2 Graphics cards, 2 motherboards, 2 power supplies. Wont Boot unless there are no graphics cards in them.

Feb 3, 2019
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I just upgraded my PC and I've got some handy downs to build my girlfriend a PC. I had a sandy bridge PC inside this small case so i took it out saw it had a pci express slot on the motherboard that allowed for a GPU. So i put it in a bigger tower and went to start it up and its like there was no power to the board. It wouldn't budge, no lights and no fans. I got no input either. So I took it out of the case to troubleshoot. After taking a lot off i found that the computer would boot if there wasn't a graphics card attached at all. It had to be out of the slot and it couldn't be put in the power supply. That lead me to believe it could be a bad pci slot, a bad GPU, or a bad PSU. So i tried my brand new PSU that is in my rig right now and it had the same results. Which told me either motherboard or GPU. I didn't believe it was the GPU because i just barely upgraded out of that card. It was working just fine days before. So i tried my old motherboard that I was also previously using in my old rig before i upgraded and I had the same results. I still didn't want to believe it was my graphics card so i tried a brand new pci e cable. Still nothing. So i assumed it was the GPU. I tried another GPU just to be certain and I had the exact same problem. I was actually kind of confused. I wanted to make sure at least one of the graphics cards worked so i put my old one (970 gtx) in my new rig and it booted right up. I'm at a loss for words. I have no idea what the problem could be. I've tried 2 different motherboards, 2 different power supplies, and 2 different graphics cards and for some reason it wont boot with the cards in them. I was thinking possibly a short but it boots when the graphics cards aren't connected in any way. So that doesn't tell me it is a short. I would like to also mention that out of both of the PSUs one of them would make a click sound like a tripped fuse when the graphics cards were put in. And i highly doubt i don't have enough power because the old PSU is a 750w 80plus gold standard PSU and my new one is a 850w corsair.

p.s when i put the 750W back in my new rig to test it only my ram rgb lights would kick on and it wouldn't boot. Though i have a 2080 in there with a z390 aorous pro that are huge power hogs so i didn't have enough slots to get my GPU powered. But you should still be able to boot the PC without a GPU considering I had integrated graphics with my i5 9600k and my board supports integrated graphics. So i'm still leaning on the side of a new PSU but the fact my new 850watt didn't work on the other boards besides my new rig tells me otherwise. So i'm really just unsure where to go from here.
 
Feb 3, 2019
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That was another thought that had popped into my head when It first happened but both my motherboards have different cpu's in them. The micro atx has a i7 sandy bridge and the atx motherboard has a i7 950. The atx board was working fine before i got my upgrade because i was using it with my 970gtx which was working perfectly a few weeks ago.