Analysis and Repair: First, check that the line transistor Q402 has been shorted due to breakdown. Try disconnecting the line transistor circuit and connect a dummy load. After turning on the machine, measure the voltages of each group of the switch power supply, which are normal. Replace Q402 and measure its collector-to-ground resistance, which is also normal. However, after a short trial run, the line transistor is burned out again. The reasons for the damage to the line transistor may include:
(1) Excessive +B voltage.
(2) Short circuit in the line output transformer T402.
(3) Insufficient line drive, increasing the collector dissipation.
(4) Abnormal line frequency changing the conduction time of the line transistor.
(5) Reduced flyback capacitor causing an increase in the flyback reverse peak voltage, leading to breakdown of the line transistor.
(6) Overloaded load.
Replace the line transistor Q402 and disconnect its collector with a probe. After turning on the machine, use an oscilloscope to measure the waveform at the b terminal of Q402. It is found that both the amplitude and frequency of the waveform are normal. Check that the flyback capacitor is in good condition, but find that the line output transformer T402 is damaged.
Technical Analysis by Beijing LG TV Repair Center, August 28, 2011.
Handling Method: After replacing Q402 and T402, the fault is eliminated.
Fault Symptoms: LG TV, no raster, no audio, and no image after turning on.
Source of Information: Beijing LG TV Repair http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3701288.