Pc won't work after trying psu in different computer

I bought second hand pc that had dead cpu or motherboard, i tested the cpu in my main rig and the cpu worked, i decided to try and hook up my main psu to the second pc with the cpu it originally had and it worked so i guessed that the issue was the psu since when i used mine it works fine, then one after a few hours i tried to turn pc on and it turned for a second on and then died, then if i unplug cpu power it will turn on and stay on, then after that i decided to hook my power supply back to my main pc and now that won't turn on, i can get it to turn on after clearing cmos but only with ssd power unplugged, if that's plugged in it dies after a second, if it's not and i clear cmos it will turn on and stay on but nothing comes up on a screen. Main pc specs
Asus z97-a, 4690k, seasonic x850, gtx970, 2 ssd's. Second rig
Asus z87-pro,i5 4460, evga 650 p2, gtx 960, ssd and hdd

There could any number of things.

1) Check CPU seating.
2) Check 12v CPU power cable
3) CPU heatsink fan
4) Motherboard stand shorts.
5) Check and reseat Memory

last thing you could do is plug in built in speaker to hear the error code beep.

Already tried all of those and main pc worked perfectly before, but then i took psu from it and hooked it up to second pc and it won't power on now

so you get the fans to spin but nothing posts

have you tried it without the gpu to see if it just posts?

you cleared the cmos but did you set the bios to see the gpu again. it might natively go to onboard graphics

I got it to go into bios with no ssds hooked up, as soon as those are plugged into mobo and psu it will turn on for a sec and shut off and then won't power on at all until unplugged and cmos is cleared

Sounds like a cable problem then. Try the ssds one at a time on independent cables. Leave the Sata cable unplugged that goes the motherboard . I believe you have a short in a cables that got damaged. Most likely the psu Sata cable. Anyways, give that a try

I tried different sata cable and that fixed it, i guess it was just a bad connector, seems to be a common issue with those seasonic psu if you look at reviews, gonna get rid of it since i already ordered evga p2 850w

Glad you got it fixed. I run those same psu in all my rigs and yet to have the problem but maybe I have been lucky. Seasonic is a company that stands by its product so contact them and get a new one. never had a problem with rma to seasonic.

If you look at reviews it seems that their psu quality isn't what it used to be, i had issues with that one since i got it, much rather try out a different brand, i went with evga p2 850 which has very good reviews