iMac no image on internal, only with external display

Hi,

My Mac computer doesn't show image, only when I connect an external screen.

Only when you get to the desk you can see a slight illumination (like a light beam) I have tried to change the power supply, inverter, video card, full LCD screen (including LCD cable) and finally change the motherboard.

I need help or it is a motherboard problem and this second hand also has the same failure or the aluminum of the computer causes failures (hahaha)

Please I need help! Thank you very much in advance

Hi,

He reviewed this but it seems fine. It is a motherboard bought on eBay and with my previous motherboard did the same thing happen do you think this part was damaged? It is possible .. I will return the motherboard and buy another one again.

From an external monitor, if I try to adjust the brightness in preferences, I see how the screen lights up a lot and turns off with the room in the dark.