SST-ST45SF-G failure 3.3/5/12 lines but not ps_on or 5vsb

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  • pope3909
    New Member
    • Oct 2017
    • 5
    • USA
    #1

    SST-ST45SF-G failure 3.3/5/12 lines but not ps_on or 5vsb

    I have a SST-ST45SF-G that has failed on all of the 3.3/5/12 lines, but the ps_on (green wire) is reading 3.7v and the 5vsb (violet wire) is reading 5v. I disassembled it and checked for any swollen or shoddy looking caps but found nothing. No components are discolored, nor do I see any scorching. It all looks perfect. It's a SFX so it's possible i missed something but i looked very closely. No sounds or fan movement either, even when the paper-clip test is done. I tested everything strait from the PSU with no cable attached to make sure that wasn't the cause. The fuse checks out as well. I checked the back mosfet but there may be one more I havent looked at yet i'll have to check. Does anyone with more experience than me (so practically anybody lol) have a probable cause to this failed PSU, or a push in the right direction?

    I'm trying to get on a good level with electronics repair but I still have a lot to learn. I got this PSU for practically nothing, but it's very expensive (new) and I'd really like to fix it for my mini-ITX build (also using salvaged parts). I've already replaced an r36 inductor on a FM2A75M-ITX (and created a custom heat-sink for all the inductors since they are famous for burning out (like this one did) on this model mobo. It's more than just saving money though, I want to be able to diagnose things better. I just need a push in the right direction.
    Tags: None
    • PeteS in CA
      Badcaps Legend
      • Aug 2005
      • 3579
      • USA, Unsure of Planet
      #2
      Re: SST-ST45SF-G failure 3.3/5/12 lines but not ps_on or 5vsb

      Pictures would be a big help.

      Basically there are two almost separate power supply circuits inside that box: the Standby regulator; the main outputs.

      The Standby circuit provides 5VSB to the MB and housekeeping Vcc for both the primary and secondary sides of the main regulator. The Standby circuit appears to be working, since you have 5V on the 5VSB (purple) wire.

      You mentioned 3.7V on the PSON# (green) wire. Is that plugged into a MB? Or is the PSU not connected to anything? For the PSU main outputs to turn on, the PSON# signal must be low. If the PSU is not plugged into a MB, short PSON# to return. This will turn on the main outputs if the PSU is OK.
      PeteS in CA

      Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
      ****************************
      To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
      ****************************

        Comment

        • pope3909
          New Member
          • Oct 2017
          • 5
          • USA
          #3
          Re: SST-ST45SF-G failure 3.3/5/12 lines but not ps_on or 5vsb

          It's apparent to me that anything beyond the obvious (physically damaged components) are out of my scope since I only have a basic electronics understanding (my professional field is clinical laboratory science.) If it's not an leaky cap or a failed diode then it's beyond me and i'll just junk it. Sorry to have wasted your time.
          Attached Files

            Comment

            • pope3909
              New Member
              • Oct 2017
              • 5
              • USA
              #4
              Re: SST-ST45SF-G failure 3.3/5/12 lines but not ps_on or 5vsb

              Disregard the images. What I suspected as a discolored component was actually just a drop of factory flux that cleaned off.

                Comment

                • Per Hansson
                  Super Moderator
                  • Jul 2005
                  • 5895
                  • Sweden
                  #5
                  Re: SST-ST45SF-G failure 3.3/5/12 lines but not ps_on or 5vsb

                  Technically it's glue: used to hold the SMD components in place as the board goes through the solder oven
                  "The one who says it cannot be done should never interrupt the one who is doing it."

                    Comment

                    • Quaddro
                      Tukang Kentu
                      • Nov 2010
                      • 141
                      • Indonesia
                      #6
                      Re: SST-ST45SF-G failure 3.3/5/12 lines but not ps_on or 5vsb

                      faulty all voltage rail, usually happens due to faulty of optocoupler area.
                      Check or replace the optocoupler(s), and also check the all component surrounded.

                        Comment

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