Built new PC and it's not starting up...

Hello!
I need help with my new rig.
Bought new CPU (core i7-3770), motherboard (Gigabyte GA-Z77-D3H) and memory (whatever). Have old PSU (700 watt FSP).
Connected everything and found that it's not starting up.
Disconnected drives, graphics card, memory, all the USB and other unnecessary stuff - same result.
Disconnected 12V power from motherboard and now when I press power, all the coolers start for a second and then turn off again.
Tried different 12V connector and slot on PSU - nothing changes.
Tried unscrew MB from the case and start it on a cardboard box to be sure there are no short circuits from screws. Same result.
I would really appreciate help, 'cause I'm stuck.
Thank you

So what exactly happens? turns on then off almost right after? points to a PSU problem.

Could be and I'd think the same, but I remover PSU from a working old PC 30 minutes before plugging it to a new one.
Of course can happen that I've damaged it in the process (I unscrewed it to remove a dust)

Thats a possiblity. I would recommend trying another PSU (borrow a crap one from a friend and attempt to boot without extra stuff or something maybe). I honestly have no explanation for that problem apart from it.

If it helps, my bro made the mistake of buying a cheap PSU/case combination, and the PC would turn on for a second or two then turn off. Unsurprisingly the PSU was a complete dud.

Will try that tomorrow. Hope it's just a PSU ... otherwise things will become much more complicated (

What you can try to do is jump the PSU outside of the case and see if it turns on and then off. To jump the PSU all you need to do is insert a bent paper-clip into the green pin on the 20+4-pin power connector and put the other end into a black pin. If you have a spare case fan to plug into the PSU I'd suggest doing that so you can see when the PSU is cycling power. Here's a picture that'll hopefully help.

(Picture isn't mine, I pulled it from google, full credit goes to the owner of the picture.)

http://c.fixya.net/fixya20/uploads/Images/8E243EE.jpg

Inserting something metal into something very electrifying sounds dangerous...Never done it though, wouldnt know. ;D

I do it all the time when I'm refilling my liquid cooling system after a nice scrub down. You'll be fine.

Solved!
Tried with a paperclip, found that it's working fine. Started checking connectors and found that I used wrong one. This modular PSU is a bit confusing.
Thanks guys!

ohhhhh.
Ill make a mental note of that for future use...xD

Hehe
As a postmortem - be careful about 12v plug - in my case only one is actually to be used for motherboard, despite that others fit by form and size!

LOL.

You're welcome Nec, have a good one. :>

Thanks, you too! =)