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286 PC keeps killing I/O cards (exploding caps)

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    286 PC keeps killing I/O cards (exploding caps)

    I have a 286 that came with 2 bad caps close to the PSU (2 10mF 16V caps that needed replacing. One was completely blown and another damaged. One was short circuit and the other open circuit).

    I replaced those 2 caps and the machine booted fine .....

    But when I inserted an I/O card in an ISA slot, I noticed that the PC did not start (not even the PSU fan would start).
    When I removed the I/O card the PC would boot fine again.

    So I started poking around the I/O card and noticed a capacitor that was short circuit. This capacitor was hooked up to the -12V rail, and caused the -12V to short to GND. Not good.

    I wanted to verify my claims that this was causing the PC not to start, so I inserted the I/O card back in the PC and started the PC. Only this time the capacitor exploded on my I/O card , and the PC was starting. (I guess the explosion of the cap removed the short).

    Now, I inserted a different type of I/O card in the PC (an IBM serial / parallel card), thinking the previous issue was caused by the I/O card and not the computer but guess what .... again a capacitor blew, this time on the IBM card. I believe the cap that exploded is again linked to the -12V rail.

    Now, I don't really feel like blowing up any more cards (thank god its bedtime), but I wanted to check with you guys if you think I just had bad luck with 2 I/O cards that had bad caps. Or if something else is at play (perhaps the PC is causing all of this).

    Any tips on how to further diagnose this ?

    The PC is booting now with the cards, and the AMI bios boot screen seems to detect the serial / parallel ports on it, although I tried to setup an LPT interlnk/intersvr session and that failed (perhaps related to the blown cap).

    Thx
    Retro enthousiast. Love everything < 486. Learning and sharing on my little channel

    #2
    Is the -12V rail actually -12 volts?

    Comment


      #3
      Were all of these cards from the same computer originally? If so, then they would have the same amount of use/wear/aging and having multiple capacitors blow on different parts would not be too surprising.

      Comment


        #4
        Sounds like two bad I/O cards to me.

        FWIW, I have several of those IBM serial / parallel cards with shorted caps on them.
        PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

        Comment


          #5
          tantalum caps explode, it's what they do

          Comment


            #6
            Here are the exploded caps in question (the blue ones and the orange / red ones) :

            2019-08-29 22.11.49.jpg
            2019-08-30 00.39.03.jpg

            I assume both are 10 microfarad 16V tantalum caps and can be replaced by this : https://benl.rs-online.com/web/p/tan...itors/5382133/ ?
            I already replaced these 3-legged ones by 2-legged ones on my IBMS as per excellent instructions on minuszerodegrees.net
            Retro enthousiast. Love everything < 486. Learning and sharing on my little channel

            Comment


              #7
              It is not uncommon for those Tant caps to short out however, it does seem like a coincidence the experience you have had.These caps are very sensitive to over voltage, much more so than an electrolytic cap. So you should check that the -12v rail is that and not any higher.

              Comment


                #8
                Unfortunately when they "Explode" they can sometimes take out something else on the card, I have 2 IBM CGA cards here, Both worked perfectly until one day a tant C8 exploded, I replaced the tants but one card is dead as a door nail and the other shows garbage on the monitor, I also have a couple of IBM serial / parallel cards which have shorted tants, Haven't looked at any of them in a while, They got put away for another day which never seem's to arrive.

                Comment


                  #9
                  Originally posted by Malc View Post
                  I also have a couple of IBM serial / parallel cards which have shorted tants, Haven't looked at any of them in a while, They got put away for another day which never seem's to arrive.
                  Same here. But before I put them away I removed the shorted ones from the circuit and the cards worked without them. But they're still waiting for replacement tants.
                  PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

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                    #10
                    Originally posted by Hugo Holden View Post
                    It is not uncommon for those Tant caps to short out however, it does seem like a coincidence the experience you have had.These caps are very sensitive to over voltage, much more so than an electrolytic cap. So you should check that the -12v rail is that and not any higher.
                    Will check that ... thx.... old PSU so you never know....
                    Retro enthousiast. Love everything < 486. Learning and sharing on my little channel

                    Comment


                      #11
                      Originally posted by Malc View Post
                      Unfortunately when they "Explode" they can sometimes take out something else on the card, I have 2 IBM CGA cards here, Both worked perfectly until one day a tant C8 exploded, I replaced the tants but one card is dead as a door nail and the other shows garbage on the monitor, I also have a couple of IBM serial / parallel cards which have shorted tants, Haven't looked at any of them in a while, They got put away for another day which never seem's to arrive.
                      Yeah that's a bummer. I actually wanted to install the I/O card to setup an interlnk/intersvr with another PC to transfer the data and couldn't get the LPT transfer to work ... Don't know if it's related to the LPT setup, but perhaps the card is in fact dead (dunno if interlnk/intersvr really needs the -12V somewhere).
                      Retro enthousiast. Love everything < 486. Learning and sharing on my little channel

                      Comment


                        #12
                        Originally posted by Robbbert View Post
                        Is the -12V rail actually -12 volts?
                        Will check.
                        Retro enthousiast. Love everything < 486. Learning and sharing on my little channel

                        Comment


                          #13
                          It is always good to check voltages but the tantalums for -12V are usually rated for 25V to 35V. It would be difficult to get levels that high out of a power supply. It is more likely just typical tantalum caps blowing up. About the only way for such voltages is for the ground lead of the 12V section to become open. Even then 24V is about as high as it'll get.
                          A while back, I was bring up a N* computer that had been off for several years. It blew two tantalums. To add insult, the +12V that blew a cap was running at about 10.5V because of a weak regulator. This was one of the TO-3 can regulators so it wasn't the short that damaged it. These are designed to take a continuous short and just get hot. It was just a tired regulator before the caps blew.
                          Dwight

                          Comment


                            #14
                            Originally posted by Dwight Elvey View Post
                            It is always good to check voltages but the tantalums for -12V are usually rated for 25V to 35V. It would be difficult to get levels that high out of a power supply. It is more likely just typical tantalum caps blowing up. About the only way for such voltages is for the ground lead of the 12V section to become open. Even then 24V is about as high as it'll get.

                            Dwight
                            The blue ones in the photo on Post #6 appear to be 16V types.

                            A while ago I built a project where there was a 5V linear regulator which unexpectedly oscillated. It had (originally 16V tants) on its inputs and outputs as per the original designer's work. I had used 35V ones I'd had in my junk box. It didn't oscillate with 16V tants.

                            So I tested the ESR on a range of tants in my stock and it appears the higher the voltage rating the higher the ESR for the same uF value tant capacitor. So it seemed to make sense when tants are used to bypass power rails that their voltage rating should be above the supply voltage by a margin, but not too high or they might be less effective at the task. I'm not sure if that is everybody else's experience with them.

                            Comment


                              #15
                              What if the OP is getting +12v on the -12v rail? Would the reverse polarity cause tants to blow?

                              Comment

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