Dodgy PSU?

I had an old PC powered by an 480W Antec power supply which suddenly stopped booting last week. Checked a few things and have the following facts;

1) The motherboard has a LED on it which is lit up with the power supply attached

2) The system wont start up even when I use a screwdriver to bridge the power on switch

3) The power supply seems to run fine when tested with a paper clip

4) When I tested the motherboard with a 250W PSU it WILL start up. The PSU jus tdoesnt have the 6 pin power supply for the gfx card so I couldnt try a full boot with hard drive.

Does all this mean that my current PSU is busted? Why does it still supply power to the mobo (LED light) but wont start from the power button?
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  1. Yes, that's a good indication the old PSU is failed. The PSU provides numerous voltages at both polarities. Just because the jumper trick causes the PSU's fan to run, doesn't mean it's putting voltage... or the necessary voltages. That is where a good PSU tester comes in handy.

    Time for a new PSU.
  2. The +5VSB LED on the motherboard is lit when the power supply's +5 Volt Stand By rail is still powered on. When this LED is lit you should not be installing or removing any devices like memory modules or expansion cards for example. USB devices should be fine.

    The +5VSB rail is actually its own separate linear power supply and will still function even if another rail on the power supply isn't functioning properly so you can't use the +5VSB LED to indicate power supply functionality. You can use it as an indication that the motherboard is receiving some power.

    PSU testers aren't that useful because they don't test a power supply under load.
  3. ...they're a heck of a lot more useful that a paper clip.
  4. I prefer my FLUKE digital multimeter.
  5. clutchc said:
    ...they're a heck of a lot more useful that a paper clip.

    Not really.
    With a paper clip I can open a stuck DVD drive as well as clip together some papers, the psu tester is singular in it's purpose.
  6. For $100 I would too! LOL...
  7. Thanks for the replies guys. I can get a multimeter and look up how to test the voltages on the PSU. Just a little surprised that it failed its only a couple of years old and a good brand, but then it did get a a lot of use.
  8. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire (standby power supply): 5 volts always on. The green wire should also have 5 volts on it. It should go to 0 volts when you press the case power button, then back to 5 volts when you release the case power switch. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.

    The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.

    Old systems used a white wire (-5 volts) that is no longer used.

    "With a paper clip I can open a stuck DVD drive ..."
    :lol: :lol: :lol:
  9. I managed to get a 500W PSU cheap. Hooked it up. Everything seems to be powering up. 6 pin power connector is plugged in, GFX card fan is spinning but no image to screen and screen wont wake. This is getting very frustrating and costly. Any ideas?
  10. What's the brand and model of the motherboard, graphics card and power supply unit?
  11. motherboard: Asus A8N Sli Deluxe
    GFX Card: XFX Geforce 7800 GT
    Old PSU: Antec NEO480
    New PSU: OCZ 500SXS2
  12. lunacyfoundme said:
    motherboard: Asus A8N Sli Deluxe
    GFX Card: XFX Geforce 7800 GT
    Old PSU: Antec NEO480
    New PSU: OCZ 500SXS2

    That's similar to one of my old builds:

    ASUS A8N SLI Premium
    BFG GeForce 7800 GT OC

    Since your graphics card isn't sending a video signal to the monitor then it would seem that the graphics card may have finally died.

    If you can get/borrow another graphics card to troubleshoot with then you should be able to confirm if that's the case.
  13. Seems like that unfortuneatley I dont have any spares so I'll have to buy a cheap PCIe to test and possibly replace it. Almost built a new a PC at this point.
  14. Did some testing with a milimeter on the two PSUs. Both checked out fine eventually. I was getting eratic readings on one when I only had the GFX card connected. Then I connected the hard drive and all the readings were normal. Checked the GFX card and the fan had stopped spinning.

    Seems like a certainty now that the GFX card is dead. Thanks for all the help, I'll update when I get a new card and test it out.
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