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Thread: Troubleshooting Compaq Portable III power supply failure

  1. #1
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    Default Troubleshooting Compaq Portable III power supply failure

    Hey folks,

    I picked up a Compaq Portable III at VCF West this past weekend, and it came with a bad power supply. If I power it on with no load, the built-in fan powers up, but none of the DC output voltages are anywhere close to correct. If I power it on with a load, it sits there and ticks, and the fan never spins up.

    There are no obviously blown or leaking electrolytic capacitors, so i'm trying to identify the first component, U2, on the PCB, which does not really have any meaningful markings on it, besides what vaguely looks Cyrillic, and "8908"

    Compaq_Portable_III_PSU_U2.jpg

    I've placed an album of other photos of the PSU at https://photos.app.goo.gl/6BhjECaeyDwiskFq5

    Any advice would be appreciated.

  2. #2

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    Quote Originally Posted by aperezbios View Post
    There are no obviously blown or leaking electrolytic capacitors
    That, in itself, says nothing about the capacitors' integrity, whatsoever. Caps can easily be blown without any obvious signs.
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  3. #3

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    Quote Originally Posted by aperezbios View Post
    ...trying to identify the first component, U2, on the PCB, which does not really have any meaningful markings on it, besides what vaguely looks Cyrillic, and "8908"
    It's a full-wave (i.e. "Bridge") rectifier (note the "sine-wave between the 2 center pins and a + and - on the edge). It converts AC in to full wave rectified voltage (likely the next component would be an inductor or large filter capacitor) The writing isn't Cyrillic---it's likely "house-markings" to determine the type and lot-code number. "8908" indicates it date of manufacture (8th week of 1989). While I've seen them fail, it's not common for them to fail without obvious signs. Remove it from the circuit and check with an ohmmeter to be certain (should not read anything less than roughly 500 ohms between any pins).

    Heres a sample of another one (but it may or not be within the specs of yours:

    https://www.digikey.com/product-deta...6FS-ND/1054251
    Last edited by T-R-A; August 9th, 2018 at 04:13 PM.

  4. #4
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    U2 looks like a full bridge rectifier to me, but I could be wrong.
    8908 is likely "8th week of 1989".

    ESR meters are a good way to check the condition of electrolytic caps.
    I'm not educated enough to help you diagnose the fault, but will be watching the thread, I enjoy learning about switch mode power supply diagnosis.
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    Quote Originally Posted by T-R-A View Post
    It's a full-wave (i.e. "Bridge") rectifier (note the "sine-wave between the 2 center pins and a + and - on the edge). It converts AC in to full wave rectified voltage (likely the next component would be an inductor or large filter capacitor) The writing isn't Cyrillic---it's likely "house-markings" to determine the type and lot-code number. "8908" indicates it date of manufacture (8th week of 1989). While I've seen them fail, it's not common for them to fail without obvious signs. Remove it from the circuit and check with an ohmmeter to be certain (should not read anything less than roughly 500 ohms between any pins).

    Given that the DC fan spins up fine, it seems safe to say that the rectifier is doing its job. There are two Sprague manufactured 330uF filter caps, rated at 105C, which look fine when removed, but need to be tested. I unfortunately can't locate my ESR meter.

  6. #6
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    Actually, the fan is connected to the mains in most of those PSU so it will spin no matter how dead the power supply is.

    They were a bad design for the whole series of portables that Compaq made and, if you find one of the portables, chances are VERY high that the PSU will be toast with about 20 different causes.
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    Quote Originally Posted by Druid6900 View Post
    Actually, the fan is connected to the mains in most of those PSU so it will spin no matter how dead the power supply is.
    That doesn't explain why the fan fails to work when the DC rails are loaded, not to mention he says unloaded he gets a DC output but extremely poorly regulated.
    My guess is the caps are visually fine but are unable to deal with the inrush current due to age. I've seen this a few times. Replace them and it should be fine.
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    Quote Originally Posted by Druid6900 View Post
    Actually, the fan is connected to the mains in most of those PSU so it will spin no matter how dead the power supply is.
    In this case, the fan is in fact DC, and very clearly labelled as a NIDEC BETA V model TA300DC, with stated input voltage on the label as 12VDC. Therefore, the bridge rectifier is evidently doing its job
    Last edited by aperezbios; August 14th, 2018 at 09:30 AM.

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