No light on display at startup, why?

Hi, I have a Macbook early 2009.

Some days ago I saw that when I move the display the screen turned off. My tech said that was the inverter cable, changed with one original part and now the screen works perfectly in every position. Works great except for one thing: when I turn on the Macbook the screen (before Apple black logo) just turns off (I see the Apple logo if I use torch from my phone). When the OS is ready I press F1 to set the backlight to 0 and then I press F2 to turn the backlight up again and then the backlight works perfectly for hours and hours. It happens only on boot and, sometimes, in the firs minute after OS ready, but I "fix" it only changing the light using F1 and F2. So I don't think it's an hardware problem..

I thought it was a software problem so I format all with Snow Leopard. Nothing. Updated to El Capitan. Nothing again.

What should I do?

Thanks all 🙂