Hisense LCD TV TLM46V86PK sometimes makes strange noises when it is turned on, and the screen displays problems when it restarts - Home Appliance Repair

A Hisense LCD TV TLM46V86PK (1933 board) LCD color TV, it is said that sometimes the screen goes black and the TV does not turn on. The user sent the TV to the dealer for transfer. The specific cause of the fault is not clear, so everything is normal when it is powered on and tested. It is probably hidden. So I decided to observe the problem for a long time. It works fine every morning when I turn it on until I get off work. Sometimes it turns off and on again. On the morning of the third day, the power-on failure occurred. The loudspeaker made a loud rustling sound, and at the same time, the screen flashed with colorful stripes. After a while, the automatic power-off indicator light stayed on and then turned on again. It returned to normal after repeated several times and never happened again. According to the phenomenon, it should be It is a cold start problem. When disassembling the machine, the voltage of the power supply board is normal first, and the fault is determined to be a problem with the main board. When the fault occurs, when measuring the power supply of each group, it is found that the N32 [AMS1117-TA] 1.8V regulated output is unstable at around 1.5--2.5 Up and down. The resistance of the measured output end to ground is normal and stable. The voltage at the adjustment end of measured pin 1 also fluctuates by a few tenths of a volt. It is suspected that the external component of pin 1 is leaking. Check that there are three main peripheral components, C777. R905. R260. The measured capacitance and resistance were all normal, so the power-on fault of the above-mentioned components was repaired and disappeared, and the output voltage of 1.8V was stable and the power was turned on normally. A few hours after the power was turned off, the power-on failure reoccurred. At this time, it returned to normal as soon as it was soldered. Replacing the capacitor C777 had no effect. It is probably caused by poor performance of the voltage regulator block or leakage of the circuit board. First, replace the AMS1117-TA and power it on normally. To be on the safe side, let the machine After cooling down, it turned on normally again. The fault was eliminated. After several days of continuous testing, the fault did not reoccur. It is analyzed that the fault is due to the abnormal operation of DDR caused by the unstable 1.8V voltage, which causes the machine to be unable to communicate normally, resulting in a program error and failure to start.