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    #1

    3COM Switch

    I have repaired this 3COM switch, it didn't power up.
    At first I thought the UC3844B was dead so I replaced it. When I saw that the external PS to feed the IC was limiting the current at 0.3A, too much for the control circuit, I checked with scope the Vcc pin and I saw it oscillating.
    I just put a 47uF 25V in parallel to the Ltec 33uF 50V and the power consumption dropped at 20mA and the voltage was a perfect line, on the PowerMosfet appeared the correct driving waveform.
    Then I powered up the PSU and it started without any problem.
    I checked the bad LTec and it was 457nF 1184Ohm, as shown in one pic.
    I replaced the Ltec wit Rubycon 47uF 25V YXF.

    Here attached few pics.
    It is a pity to see this PS full of good caps, Rubycon - UCC on secondary side and Hitachi as input bulk cap, failing for a small Ltec.

    Ciao
    Gianni
    Attached Files
    "In the confrontation between the stream and the rock, the stream always wins...Not through strength, but through persistence."
    H. J. Brown
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    #2
    Re: 3COM Switch

    Isn't it a damn shame how a nice bit of equipment can fall due to crap caps....even the tiny ones?

    I'll keep this in mind if I have any of my 3com stuff go out.....for now its been all smiles.

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      #3
      Re: 3COM Switch

      Originally posted by hardwareguy
      Isn't it a damn shame how a nice bit of equipment can fall due to crap caps....even the tiny ones?
      Yes it is.
      I remember in '90 we had a lot of failure due to a 100uF 25V mounted as filter on SG3524 Vcc pin, in that case probably the caps, the brand was ROE, were faulty.
      The controller was on a redundant PSU with output +5V 25A, +/- 12V 6A, they were for industrial application and mounted inside this kind of cabinet.

      Ciao
      Gianni
      "In the confrontation between the stream and the rock, the stream always wins...Not through strength, but through persistence."
      H. J. Brown

      Comment

        #4
        Re: 3COM Switch

        I have the same switch and after a power failure it doesn't start anymore. I also replaced the 3844 but didn't work. I'm going to check this cap you mentioned , 4 years ago but thanks!

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