TCL55 inch LCD (MS68) does not boot maintenance 1 case - android - Phones Developers

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A TCL55-inch LCD, the logo of the boot machine panel "TCL" is on, the whole machine does not respond. First measure 5vCPU power supply, found only 3.2v, the board is marked with 5v, suspected 5v standby circuit has a problem, check for a while did not find the problem, replaced 2 filter capacitors or 3.2v, and finally determined its standby voltage is 3.2v, White was busy for a while. Measure the voltage of other groups and find that 24v has normal output. Then measure the voltage of the power supply filter capacitor at 390v, indicating that the whole machine is already in the power-on state, but the backlight is not bright, and the boot music is not used, indicating that the motherboard has not officially worked yet. The measurement does not have a backlight to turn on the voltage, and the fault is locked on the motherboard.
The working principle of this machine is that after the CPU works, the main power output 24v voltage to the main board, converted to 12v by DC-DC, 12v and then 5v through DC-DC output to each voltage regulator for each group of chips to work.
Measuring 12v voltage is normal, but the voltage of 12v to 5v output is unstable, and it oscillates between 5.8-6v. As the voltage sways, the logic board emits a “beep” sound, which is estimated to be the DC-DC conversion circuit on the logic board. Noise caused by unstable work. At this point, the fault is basically determined at the conversion position of 12v to 5v. After replacing the 5U filter capacitor 330U/25V, the output voltage drops to 5.3v and is no longer shaking. At the same time, the music sounds, the backlight is illuminated, and the whole machine starts to work normally. After measuring this 330U capacitor is only 56u.