Bad computer PSU?

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NickB

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Hi folks...always find good advice in these pages but this time im stumped...well...hoping my psu or mobo isnt toast.

My computer is essentially dead...no display or booting.
I unplugged everything! And only attempted to power MB and case fans.
If I switch the psu on at the back everything powers up...without touching the power button.
I tested the psu by shorting green and black...nothing.
If I plug it back in the mobo the mobo powers up.
In this weird "on" state I read the voltages on all the leads dangling in the case and I get good 12 and 5 volts on all the pins...

Is my psu dead? Thanks for any suggestions.
Nick.
 
Unplug all leads to the motherboard and other devices, except CD-ROM (but unplugged data connector). Now plug power cord in on the back. Does PSU start up? If doesn't, then PSU should be OK.

Now plug in all neccessary connectors to the motherboard, power on and immediately go to the setup. Load setup (or optimized) defaults, save settings and restart. I assume hard drive is not connected, it will stop on boot error. Press power button, it should shutdown. Now you can test whether it helped.
 
So I got another psu and plugged it in...switching seems ok...niw power button works normally and I can switch mobo off...but system wont boot...mobo doesnt beep like it should. Think I have more issues here.
 
Ok blonde moment...forgot the additional 4pin plug on the MB...all working now. ..psu was to blame! Thanks guys!
 
I suspect the 3.3v as I measured 5 and 12v while it was "on".
Just a question on the paperclip test from green to black...couldnt get it to work on new psu...but psu works just fine...i tried various green to black combos...didnt work. How much load does one need to kickstart a psu?
 
NickB said:
I suspect the 3.3v as I measured 5 and 12v while it was "on".
Just a question on the paperclip test from green to black...couldnt get it to work on new psu...but psu works just fine...i tried various green to black combos...didnt work. How much load does one need to kickstart a psu?
Click to expand...
Some PSU won't start without load, as ovevoltage protection kicks out. I is why initial load like CDROM (as suggested in my initial post) or hard drive is required.

Also check voltage under nominal load, i.e. with connected motherboard.
 
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