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How to Repair LCD TV Auto Shutdown RandomlyModel: LG 32LB9R-TB LCD TVSymptom: The TV will Auto Shutdown Randomly

This is an LG 32LB9R-TB LCD TV. The problem was it will auto shutdown after 5 minutes or 30 minutes, sometime after 1 hours then power shutdown. When try to press again the TV power on switch, the Tv will directly shutdown again or after few seconds. When using table fan to blow the Power Supply board (PSU), and using a piece of card board to block the wind blow to mainboard. The TV can last 2 hours still working properly. After that the table fan change to blow the mainboard and the card board block the PSU. The TV was shutdown just after about 8 minutes! This power supply board is an IP board, the part number is: EAY38637401. Even change the table fan blow the PSU, the TV can last longer to working!

With this method, we know that the problem was inside the power supply board. So I can replace a working power supply board to solve this TV problem. But I want to repair it in components level and want to find out which component is causing the symptom like this. Since we know that the problem was inside the PSU board, so we can concentrate to testing or troubleshoot this board. Using the hot hair dryer to blow the PSU board to find out which section is sensitive with the heat blow and shutdown the TV. Before that, checking their ERR pin and it was located in PSU board connector. Where the ERR pin features are to detect any high temperature, abnormal current and voltage activity occurs and the mainboard CPU will shutdown the TV to prevent any serious damage in it. I found that their ERR pin voltage was starting (when power on the TV) from 4.68V slowly drop to 3.19V. When the ERR pin voltage drop to 3.28V the TV will shutdown. Thats mean when ERR pin voltage drop to 3.28V, the TV will in protection mode (TV automatic shutdown) .So we need to know which section is causing this voltage drop and causing the TV in protection mode. This PSU board is an IP board, so their inverter section was built-in into the board. Inside this PSU board, it has PFC section, PWM section, Voltage output section and the High Voltage section (inverter). So using the hot hair dryer to blow to the PSU board carefully and found that the inverter section was the culprit! After that using soldering iron and freeze/Coolant to use interchangeably to testing their small components inside the inverter section. Finally found the Q311 (1A) smd NPN transistor was defective and it is nearby the U302 (LM393). Replaced it with a H1A (2N3904) smd NPN transistor and the TV working properly now.

After changed the Q311, the ERR pin voltage is about 3.53V. Even this TV working about 9 hours, their ERR pin voltage still stable and its about 3.56V.

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