Is it the inverter or a faulty display or anything else?

I have an iMac 2009 EMC 2308 with the same symptom as many others in this particular post. I simply can’t make it work. I had to replace the video card and the iMac started to work but only with an external display. The internal one does not show any image, however I can see the backlight working after it boots. All 4 leds light up and it all looks fine when I work with it on the external display. I changed the display cable and the V-Sync one too. When I try to put the image clearer or darker using F1 or F2 I can see the light on the internal display responding to the commands but still no image at all. Is it the inverter or a faulty display or anything else? Please can anyone help me out here? Thank you so much!