i found the problem
the pci port on the motherboard is broken
the rx470 what i send back , smell burn but was still working (but unstable)
so it s actually strange that something happen also to the pcie port (because gpu was still working when i send it back)
now if i put there a gpu, amd driver won't start
i tried installing win10 but only blue screen and reboots
back to 7 but each time i try to install that gpu on pice3, all gpu stay in vga, or bleu screen
i change power rise few time, switch gpu's
also if i disable onboard gpu and a gpu is connected to the broken port, the pc won't boot unless i unplug it or activate intel onboard gpu
motherboard is an asrock b85 anniversary, had so much trouble with that board, before i used rx470's i put 6x 380 on it but only 5 could run stable
still on warranty but can't send back i don't have an other one