Is it possible for the backlight to be faulty if it is ok when running on battery only?

I tried the fn key but it makes no difference.

I have located an inverter but at £80 I would like to feel a little more confident this it will fix the problem before buying it.

Is the power regualtor board asomething that can be replaced easily and at low cost?

 

 

 

It has not switched to external but works fine if I connect an external monitor to it.

The screen is very dark when on mains power but not invisible.  When I remove the power lead the screen returns to normal brightness.

This afternoon I removed the hard disc and battery and got the same result right from boot up.

 

thanks for the info...What puzzles me though is the same soldered cables are used when powered by battery or mains. 

I like the idea that the power regulator on the motherboard might be reducing the mains power supply to the invertor although I have never seen it before and am surprised that it does not show up as a problem elsewhere such as when charging the battery.

we can discount the backlight because it works on battery

we can discount the screen because it works on battery

I have tried two mains power supplies so we can discount those

We can discount the cables because it is ok on battery

We can discount windows drivers because it is the same on boot up without the hard disk installed.

The Bios has been updated.

That leaves the inverter and the power regulator.

I have discounted the video card because everything is ok when using an external monitor but am I right to do so?