Turning on motherboard by grounding the green wire

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  • larrymoencurly
    Badcaps Veteran
    • Oct 2004
    • 960
    • USA
    #1

    Turning on motherboard by grounding the green wire

    Can grounding the green PSU wire while its connected to a motherboard ever damage a motherboard? I've made a few motherboards show signs of life by doing that, and two of them even managed to show the BIOS boot screen (but couldn't make them turn on the usual way).

    A friend of mine had a Socket 754 Nforce3 (MCP250) motherboard suddenly shut down on him, and it wouldn't turn on when its power switch jumpers were shorted together. So I hooked it up to a known good PSU (300W Delta with all Rubycons and Nichicons) and grounded its green Power-On through a 200 ohm resistor. That made the CPU fan turn on and stay on, but there were no beeps or video (no built-in video; used AGP and PCI video cards). Also the Nforce3 chip got hot enough to make the circuit board reach 110C on the bottom side (not measured until probably 3 seconds after turn-on). The CPU, video card, and memory work fine in another motherboard. Nothing looks discolored. I've tried pressing on the Nforce3 chip while trying to turn on the motherboard through its power switch pins, but nothing happens.

    I don't think it's a matter of bad caps because he recapped it a few years ago with all Nichicons, and his soldering looks good under magnification.
    Tags: None
    • gilly1984
      Badcaps Veteran
      • Jun 2012
      • 751
      • UK
      #2
      Re: Turning on motherboard by grounding the green wire

      110c seems very high.
      Do NOT touch heatsinks when testing for voltages as they may be LIVE!

        Comment

        • selldoor
          Slow Learner
          • Dec 2010
          • 7870
          #3
          Re: Turning on motherboard by grounding the green wire

          Check the caps are the right way round - if it is an asus or asrock board I believe the markings are not as usual. The white line on the cap does not go on the black semicircle side.
          Last edited by selldoor; 11-13-2012, 05:41 AM.
          Please upload pictures using attachment function when ask for help on the repair
          http://www.badcaps.net/forum/showthread.php?t=39740

            Comment

            • larrymoencurly
              Badcaps Veteran
              • Oct 2004
              • 960
              • USA
              #4
              Re: Turning on motherboard by grounding the green wire

              Originally posted by selldoor
              Check the caps are the right way round - if it is an asus or asrock board I believe the markings are not as usual. The white line on the cap does not go on the black semicircle side.
              It's an ECS and had been recapped a couple of years ago, apparently very well. I didn't check the MOSFETs while the mobo sort of ran, but I didn't find any gate-channel shorts.

              I have a feeling the problem is the common Nvidia chipset failure:

              http://www.theinquirer.net/inquirer/...hips-defective

                Comment

                • lti
                  Badcaps Legend
                  • May 2011
                  • 2548
                  • United States
                  #5
                  Re: Turning on motherboard by grounding the green wire

                  If the chip gets that hot, it's fried. That chipset was made long before the defective chip issue.

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