Power Supply or Motherboard 2

[jeremybarker]

I was trying to determine if an issue with "computer 1" was with the power supply or the motherboard. "computer 1" wouldn't turn on, no fans, stripped all components out (onboard video) and the only thing that would indicate it was plugged in was a single green led on the motherboard. I pulled the power supply from "computer 2" and put in "computer 1" still nothing. I put "computer 1" power supply into "computer 2" and the power supply fan spun for a moment, then died. I put "computer 2" power supply back into "computer 2" and now I get the same green led on the motherboard, but nothing else. Have I fried both boards or am I missing something? Thanks!

 

[G0AOZ]

With mains power disconnected from the computer, press the start button on the computer for several seconds. This will reduce any residual voltages within the PSU. Disconnect the PSU from the motherboard, from all drives and peripherals. Connect an old known working hard drive to the PSU. Plug in and switch on mains power. Short out pins 13 and 14 if it's a 20-pin PSU, or 15 and 16 if it's a 24 pin PSU. If the basic circuits in the PSU are still ok, the hard drive will spin and the PSU's internal fan will rotate. If it works, it does not necessarily mean ALL rails in the PSU are functioning correctly, but at least it gives you an idea if the PSU is completely dead or not. Measure and check the votages of all output rails. This test won't tell you if the PSU breaks down under normal load conditions, only a more expensive piece of test gear will determine that.

PSU's can deteriorate over time - they can take a battering from mains supply variations, glitches and spikes. Are these two PSUs you've tried a few years old? You could also try a new one if you're brave enough...

ROGER - G0AOZ.

 

[goombawaho]

Your situation is dying for a third "known good" power supply to be plugged into both motherboards to get a good idea of what's going on - power supply vs. mobo.

 

[jeremybarker]

Thanks for the replies.

I'll try what you suggested G0AOZ this weekend.

Forgot to mention that "computer 2" was operational until I started swapping power supplies, but thanks for the suggestion goombawaho.

I'll post back with the results hopefully tomorrow.

 

[G0AOZ]

... how old?

ROGER - G0AOZ.

 

[jeremybarker]

computer 2" that was working prior is about 8yrs, "computer 1" I have no idea, but older than that. Neither have blown caps or any other damage that I can see on the board.

 

[goombawaho]

Hopefully you didn't "cross-pollinate the destruction" of the formerly working mobo by putting a bad power supply on it!!!

If a good (third) power supply won't get either board to work, yet will work in another PC, it looks like you have two dead boards.

Before pronouncing death though, I would pull both boards out of their cases, put them on a non-conductive surface and only have the bare minimum connected (mobo, power supply, memory, processor/processor fan, video card if not integrated) and see if you can get any action. Start the system by shorting across the power jumpers with a screwdriver.

This will rule out any shorting against the case AND any problem with USB ports or other "extra" items that are often plugged into motherboards. If the bare minimum don't work - it be broke.

 

[G0AOZ]

..."Neither have blown caps or any other damage that I can see on the board.". By this, I take it you mean the motherboard rather than the circuit board inside the PSU. And 8 years could take you back to the time when 'bad caps' was very prevalent in all manner of things electronic. Also, many PSUs expire before they reach that age due to mains power fluctuation stressing...

Worth trying another a good modern PSU with a higher Wattage rating than what you've got.

ROGER - G0AOZ.

 

[jeremybarker]

I did mean the motherboard when I was referring to the capacitors.

I was able to hookup a HDD and short pins 13 + 14 and the PSU fan spun and the HDD powered on. After this I removed the HDD and tested the voltage of "computer 2" PSU and all pins gave me correct voltage.

I wasn't able to identify the motherboard power jumpers, but with the motherboard out of the computer and just the video card plugged in, I tried to power it on using the power switch and I'm still getting nothing.

At this point I'm assuming I fried "computer 2" motherboard knowing that the power supply voltage checked out. The only other 2 power supplies I have here are being used and I can't risk causing any potential problems with them.

Thanks for the time both of you have put into helping!

 

[G0AOZ]

Bear in mind that the check you've done does not test the PSU under normal load conditions, i.e. when it's powering motherboard circuitry and peripherals etc. This test tells you that the PSU is not completely dead. When all the output rails are under load, a PSU that's starting to fail may exhibit reduced, unstabilized, or non-existent voltages on some rails.

You can still use the shorting pins 13 + 14 method even if you've connected the PSU to the motherboard. Just poke a shorting link down the side of where the appropriate wires from PSU go into the 20-pin multiway plug onto the motherboard. You should be able to trace through from the front panel power switch the connections to the motherboard - look closely at the motherboard and you may see PWR SW or something similar. Also, you don't need keyboard, mouse or video card plugged in to determine if it's starting - you'll see or hear the CPU fan turning.

Good luck with it...

ROGER - G0AOZ.

 

[goombawaho]

G0AOZ - the mobo might be "starting" but it could still be toasty. I've seen that. That's why I recommend everything be connected to allow output on the screen vs. just seeing the CPU fan spinning.

 

[G0AOZ]

Goom, sure... Just wanted to make sure the video card wasn't sucking the life out of it!

ROGER - G0AOZ.

 

[goombawaho]

Yeah - that's possible too. Vampiristic video.