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So you turn on the computer and nothing happens, it could be a broken case switch and therefore the PSU never gets the signal to turn on. Or it could be a dead PSU or something else dead pulling down the output of the PSU.
Here's what you do:
1) Unplug the leads to the case switch at the mobo and jumper them with a small piece of wire. Now turn on the main power switch at the back of the PSU. If nothing happens, we're a long way to proving the PSU is dead.
2) If above procedure caused nothing to happen, now we get a little more into it. Remove the 20pin PSU connector and the 4pin CPU connector to the mobo, all other psu connections can stay put. Using a small wire (a paper clip will do) jumper pins 14(the only green wire) and any BLACK wire (let's say pin 13 or 15) on the 20 pin PSU connector. Now turn on the main PSU switch. Your PSU should come to life (HDD spins up, and fans spin).
Below is a pic of the 20 pin connector

From the pic you can see that pins 13 and 15 are ground pins, that's why you can use either one. You want to ground pin 14. This is all the case switch does via mobo traces out to the case switch leads. Well it does go thru a FET or Bipolar transistor to make the actual connection, this is how Windows can soft shut down the PC.
3) If nothing still happens then start removing 4 pin molexs one at a time, starting with the video card (if it has one). This will eliminate any peripherals "holding" or "pulling" down the PSU. After all connections are removed and still nothing, then your PSU is dead for sure.
4) Edit: I forgot this possibility. If after step 2, all fans spin up and HDD spins, but at step 1 nothing works, then it's most likely a faulty mobo. And most likely that little FET or Bipolar transistor or at least the circuit that turns that on. Either way at this point you're best to take it in for service or RMA your mobo back if applicable.
Hope this helps, Cheers.

Antec sells a power supply tester for $12 available at circuit city. It has 5 LEDs to tell you if you're getting good power on all rails plus it has voltmeter ports in the back to plug in a voltmeter.

Good investment and beats swapping out a PSU (time and money) if yours is still good.

hi there,
after i put jumper between 14 and ground my HDD kicked in but when i removed jumper my computer still didnt work,my question is if your HDD works when you put jumper what does that mean and what is next step

thanx

Hello and w elcome to Techspot.

It means you need to try another psu. If that doesn`t help, there`s a very good chance your mobo is faulty.

Regards Howard

when i try to boot my computer you can hear fan running and my dvd ram light flashes and red light on case stays on and no green light which usualy comes on normaly start up,my mobo is Aopen ax6b .any ideas what cause that .prior to that i swapped psu with another psu then when i put psu back on this problem showed up..thanx

As I said earlier, if another psu doesn`t solve your problem, your mobo is probably faulty and will need to be replaced.

Regards Howard

Good tutorial, I used the same technique on my friends pc after he got his mobo replaced, at the time it was top of the line mobo. Turns out PSU was just bugered so it ended up sending whole pc back to the shop and got fixed.

Maybe someone should include some pictures, or even a video of this just to make it more clear.

I'm a little confused about point 1) in the tutorial. Can anyone translate it better for me please.
Does it mean the 2-pin power switch lead that is attached to the case?

The other points I understand.

Yes you are correct. unplug the case switch leads to the mobo. On the mobo where those wires just came off, use a piece of wire to jumper those 2 pins. This is eliminatiing the Case Switch, or faulty leads to it, as the problem. When you turn on master power switch at back of PSU and nothing happens, then you need to continue to step2. If all is fine then it's either a bad case switch or bad leads going to from mobo.

Cheers.

I made a video tutorial on the same subject how to jump the PSU pins,
If any one needs a visual guide here ya go!
http://www.metacafe.com/watch/729606...er_supply_psu/

So i tested my PSU to see if was dead. The test was awesome and learned that there was nothing wrong with the PSU. So I'm assuming what's wrong with my PC is either the CPU? or the mobo. How can you be sure the mobo is faulty? When everything is connected, a green light on the mobo is on.

You can have the cpu tested at your local pc repair shop. This should only cost a few dollars.

If that checks out ok, then it has to be the mobo that`s dead. the fact that there`s a green light on the mobo, is no indication that the mobo is ok.

Regards Howard

Howard is correct, but I just want to be clear.
Step 1: removed case switch leads......: what happened?
Step 2: removed 20pin connector........: what happened?

If the PSU doesn't come on after Step1, but does come on at Step2, there's a really good chance the mobo is screwed. But there's noway of telling if the CPU is dead (it's part of mobo) or other parts of mobo. So best thing now it take CPU to some where and have it power up a PC. All you need is a friend with the same socket type mobo (but be careful, if your CPU is really dead you might damage his mobo/PSU).

Cheers.