Question ASUS rampage IV formula black screen

Apr 4, 2020
HI all. I have problem with my pc and i am desperate- to this time i didnt find same problem on the internet. My hardware is: Asus rampage IV formula, I7 3930k, gigabyte HD 7870 OC, SSD with windows 7 64 bit intel 530, fortron au750w. Last week i had MB Gigabyte x79 ud7 and my pc shut down for no reason- black display. After servis repair we recognize the motherboard was dead. Every other components were running normally. So i bought asus rampage 4 formula. When i changed MB still i cant start pc= on MB is code 34 and vga+boot led is lightning. Screen is black and says no connection. I plug it with hdmi. So i tried change graphic from my old pc nvidia 6600gt with vga cable and it runs. With this card i can go to bios settings and also start windows. So i update bios to the newest version, and set defult in bios. But 7870 still not working. I tried all pcie slots, also other cables from psu, also i tried use older bios, diffrent hdmi cables but not working. When i use both card together i can see them both in bios but only with 6600 gt i can boot system. Please can someone help me what to do ? Thank you
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