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    #1

    12 volt leakage on outer cap case

    Hi,
    Currently trying to sort out a non posting N780i motherboard. Found one leaking cap ( Samxon 100uf 25v) and have replaced that but no joy. as the board has a lot of this particular cap fitted and knowing the difficulty of removing and testing each one I tried the crude test of measuring the voltage on each cap outer case in situ with the board fired up in the hope I might find a dead cap or one differing greatly from the others. I have found one of these 100uf caps showing 9 volts on its outer casing whereas every other cap on the board showed something in the region of 0.1 - 0.3v or less.
    Am i missing something here or is that a weird reading for a cap to give from the case to earth?
    Happy to take that cap out even though it is very innaccessible but dont want to do it if I am simply wasting my time because such a measurement is normal in some circumstances
    John
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    #2
    Re: 12 volt leakage on outer cap case

    I can see caps could blow and short against the case, though there should be some capacitive effect from a pin to the case (There are some caps that have cases explicitly grounded - but it should clearly say so.)
    See if it's really conductive or not using a lower ohms mode.

    Comment

      #3
      Re: 12 volt leakage on outer cap case

      Hi
      Thank you for the lone reply and your thoughts. It remains a mystery at the moment as given no early replies I decided to change the cap and see what happened. The old cap tested ok out of the board and I put in a new one.
      The board symptoms remained (ie no post) and when I repeated my earlier crude test on the new capacitor the result was the same - some 9 volts between case and ground. I am sure there must be a logical explanation but as a tinkerer rather than an electrical expert I am not sure what it might be.
      thanks again for your reply though - its much appreciated
      regards
      John

      Comment

        #4
        Re: 12 volt leakage on outer cap case

        that is a bit strange as the can is normally at ground potential.. only thing that springs to mind immediately is a broken trace on the board which should be taking the negative leg of that cap to ground.. what line is the cap on?.. what is it decoupling? does the pin marked negative actually connect to a ground point, like say a screw hole? finding 9v where there should be ground will be a very good reason why the board won't post.. possibly a shorted mosfet (buck regulator ic etc) passing current which has also blown a trace to ground..

        so thinking out loud.. I would check for shorts from the various voltage rails to different ground points.. and also check the ground planes are actually at ground and not floating..

        Comment

          #5
          Re: 12 volt leakage on outer cap case

          Just tested a Panasonic 1000uF 10V cap (HFQ?) ... the case is isolated from both pins.
          Tested a fake Nichicon... case has weak capacitive coupling to both pins, but otherwise isolated.

          And tested a Fuhjyyu... same as the fake. Weak capacitive to both pins.
          NTE Teapo (allegedly) ... seems isolated.
          Last edited by eccerr0r; 06-15-2016, 04:14 PM.

          Comment

            #6
            Re: 12 volt leakage on outer cap case

            Thank you both - that gives me some ideas to work on. Since I reported this ( and despite that 9v issue) I have managed to get the board to fully post using a different cpu and cleaning the socket and have installed XP just as a test. However that has lead to a different but associated problem in that when up and running if you hit the board mounted reset button it will go though post fully and complete and will do the same if you restart in windows. It will do this on every occasion tried.
            However if you turn off the board at its power button or shut down in windows and then press the power button to start it only runs up to code 25 and not through to post which is exactly what it was doing before. Leave for 30 mins or so and hit the power button and it fully posts again!
            I am sure whoever designs and builds motherboards installs some kind of insanity bug to turn people like me insane!
            Now not sure if the leaking 9v is the cause of this particular problem or whether it is a duff capacitor somewhere which has enough power to boot up when rested but not after a shutdown until it has again rested.
            I will not be beaten by an inanimate piece of plastic and metal!!

            Comment

              #7
              Re: 12 volt leakage on outer cap case

              u might also wanna check out your power supply. the inability to cold start or cold boot the system is indicative of bad caps. but whether they are bad caps on the mobo or psu or both is for u to check and find out...

