repair a viewsonic lcd monitor that keep on/off

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I just received a 17 inches viewsonic lcd from my friend
,but it keeps on/off, the green led keep blinking on and off.

I bypass the internal power supply and power the lcd up using an
external power supply and the lcd works great.

If the internal power have no load(not connected to video board), the
12V output is stable.
But once it connected, the voltage keeps jumping up and down (0-12V)

I also check the all of the caps with esr meter, they are good.
All of the diodes have passed the diode test on the multimeter.
Threre are no visual damage to the board.

i know lcd are cheap now and repair is not costworthy, but i want to
learn.

Any suggestion of what cause the internal power supply to fail under
load?
What parts should i check?
Anyone familiar with the lcd power supply?