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Recapped PowerMac 8100 PSU and now unstable

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    #21
    Re: Recapped PowerMac 8100 PSU and now unstable

    Originally posted by stj View Post
    not sure about that 2.2ohm one - it sounds high but i'd needto check a good one to compare
    I would second that
    9 PC LCD Monitor
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    These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

    1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
    2 Hem Saw Computer Stack Board

    All of these had CAPs POOF
    All of the mosfet that are taken out by bad caps

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      #22
      Re: Recapped PowerMac 8100 PSU and now unstable

      So I pulled that cap and some of the blue ones and the other larger gray box one as well. My ESR meter couldn't read them because they are just too low capacitance (0.1µf, etc), but my DMM at least verified their capacitance to be good. As for the high ESR reading, that was in-circuit and it was misreading as the capacitance reading was off as well.

      Should I just shotgun replace some of these? I'll keep pulling the filter caps up to and including those around the massive ones.

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        #23
        Re: Recapped PowerMac 8100 PSU and now unstable

        no, check the 2 big filter electrolytics next.
        i know you replaced them - but i dont know what with

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          #24
          Re: Recapped PowerMac 8100 PSU and now unstable

          I’ll do that. One thing also to remember is this problem existed prior to my changing out the caps. The problem existed 20 years ago when I acquired the machine, I just never got around to it till now!

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            #25
            Re: Recapped PowerMac 8100 PSU and now unstable

            Sounds like maybe the PSU temp is too high. (or a thermal sensor problem)
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              #26
              Re: Recapped PowerMac 8100 PSU and now unstable

              Re CX1, measuring its capacitance in circuit will include any other X caps in parallel with it. So, for example, if there's a .47uF that is also across the AC line your .1uF cap will measure ~.57uF.
              PeteS in CA

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                #27
                Re: Recapped PowerMac 8100 PSU and now unstable

                caps across the line usually have a parallel discharge resistor too.
                that will make it read higher.

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                  #28
                  Re: Recapped PowerMac 8100 PSU and now unstable

                  I took that box cap out and it tested fine. Now, I have removed the blue disc caps and tested them out of circuit with the DMM, and they all test within tolerance for their specified capacitance. They are all far too small to test ESR. I tested the large filter caps, both published at 680µf, both with 0.17 ohm ESR, one 601µf, the other 592µf, so all around good. I also took out and tested the thermistor within the circuit...It appears to be rated for 7 ohm (it is 15SP M007), and tests at 11 ohm.

                  I can test more ceramics as I go, but I m running out of ones to test at this point.

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