LG TV Repair Case: After the TV is turned on, the indicator light is on but there is no raster on the screen.
Analysis and Maintenance: A static inspection found that the line output transistor (Q402) was already broken down. After replacing the line output transistor and testing the machine again, it was damaged once more, indicating that there are other faults in the circuit.
The main reasons for the repeated burning of the line output transistor are as follows:
(1) Insufficient line excitation increases loss and current. The temperature rise of the line output transistor is relatively fast. When the temperature rise exceeds the limit value, the line output transistor will be damaged.
(2) Instantaneous increase in power supply voltage causes the supply voltage to be too high. When it exceeds the voltage endurance value of the line output transistor, the line output transistor will be damaged.
(3) Severe leakage or short circuit in the line output circuit increases the current in the line output loop, causing the line output transistor to be damaged due to overcurrent.
After the above analysis, first measure the +B voltage without fluctuation or jump-up phenomenon, indicating that the +B voltage is normal. After working for a period of time after turning on the machine, touch the newly replaced Q402 with your hand and find its temperature rise is normal. Therefore, the possibility of damage to the line tube due to excessive current is ruled out. It also indicates that the line excitation voltage is normal. Suspect that the line output transistor was broken down due to an excessively high instantaneous reverse peak voltage. Upon checking, the line retrace capacitor C412 has lost capacity, causing the reverse peak voltage to rise, exceeding the limit value of the line output transistor, leading to the damage of the auxiliary line output transistor.
Solution: Replace the line retrace capacitor C412 to eliminate the fault.
Beijing LG TV Repair Center, August 10, 2011.