Bad PCI-E power adapter somehow "broke" two motherboards

[nobody647]

My friend is building a new PC with a GTX 1080, i7 6700k, Gigabyte GA-H110M-A motherboard, and a Corsair CX750M PSU (I don't have the full pcpartpicker list). It booted without the video card, and it booted with the video card without supplying power to the video card (The GEFORCE logo lit up). The 1080 requires an 8-pin PCI-E power cable, but it came with an adapter from 2x 6-pin to a single 8-pin (pacificgeek.com/productimages/xl/CB-2X6F8M.jpg). We found two 6+2 pin cables for the modular PSU, and decided to plug both of them into the adaptor, leaving the extra two pins out and using them like 6-pins. It booted, but the corner of the motherboard where the power switch, reset switch, HDD light, etc. plug into got extremely hot and the next time we tried to start it, it wouldn't boot. We removed the video card and it still didn't boot. The power supply still worked on another PC, but another PC's power supply would not work with this one, so the problem was not the power supply. We figured that somehow we broke the motherboard by giving "too much power" to the video card, and exchanged the motherboard for a GA-Z170M-D3H. We got it to the same state as before, the GEFORCE logo on the side of the video card lit up. This time when we tried to power it, we put the 6+2 into the 8-pin slot on the video card. But now when we tried to boot it, the case light would come on and the case fans would jolt and move a few millimeters, but the PC would not boot. After removing the power to the video card, it booted as it did before. We then tried to use the adapter again, only putting a single 6+2 connector in, leaving out the extra 2 pins. The PC did not boot, and continued to not boot, even after removing the cable. We tried different slots on the PSU, different PCI-E slots for the video card, and eventually removed everything except the mobo, CPU, and RAM, and the PC still refused to boot. Using another PSU still doesn't work, and our PSU will power another computer. Both times, the motherboard seemed to break only after using the adapter. I'm not sure if what I'm describing is possible, or if there is a problem with another component. I do not believe there is a problem with the CPU, as after we cleaned it and installed it in the new motherboard, the PC booted, only to stop working after the adapter was used. Some additional information: I'm not sure if the 1080 is in good condition, as it was purchased but then returned to the Microcenter where we bought it before we purchased it. Whoever owned it previously didn't even remove the plastic wrap on the sides of the card, so we thought it was never used. It's possible that whoever bought it first realized it was broken and instead of RMA-ing it, returned it to the Microcenter without telling them that it was broken. Thank you so much for reading this and taking the time to help us.

 

[Dark Lord of Tech]

That setup with that card and PSU requires no adapters to run.

The PSU has the needed PCIE cables for the 1080.

 

[nobody647]

The first time we tried this, I wasn't there, and my friend decided that he should use the two 6+2 pin connectors on the adapter because more power = better (He thought that 12 pins worth of power would make it run faster than 8 or something like that). After we replaced the motherboard, the PC would boot only if the 1080 was not connected to power. When I directly plugged in the 6+2 to card, the case lights would come on and the case fans would move slightly, but it would not boot. I tried to use the adapter to see if somehow it might work