No video on internal and external display

Hello!

I have 2 iMacs, same model: iMac Intel 21.5" EMC 2428, one of them was working fine and the other one wasn't showing any image.

I've replaced graphic board, logic board, memory, display and the display cable's, and still no image. Then I've switched the graphic board with the iMac which was working, it worked fine, i did the same with the memory, display and display cable's. No problem, it still works.

Then, when I did the same process with the logic board, the iMac that was working, turned on, with no image, and for a short time, the power supply begin to release some kind of smoke, after that, the same problem that I was having on the first iMac, start to occur on the second one, that was working fine. So, I ordered, all the parts again, MLB, power supply, graphic board, display, display cable's, memory, all brand new, working fine, then I've replaced those parts, and again, the power supply released some kind of smoke, and again, no image.

All diagnostic LEDs are on, but I have no freaking image, both internal and external display. Now both iMacs have no image, they start normally, USB gadgets works, iPhone charges, but no image. Anyone can help me?

I have brand new MLB, power supply, display, display cable's, memory, graphic card, but I didn't tested those yet.