Sunday, January 03, 2016

LG MC-766Y MICROWAVE OVEN Test procedure and component test – Magnetron Test – High voltage transformer test – High voltage diode test – Microwave oven repair

LG MC-766

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 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 190 mm.
# The oven and the empty vessel are at ambient temperature prior to the start of the test.
# The initial temperature (±1) 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 shelf 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 5°K.
# The microwave power output P in watts is calculated from the following formula:
P = 4187 X (∆T) + 0.88 X (T2- T0) X M/ T
# T2: Temperature after heating
# T0: Temperature of bowl
# M: Weight of bowl
is measured while the microwave generator is operating at full power. Magnetron filament heat-up time is not included.
# 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.
PRIMARY INTERLOCK SWITCH TEST
When the door release button is depressed slowly with the door closed, an audible click should be heard at the same time or successively at intervals. When the button is released slowly, the latches should activate the switches with an audible click.
If the latches do not activate the switches when the door is closed, the switches should be a adjusted in accordance with the adjustment procedure. Disconnect the wire lead from the primary switch. Connect the ohmmeter leads to the common (COM) and normally open (NO) terminal of the switch. The meter should indicate an open circuit in the door open condition.
When the door is closed, the meter should indicate a closed circuit. When the primary switch operation is abnormal, make the necessary adjustment or replace the switch only with the same type of switch.
SECONDARY INTERLOCK SWITCH TEST
Disconnect the wire lead from the secondary switch.
Connect the ohmmeter leads to the common (COM) and normally open (NO) terminals of the switch. The meter should indicate a open circuit in the door open condition. When the door is closed, meter should indicate an closed circuit. When the secondary switch operation is abnormal, make the necessary adjustment or replace the switch only with the same type of switch.
MONITOR SWITCH TEST
Disconnect the wire lead from the monitor switch. Connect the ohmmeter leads to the common (COM) and normally closed (NC) terminals of the switch. The meter should indicate closed circuit in the door open condition. When the door is closed, meter should indicate an open circuit. When the monitor switch operation is abnormal, replace with the same type of switch.
NOTE: After repairing the door or the interlock system, it is necessary to do this continuity test before operating the oven.
COMPONENT TEST
HOW TO TEST HIGH VOLTAGE TRANSFORMER (Wire leads removed)
1. Measure the resistance.
(Ohm-meter scale: Rx1 and Rx100)
# Primary winding - Approx.: 1.4 ohm
# Secondary winding - Approx.: 90 ohm
# Filament winding - Less than: 1 ohm
2. Measure the resistance.
(Ohm-meter scale: Rx1000)
# Primary winding to ground - Normal: Infinite
# Filament winding to ground - Normal: Infinite
HOW TO TEST MAGNETRON (Wire leads removed)
1. Measure the resistance.
(Ohm-meter scale: Rx1)
# Filament terminal - Normal: Less than 1 ohm
2. Measure the resistance.
(Ohm-meter scale: Rx1000)
# Filament to chassis - Normal: Infinite
NOTE: When testing the magnetron, be sure to install the magnetron gasket in the correct position and be sure that the gasket is in good condition.
HOW TO TEST HIGH VOLTAGE CAPACITOR
Measure the resistance.
(Ohm-meter scale: Rx1000)
# Terminal to terminal - Normal: Momentarily indicates several ohms, and then gradually returns to infinite.
Measure the resistance.
(Ohm-meter scale: Rx1000)
# Terminal to case  - Normal: Infinite.
HOW TO TEST HIGH VOLTAGE DIODE
Measure the continuity (Forward).
(Ohm-meter scale: Rx10000) - Normal: Continuity. Abnormal: Infinite.
Measure the continuity (Reverse).
(Ohm-meter scale: Rx10000) - Normal: Infinite. Abnormal: Continuity.
NOTE : Some inexpensive meters may indicate infinite resistance in both direction.
CIRCUIT DESCRIPTION
GENERAL DETAILS
# The low voltage transformer supplies the necessary voltage to the micom controller when power cord is plugged in.
# When the door is closed, the primary switch is ON, the secondary switch is ON, and the monitor switch opens (contact COM and NO).
WHEN SELECTING COOKING POWER LEVEL AND TIME
# The micom controller memorizes the function you set.
# The time you set appears in the display window.
# Each indicator light turns on to indicate that the stage has been set.
WHEN TOUCHING THE START PAD
# The coil of the relay is energized by the micom controller.
# Power input is supplied to the high voltage transformer through the fuse to the primary switch and relay 2.
# Turntable rotates.
# The fan motor rotates and cools the magnetron by blowing the air (coming from the intake on the baseplate).
# The air is also directed into the oven to exhaust the vapor in the oven through the upper plate.
# Cooking time starts counting down.
# 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 high voltage diode and charging of the high voltage capacitor.
# The negative 4,000 Volts DC is applied to the filament of the magnetron.
WHEN THE OVEN IS SET AT ANY LEVEL EXCEPT MAXIMUM.
# The micom controller controls the ON-OFF time of relay 2 by the applied signal to vary the average output power of microwave oven as POWER LEVEL.
# One complete cycle of relay 2 is 22 seconds.
WHEN THE DOOR IS OPENED DURING COOKING
# Both the primary switch and relay 2 are cut off primary winding voltage of the high voltage transformer.
# ON-OFF of relay 2 is coupled electrically with opening and closing of the secondary switch.
# When the door is opened, the secondary switch is opened and when the door is closed, the secondary switch is closed.
# The cooking time stops counting down.
# Relay stops functioning.
# As the door is opened, if the contact of primary switch fails to open, the fuse opens due to the large current surge caused by the monitor switch activation, which in turn stops magnetron oscillation.
WHEN TOUCHING THE START KEY WITH THE GRILL COOKING FUNCTION SELECTED
# The contacts of the primary switch and the secondary switch close the circuit.
# A.C. voltage is applied to the grill heater through grill thermostat as shown by the solid line.