PC Powersupply Be Quiet Dark Power Pro 6 Pc wont start

[JoG3]

Hey

I recieved my friends old PSU. He said it worked in his old pc but it didnt work in his new build.
He said when he started the new pc with this PSU the fans would start to spin but nothing else would happen.

So i decided to test it because my current PSU is underpowered.

I measured the psu before putting in to my case. All the voltages were right.

So i swapped the psus over and tried to start pc up. All that happened was that the fans started but nothing else happened.
I then stripped all the components off the motherboard except processor and the psu.
Still nothing... Not even the usual beeping wich comes if there is no ram memory and video card. 

So I measured the voltages again when the psu were plugged in the motherboard and the computer were turned on. Still the voltages were good. I tried my old psu and the pc booted fine and the damn beeping started so i knew my pc was still alive

What should i check next? Im kinda new to electronics.

Cheers from Finland

[Neverther]

Power Good/OK signal missing?

I'm not sure how motherboards process that signal, but maybe it wont try anything as PSU is telling the voltage/power input is bad.

[Rerouter]

though it should not be the case, is this or the old computer it was taken from a Dell? (dell liked to have there own ATX plug pinout)

Next up is the 4/8 pin power plug that is normally towards the back side of the CPU, without this the CPU never gets its core voltages and simply wont boot,
(big 20 + 4 pin plug, and small 4+4 or 8 pin plug and the +4 of the 24 pin plug is not the same but will fit! )

check that all the plugs are in, if its still not working, unplug the power supply from the board, jump start it (green wire to ground) and measure that the voltages match the atx standard, as that power supply sounds so cheaply generic it could well be the problem (I've been on whirlpool forums for years and all the violent destruction of hard drives from cheap supplies has rubbed me the wrong way on them)

[JoG3]

The psu wasnt orginal one in my friends old pc. This is a store bought "high end" one.

I measured the psu off from computer.

• Red 5v measured 5.6v
• Yellow 12v measured 12.04v
• Orange 3.3v measured 3.4v
• Blue -12v measured -11.97v
• Purple 5v measured 5.15v
• Grey 5v measured 0.1v

When I tested the psu plugged in to the computer, all the voltages were the same except the grey "Power_OK" wire.
It peaked 4.5v at startup but then it dropped to 0.1v. Should this happen or does it need to be constant 5v when computer is turned on?

And i compared the psu to my old one and the pins were identical 

[Rerouter]

http://whirlpool.net.au/wiki/power_good_signal

it means the psu is telling you it isn't happy with its output, thus the motherboard will never boot,

[JoG3]

Ooh... I didn't know that. Thanks!
But what I should do next? How to diagnose the problem or is there some components with usually are the cause of this?

[Rerouter]

Take another known working power supply, e.g. your computers one and put it in his to test,

[JoG3]

I meant how to diagnose why there is the +5v missing from the grey wire?

[Melkor]

Because it's trying to protect itself. After you measured a voltage on the red 5V rail that is over 10% higher then it should, it is normal that will not want to expose the rest of the components to this overvoltage.

[Shock]

Possible shunt regulator or zener diode problem depending on the design.

As your probably aware it can be dangerous working on near exposed mains voltages.
It's not an ideal first project. So unless you know how avoid getting electrocuted, buy a new one.

The next step involves making a dummy load and dismantling/reassembling the PSU, soldering and testing around exposed mains voltages.
So you should be familiar with your test equipment and proper electrical safety.

[JoG3]

Ok thanks. I will start figuring it out.
And dont worry, i have had my fair share of elecrtocutions along the way. So i know where not to poke