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Thread: Pericom terminal power supply - ideas ?

  1. #1
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    Default Pericom terminal power supply - ideas ?

    Folks,

    this is the power board from a Pericom 6807AAB Serial terminal.

    The bridge rectifier in the top right starts smoking on power up - it does this whether the power supply is connected to its downstream load in the terminal or not. I suspect a short somewhere downstream in the power supply itself.

    I have no circuit digram for this device so am having to try and work through to determine what is causing the problem.
    The BR (a PW02) is sourced from a transformer 24AC tap and has the gold coloured 4700uF 25V smoothing cap across its output.

    I'm suspicious about the capacitor as its the same specification as the others shown but a different type - from the silkscreen possibly looks like it should be physically much bigger.

    There is not much evidence of any excess load related heat further into the PSU circuit.

    Thoughts please ?



    IMG_20190527_104246455.jpg

  2. #2

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    Quote Originally Posted by zippysticks View Post
    Folks,

    this is the power board from a Pericom 6807AAB Serial terminal.

    The bridge rectifier in the top right starts smoking on power up - it does this whether the power supply is connected to its downstream load in the terminal or not. I suspect a short somewhere downstream in the power supply itself.

    I have no circuit digram for this device so am having to try and work through to determine what is causing the problem.
    The BR (a PW02) is sourced from a transformer 24AC tap and has the gold coloured 4700uF 25V smoothing cap across its output.

    I'm suspicious about the capacitor as its the same specification as the others shown but a different type - from the silkscreen possibly looks like it should be physically much bigger.

    There is not much evidence of any excess load related heat further into the PSU circuit.

    Thoughts please ?



    IMG_20190527_104246455.jpg
    Looks like a straightforward linear regulated power supply. A common mistake on these supplies (I don't know why o earth) is fusing *after* the smoothing capacitors, while one much better approach would be fusing one leg of the AC input, but I digress.
    Basicly these supplies have the following configuration:

    AC -> bridge rectifier -> smoothing capacitor -> fuse -> linear regulator (IC, discrete, whatever) -> smaller capacitor

    If the rectifier smokes even with no load and with its fuse removed, it's probably either the bridge that's shorted or its smoothing capacitor that's shorted, or both (since a diode operating into a short, becomes another short usually for overcurrent junction failure).

    One can easily check if a capacitor and/or the diodes are shorted with a multimeter.

    HTH

    Frank IZ8DWF

  3. #3
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    Default

    Thanks Frank,

    yes the smoothing orange cap for that rectifier is shorted so I have ordered a full replacement set.

    While they are being delivered I will deal with the screen 'cataract' problem. (see picture).

    One thing that puzzles me is this brown liquid leak - apparently from around the bottom of the tube. It's a sticky brown odourless liquid that I'm not sure is some sort of lubricant/adhesive from around the copper tube band that has heated and run. Anyone seen this before ? Is it 'normal' ?

    You can see this in the second picture below:

    IMG_20190527_131407519.jpg
    IMG_20190527_131045027.jpg

  4. #4

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    I don't usually replace what hasn't failed (I mean the other capacitors, in this example), but that's an habit that few have I think.
    The brown liquid might be foam or a glue that degraded into that form with the age and the thermal cycles probably, there should not be any liquid substance in a typical CRT.

    Frank

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