
2. Input power voltage should be exactly specified voltage.
3. Ambient temperature should be 20 +/- 2 Degree C (68 +/- 3.6 Degree F)
Heating time for power output:
A (second)
B (W)

1. Make sure to check the microwave leakage before and after repair of adjustment.
2. Always start measuring of an unknown field to assure safety for operating personnel from microwave energy.
3. Do not place your hands into any suspected microwave radiation field unless the safe density level is known.
4. Care should be taken not to place the eyes in direct line with the source of microwave energy.
5. Slowly approach the unit under test until the radiometer reads an appreciable microwave leakage from the unit under the test.
1. Prepare Microwave Energy Survey Meter, 600cc glass beaker, and glass thermometer 100 Degree C (212 
Degree F).
2. Pour 275cc +/- 15cc of tap water initially at 20 +/- 5 Degree C (68 +/- 9 Degree F) in the 600 cc glass beaker with an inside diameter of approx. 95 mm(3.5 in.).
3. Place it at the center of the tray and set it in a cavity.
4. Close the door and operate the oven.
5. Measure the leakage by using Microwave Energy Survey Meter with dual ranges, set to 2450MHz.
1) Measured radiation leakage must not exceed the value prescribed below. Leakage for a fully assembled oven with door normally closed must be less than 4mW/Cm2.
2) When measuring the leakage, always use the 5 cm (2 in.) space cone with probe. Hold the probe perpendicular to the cabinet and door. Place the space cone of the probe on the door, cabinet, door seem, door viewing screen, the exhaust air vents and the suction air vents.
3) Measuring should be in a counter-clockwise direction at a rate of 1 in./sec. If the leakage of the cabinet door seem is unknown, move the probe more slowly.
4) When measuring near a corner of the door, keep the probe perpendicular to the areas making sure the probe end at the base of the cone does not get closer than 2 in. from any metal. If it does not, erroneous reading may result.

# High voltage is present at the high voltage terminal of the high voltage transformer during any cooking cycle.
# It is neither necessary nor advisable to attempt measurement of the high voltage.
# Before touching any oven components or wiring, always unplug the oven from its power source and discharge the capacitor.
High voltage transformer
1) Remove connections from the transformer terminals and check continuity.
2) Normal readings should be as follows :
Secondary winding ... Approx. 100 Ohm +/-10%
Filament winding ... Approx. 0 Ohm
Primary winding ... Approx. 1.2 Ohm
High voltage capacitor
1) Check continuity of capacitor with meter on the highest OHM scale.
2) A normal capacitor will show continuity for a short time, and then indicate 10MW once the capacitor charged.
3) A shorted capacitor will show continuous continuity.
4) An open capacitor will show constant 10MOhm.
5) Resistance between each terminal and chassis should be infinite.
High voltage diode
1) Isolate the diode from the circuit by disconnecting the leads.
2) With the ohmmeter set on the highest resistance scale measure the resistance across the diode terminals.
Reverse the meter leads and again observe the resistance reading. Meter with 500V dc or higher voltage batteries should be used to check the front-back resistance of the diode, otherwise an infinite resistance may be read in both directions. A normal diode's resistance will be infinite in one direction and several hundred kOhm in the
other direction.
For complete magnetron diagnosis, refer to "Measurement of the Microwave Output Power." Continuity checks can only indicate and open filament or a shorted magnetron. To diagnose for an open filament or a shorted magnetron,
1) Isolate magnetron from the circuit by disconnecting the leads.
2) A continuity check across magnetron filament terminals should indicate 0.1 Ohm or less.
3) A continuity check between each filament terminal and magnetron case should read open.
If the fuse in the primary and monitor switch circuit is blown when the door is opened, check the primary and monitor switch before replacing the blown fuse. In case the fuse is blown by an improper switch operation, replace the defective switch and fuse at the same time. Replace just the fuse if the switches operate normally.