              Comment

                #8
                Re: 12 volt leakage on outer cap case

                Originally posted by johnboy183 View Post
                Thank you both - that gives me some ideas to work on. Since I reported this ( and despite that 9v issue) I have managed to get the board to fully post using a different cpu and cleaning the socket and have installed XP just as a test. However that has lead to a different but associated problem in that when up and running if you hit the board mounted reset button it will go though post fully and complete and will do the same if you restart in windows. It will do this on every occasion tried.
                However if you turn off the board at its power button or shut down in windows and then press the power button to start it only runs up to code 25 and not through to post which is exactly what it was doing before. Leave for 30 mins or so and hit the power button and it fully posts again!
                I am sure whoever designs and builds motherboards installs some kind of insanity bug to turn people like me insane!
                Now not sure if the leaking 9v is the cause of this particular problem or whether it is a duff capacitor somewhere which has enough power to boot up when rested but not after a shutdown until it has again rested.
                I will not be beaten by an inanimate piece of plastic and metal!!
                That is absolutely bad caps. The ones the same as the one you have changed are probably all duff.. maybe not on a capacitance test, but under proper working voltages.......

                I have an old machine which will only start if the psu is completely powered off for about 20 seconds.. then you have to be quick on the power button.. It won't reboot either on the reset switch or from the os.. needs the power down treatment.. Lovely bulging caps all around the cpu vcore circuits.. waiting for them to explode so I can post the results in the Hall of Shame

                Am I going to bother fixing an athlon 1900 board from late 2000??.. not likely unless I'm bored and have a pile of salvaged caps lying around..

                Proper evaluation of the board needed.. will it cost more in caps than going for an upgrade? .. Will replacing every cap clear all the faults?? .. that spurious 9v seems to indicate something more than caps wrong here somewhere..

                Comment

                  #9
                  Re: 12 volt leakage on outer cap case

                  I think I can exclude the psu as I have tried two different ones and both have no effect on the issue. If this was a TV I would be almost sure it was caps but motherboards I am a relative stranger to though the symptoms heavily suggest a cap ( or caps ) that recover on standing. What I don't understand though is why it will fully reset and run to boot if the reset button is pressed whilst the pc is running or when restart is pressed in Windows and will do this any number of times I want!. I would have thought the same duff caps would have to kick in to run it up again whether from reset, restart or from off to on. But my knowledge of the run up sequence of the hardware in a mobo is non existent so maybe it is possible.
                  It is only a project ( though an annoying one!) so the question of a new board is not relevant - I either re-cap it or put it away for another day. The cost of the caps is not an issue either as I have lots of high quality caps pulled from other boards to use when tested. What is a real downer on this board is the difficulty of removing the caps. Even when the solder is removed ( which is hard as EVGA seem to use something that is a b***** to melt) I find the pins themselves are so tight in the holes I am spending twice as long as usual on each cap as i want them out undamaged so i can test them to find a culprit.
                  Thanks again for the thoughts and ideas
                  John

                  Comment

                    #10
                    Re: 12 volt leakage on outer cap case

                    Originally posted by juliadream View Post
                    That is absolutely bad caps. The ones the same as the one you have changed are probably all duff.. maybe not on a capacitance test, but under proper working voltages.......

                    I have an old machine which will only start if the psu is completely powered off for about 20 seconds.. then you have to be quick on the power button.. It won't reboot either on the reset switch or from the os.. needs the power down treatment.. Lovely bulging caps all around the cpu vcore circuits.. waiting for them to explode so I can post the results in the Hall of Shame

                    Am I going to bother fixing an athlon 1900 board from late 2000??.. not likely unless I'm bored and have a pile of salvaged caps lying around..

                    Proper evaluation of the board needed.. will it cost more in caps than going for an upgrade? .. Will replacing every cap clear all the faults?? .. that spurious 9v seems to indicate something more than caps wrong here somewhere..
                    Those old socket a 462 motherboards that can run high end Palominos with a agp slot at 2x are worth keeping. Perfect to pair up with a Voodoo5 5500. Each day that goes by means less of them surviving.

                    Comment

                      #11
                      Re: 12 volt leakage on outer cap case

                      780i suffers from the wonderful bumpgate defect - likely the northbridge has gone the way of the dodo. Not much point looking for bad caps here. Give it a bit of heat and see if it does anything interesting.
                      Originally posted by PeteS in CA
                      Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                      A working TV? How boring!

                      Comment

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