Yeah, ATX is showing 3/5/12 as it should. Well, 12.5. Maybe one duff HDD molex cable was enough to cause this, plugging the other into the same bit works as well for the case fan. Weird. Time to put it all back together.

And... Still nothing ffs. Going to just smash it all with a hammer. Think Truffy had something about that in his advice.

Have you got an old/another PC tucked away somewhere, as in try powering it with your BeQuiet PSU or try powering your current PC from the other PSU? The BeQuiet does look as though it has all sorts of fancy dohickeys and magic electric juju inside.

Do you have a hard drive (not solid state) you can use as a load i.e. plug in directly? I think these need 12V to spin up which would show whether it's supplying enough.

Edit: OK, I see you've moved on beyond this. Serves me right for taking a break to watch the telly before clicking on post.

Ha. Yes. I am a bit bereft of spare things (small flat plus baby means clear outs were enforced) but I'm back to being flummoxed now anyway.

Rrrgh. So I started again thusly: PSU connected to nothing, ATX shorted with paperclip. Powers on, volts read as they should. Plugged in the 'other' motherboard power (that fires up the CPU I think) and powered up, volts read as they should. Plugged in PCIe for the graphics, volts all good. Plugged in AIO CPU cooler power, volts all good. Plugged in hard drives power, NOTHING. Hello. Went back a step, nothing. Mixed and matched a few steps, nothing. Went back to just PSU With ATX cabLe shorted, nothing.

*flings hands in the air*

*goes to bed*

When it does nothing, the PSU doesn't power up?

Sounding like a PSU on its way out.

Broken and/or stuck power switch? You can bypass the switch by stabbing wildly at the motherboard shorting the two pins that the power lead from the switch is plugged onto, with a screwdriver. If the PC turns on, the switch is faulty.

The relevant wire will be a very fine twisted pair and probably have PWR written on the plastic plug bit in itty bitty writing. It may also be silk-screened on the mobo near the pins.

I've had bad hdds totally nix a pc posting (not stopped power on though). 

Nah, forget the pins thing, I didn't spot that you left the paper clip in. Although shorting the pins will work.

So all the power stuff worked, fans on etc. until you plugged a hard drive in?

Yes... But once I unplugged it again (and I'd had the same HDD on moments earlier no problem) it still wouldn't work. So unless it outright killed the PSU, I dunno.

My mobo has a power button on the board itself so I'm reasonably satisfied the case isn't a problem.

This morning, I tried PSU with paperclip plugged into nothing and it switched on at correct voltages again. Then I went to work. 

When I found myself with a similar dilemma a few weeks ago, I was happy that the motherboard was faulty as the multimeter readings for the PSU were fine and it powered a hard drive, fans etc perfectly well when not connected to the board. It was also a reasonably expensive item even if not quite up there with yours.

However, I decided that I couldn't be sure, and as I hadn't done any PC building for ages I bought a new motherboard/CPU/memory and a new PSU. The old CPU was an early i5 and as compatible second hand motherboards are stupidly expensive, I couldn't reuse it.

I was then left with the old PSU which I used to build a second PC/Server using an old Athlon 64 and motherboard from the I-can't-throw-that-away pile. I learned a lot about the fibs told by Nvidia and several mobo manufacturers about how much memory the early Nforce chipset/bios combo could support (up to 4GB but only if you could find memory modules that nobody actually made) and had fun playing with it until it started making crackly noises and finally the PSU burst into flame with a loud bang.

So my present theory is that the PSU may well have killed the original motherboard by serving up generous portions of voltage at inappropriate moments.

Of course, I then decided to buy an ultra cheap PSU and discovered that I could make use of my old i5 and memory with a new motherboard from Gearbest who manufacture a whole range of boards based on old chipsets for a far more reasonable price than second hand ones fetch on ebay. Expensive things computers.

My motherboard, CPU, RAM and PSU are all second hand from the same bloke, who has the most ridiculous leading-edge kit all the time and sells it on as soon as the bleeding edge moves on. While this means I got good stuff cheap it does leave me with a bit of a replacement dilemma, should I ever get to the bottom of this mystery. I doubt I'll go like-for-like in quality on any of it.

But yeah, first I have to figure out this nonsense. I suppose tonight I'll go through the whole chain again but maybe only plug in the SSD drive and not the mechanical one at first. Or something.

Ok. Set the whole power chain up again. If I plug in everything but the motherboard, and short the ATX pins with a paperclip, it powers on. So, motherboard failing, right? Switched it off again to think about the next thing I can do. Except I just thought "oh yeah, recheck voltage" and went back to it and now it will NOT power on. Wtf? Argh!

Did you try another psu already? Starting to sound like it's your problem.

Would have to buy one to do that I think (though I'll raid the server room at work tomorrow just to check) so I'm trying to avoid it until I'm sure. It is increasingly more likely... But I kinda feel like the motherboard or something on it could yet be the culprit somehow.

After leaving it a while then trying again it switches on and let's me read a good voltage. It really seems that connecting the motherboard is the step that jiggers it and puts the PSU into some kind of safety mode that shuts off the power for a bit.

Maybe it has overheating protection. Does it get noticeably warm?

Nope. Also a cold start plugged in is a fail, but while I've got it running in whatever form I can leave it for plenty of time no trouble.

Your theory looks pretty good. I see that the BeQuiet P8 1200W has several clever protective measures which a broken motherboard would probably trigger - like short circuit/under voltage/over voltage etc. etc. all of which turn the PSU off (and probably keep it off for a while).

http://www.bequiet.com/en/powersupply/10


Edit: not sure that link's right, but I think the tech details are the same.

Edit Edit: the link's OK

Yeah. Looks like. Hm. Nevertheless, I've ordered a PSU, my thinking being that I can potentially be careful about unpacking and connecting it to the minimal setup (including motherboard) and if it still fails, return it. Whereas if I get a new motherboard and that still fails I'm stuck with it.

No luck on a PSU scrounge in the office so it's done. I really hope it is the PSU because the motherboard's current version costs £275 fucking pounds, there is no way that's getting matched. I wonder what level of downgrade I can get without missing something noticeable.