In reply to a post by 12eason:

I would test the PSU in isolation, to rule out the motherboard. Unplug all the cables including the two motherboard ones. Get the 20/24 pin main motherboard cable, bridge pins 13 and 14, 14 is what tells the PSU to switch on, 13 is just ground (you can use any black wire). It's an easy wire to spot as it is the only green one and it's right next to the clip. Pinout; http://pinouts.ru/Power/atxpower_pinout.shtml
Once you've bridged it, turn the PSU on and either the PSU fan will spin up or it won't. If it doesn't it's knackered, but even if it does, it could be that the PSU is putting out low voltage on one or more of its rails, but that is pretty rare tbh. Usually they just go completely in my experience. You'd need another PSU or a multimeter to go any further.

Thanks for this and for the helpful link.

So to make sure I understand correctly before carrying out this tricky task. I hate having to remember where all the various wires go. What I need to do is disconnect every wire from the PSU - presumably I can do that from detaching at the PSU cable end of things rather than removing all the small wires to the motherboard and 4 independent fans. Once achieved bridge pins 13 and 14 and turn on the PSU and see what happens?

Await confirmation before attempting. I should have just waited until I really needed the extra capacity on my hard-drives but I have had it sitting there for 12 months waiting for an opportune moment - clearly it wasn't last weekend!!!

I have also seen corrupted cmos values make a machine fail like this, ( for example, the processor thinks it should run at 14tHz... )
testing the psu in isolation( following 12easons advice) should help determine psu or elsewhere, but generally psu's should be tested with some load.

Another thing that can cause this sort of problem is a motherboard shorting to the chassis. The only real way to test that is to take the motherboard and PSU out of the case, put them on an insulated surface with the power connectors plugged in and see if it works (or at least the fans spin). But that's a pain and I can't see how just plugging a new drive in can have made that happen (but it could easily have led to a disloged cable).

Hopefully it will turn out to be something trivial. Good luck.

Good point. It may be worth clearing the CMOS by shorting the appropriate jumper.

reseat RAM, unplug HDD/optical disk drives, replug connections, remove power fully, remove stuff plugged into back of mobo, remove CMOS battery for 30 min. Put bios battery back only, try again.

In reply to a post by lenaspell:

So to make sure I understand correctly before carrying out this tricky task. I hate having to remember where all the various wires go. What I need to do is disconnect every wire from the PSU - presumably I can do that from detaching at the PSU cable end of things rather than removing all the small wires to the motherboard and 4 independent fans. Once achieved bridge pins 13 and 14 and turn on the PSU and see what happens?

If you have a modular PSU with detachable leads, then detaching them at the PSU would be a whole lot easier. You only need the main motherboard cable. Just make sure the 4pin processor cable is unplugged too, those usually aren't modular.

In reply to a post by 12eason:

In reply to a post by lenaspell:

So to make sure I understand correctly before carrying out this tricky task. I hate having to remember where all the various wires go. What I need to do is disconnect every wire from the PSU - presumably I can do that from detaching at the PSU cable end of things rather than removing all the small wires to the motherboard and 4 independent fans. Once achieved bridge pins 13 and 14 and turn on the PSU and see what happens?

If you have a modular PSU with detachable leads, then detaching them at the PSU would be a whole lot easier. You only need the main motherboard cable. Just make sure the 4pin processor cable is unplugged too, those usually aren't modular.

Excellent - yes the Zalman ZM600 is a modular PSU so that makes life much easier. Will have a quick cuppa and then got on with the task. Thanks to everyone else with ideas - I will work through them all to eliminate things. Even in large towers there never seems enough room to do things easily!

In reply to a post by Pipexer:

reseat RAM, unplug HDD/optical disk drives, replug connections, remove power fully, remove stuff plugged into back of mobo, remove CMOS battery for 30 min. Put bios battery back only, try again.

Thanks - I will give it a try later.

12eason - the PSU blue light glowed for a couple of seconds and the fan spun for the same length of time and then stopped. I tried it a couple of times and the same thing happened but it doesn't remain on so I guess it is in fact a fault on the PSU.

I have to say it is a lot easier removing the modular cables from the PSU

I now have the sides off and nothing else plugged into the back so it is only the internal bits of kit that are drawing power and more importantly can see exactly what does or does not happen when I press the on switch. The CPU fan and several other fans make an attempt at starting but merely twitch the once. A small blue light by the side of the one switch flickers for the merest of time. So I am still perplexe

Sounds like one of the PSU protection circuits is kicking in - you get a brief surge of power, then the PSU measures something out of spec (voltage, current, whatever) and turns itself off to protect itself and/or the motherboard.

EIther that or the motherboard power on circuitry is faulty.

Really need to check the PSU in isolation as others have suggested, or at least with the M/B out of the case and minimum hardware installed (CPU, some RAM and Video)