Dead CPU or motherboard

[8bit]

Old PC was an i7 3770 on a Gigabyte GA-Z77X-UD5H. One day it stopped working, it would power on for like 0.5 seconds then off again immediately, then a few seconds later I'd get another 0.5 seconds of life then off, repeat ad infinitum.

Process of elimination led me to either dead CPU or motherboard (removed all other components and tried another two PSUs). Deciding the motherboard was the more likely casualty I bought new motherboard, CPU and RAM for the main PC and decided to repurpose the old i7 into a home server, so bought a Mini ITX board with the right socket for it.

Same problem there, powers on for a split second and off again then goes into this loop of doing that every few seconds, just as it did on the Gigabyte board. So it looks like the CPU died then.

Decided I still liked the home server idea so I went back to ebay and bought a 3rd gen i5. I've just tried that tonight, in both boards, and I get the same symptom as above. Tried two different PSUs as well.

So either I've been unlucky and I've either bought a duff CPU or motherboard off ebay or I'm missing something, doing something stupid etc. here. Strange thing is, I get the same behaviour with two motherboards, my old Gigabyte one and the Asus mini ITX board. If the boards were faulty then I'd expect them to exhibit different failure characteristics.

Anyone help me out here?

[kalniel]

Are you testing them outside of a case? Any peripherals (eg keyboard) plugged in?

[8bit]

Yes, on the kitchen worktop (granite). All that's plugged in currently is the CPU with a heatsink on top and the two PSU cables.

[kalniel]

Paper clip trick to see if the PSU stays powered up by itself first? Sounds like you've had a lot of bad luck - lack of testing a known working CPU suggests to me that could be the problem, but hard to definitively prove without finding a known working part to check.

[8bit]

Just tried both PSUs, they both spin up fine

My brother has a PC with a socket 1155 board, I've emailed to ask if he'd mind testing the CPUs for me.

[mistral_blue]

Did you do anything wacky like overclock stuff/alter bios settings? Have you reset to defaults/removed bios battery to reset it all? In ADDITION remove and re-set all your components including your RAM/GPU, have had duff contacts cause similar issues in the past...

*re-seat

Also make sure you have PSU connectors going to all the relevant bits of the mobo & GPU, have had boards refused to post after cleaning because I hadn't put all the connectors back on again! Have you tried googling for 'post boot loop' or similar for more ideas...

And are all your jumpers in the right place?

Hang on, do you have on board graphics? If not I suspect the system won't POST if you have no GPU card inserted...

I'vd had it before if graphics card not powered but plugged in, it will boot loop, so if your GC is duff and you've removed everything else, it will not boot unless you have a working GC or on board graphics. Sometimes boards give an error code via bios beeps (assuming you have 10p PC speaker connected to the relevant jumper), others have an led flash pattern or display a number code to show which component is causing the issue...

I apologize if I'm stating the obvious by the way but I'm just coming up with stuff that's caught me out in the past. Nothing scarier than going from a fully working computer to one that won't even POST