No video after motherboard change

Hello,

Let me preface this by saying I’ve worked on computers and phones for ten years, and I make a living doing this.

So I got a iMac second hand, and it had a firmware lock, so I decided to buy another motherboard and change it, I changed the board, and everything boots up but total black screen not any illumination and all 4 LEDS are on for the diagnostics, so I figured I got burned by bad eBay seller.

So I tried my old board which would boot to lock screen before changing motherboard, I tried that and same thing black screen nada, all 4 LEDs are on. So I figured maybe video cable went bad in process of removing it from iMac, and I changed that and still black screen

Any ideas?

Thanks