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Caps loadly popping on motherboard

Discussion in 'Hobby Electronics' started by Michael C, Dec 5, 2007.

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  1. Michael C

    Michael C Guest

    I've got a motherboard that runs off a single 12V supply (Via ITX board). It
    provides power to the hdd and cd-rom via a power out plug. I hooked it up
    today with a 12V supply and shortly after starting it up one of the caps in
    the onboard power supply popped quite loudly. Then a minute or so later
    another one popped. I was only running a single hdd and no CD rom so
    wouldn't have thought I was overpowering it. Can anyone tell me what is
    likely to have happened?

  2. Michael C

    Michael C Guest

    You'd think I would have learnt how to spell loud by now :)

  3. **It wasn't really a 12 Volt supply?

    Did you measure the "12 Volt supply"?

    Trevor Wilson
  4. L.A.T.

    L.A.T. Guest

  5. **Good point, though I doubt anyone could be THAT stupid.

    Trevor Wilson
  6. So this is an offer to better that? I'll bite. :)

    A small "Squirrel" printer used in POS applications come in for repair.
    Report said "Dead". Apon closer inspection and preparation to apply 12v to
    power up the unit, we notice the PCB is completely cooked. Nothing that a
    mere 12v could do, this needed more juice than that. Turns out client lost
    the adaptor for one of these things, and made up their own - by plugging it
    into the mains.
  7. **That's pretty good. I have photos (somewhere) of a turntable, where the
    client accidentally wired the phono leads to the mains supply......

    The headshell was one of those plastic ones and arrived as a blob on the end
    of the tone arm. It was a beaut. The other one (which I also have photos of)
    is where a client's wife/girlfriend arrived home early, to find my client in
    bed with another woman. She grabbed a hammer and took to his hi fi system.

    Hell hath no fury and all that.

    I'll try to locate the pics, scan and post them somewhere.

    Trevor Wilson
  8. L.A.T.

    L.A.T. Guest

    A million years ago, or thereabouts, I bought a single-board computer,
    called AIM65. Based on a 6502 chip. It needed a power supply that provided,
    I think, +5, and +24 volts. I was dead tired, and it was about 1:00AM, and I
    was wiring and soldering a breadboard rat's nest of a temporary power supply
    , sitting on the floor. I fired up the finished supply, and it didn't work.
    I bent over it to look at what was wrong, and a big electrolytic (about the
    size of a D cell) exploded and shot past my head and made a big dent in the
    ceiling which is still there. It could have blinded me. I had wired it back
    to front.
    Yes, some of us are THAT stupid. Sometimes.
  9. Suzy

    Suzy Guest

    Yes, well... We can all be stupid at times. Recently I had to cut the output
    wires of an *AC* plugpack. Then I spent a fruitless couple of hours trying
    to determine what polarity was each lead...
  10. Sometime back another young poster to this NG had a similar experience with
    an exploding electrolytic.
    Difference was it hit him square in the forehead, leaving a crosshair
    pattern resembling the vent release on the top of the cap.
    Lucky he didn't cop it in an eye !

  11. Michael C

    Michael C Guest

    Yep, measured at 12V and polarity is correct, positive in the centre like
    most of em. It's the same brand and model I have used previously. Possible
    it's faulty somehow?

  12. Bob Parker

    Bob Parker Guest

    I installed a 10uF electrolytic backwards in an amp I was repairing
    once. It was a bypass cap on an STK package.
    There was a very loud bang as I was leaning over the unit, and the
    cap's can flew past my face, missing my eye by maybe an inch before
    hitting the ceiling then bouncing around the room.
    Nice to know I'm not the only one's who done that. :)

  13. Dorfus

    Dorfus Guest

    Best one I heard was a friends sister was using a handheld cake mixer
    when the mains lead somehow worked loose and fell into the cake mix.
    Without thinking she picked up the leads and licked the cake mix of them
    and got one hell of a boot on her tongue.

  14. Dorfus

    Dorfus Guest

    So did you get it right in the end?

  15. Marc

    Marc Guest

    A looooong time ago I used to work on welding machines
    at a manufacturer, and hooked up the enormous ~20 x 10cm dia.
    Mallory can electrolytic used on the rectified output of the
    DC MIG welding output section backwards...

    The sides were open on the machine in production,
    luckily I was standing just behind the rear plate when I
    powered it up (415V 3 phase too)...
    The cap blew like a grenade, exploding its guts everywhere
    with a bang and force that scared the absolute shit out of
    me and everyone else in the factory.

    Took me a while to clean up the inside of that machine and
    all around :)

    Since then and a smaller occasion (not backwards cap)
    I always wear safety glasses when powering up a suspicious
    or repaired power supply or other device with electro's
    and high current supply, don't want to lose an eye either...
    My safety glasses sit on the shelf just above the workbench.
    I've had tantalum caps throw burning shards around too.



    * My email address requires the identical words and
    * underscores removed to email me
  16. Suzy

    Suzy Guest

    Yes, I managed to identify the polarity in a 50th of a second, although it
    changed immediately afterwards, but it's OK because it changed back again...
  17. Franc Zabkar

    Franc Zabkar Guest

    The caps in the onboard PSU are probably 6.3V types. Maybe the PSU
    failed in such a way that the input voltage was shorting to the output
    ??? I've never tried powering a motherboard without a CPU, but could
    doing so bring about such a failure ???

    - Franc Zabkar
  18. Dorfus

    Dorfus Guest

    Thank goodness for that!

    What are you working on lately Suzy? You're always doing interesting
    things. Did you design an antenna?

  19. Suzy

    Suzy Guest

    Yes, it receives its own transmissions, so lessening spectrum congestion,
    not to mention propagation delay...
  20. kreed

    kreed Guest

    in the very early 1980's I recall doing the occasional repair jobs for
    a bloke (now retired 16 years back) who amongst other things, had a
    quantity of coin-operated video games in bars and shops. One day I
    remember having to service a game called "asteroids" that had a
    "vectorbeam" or "XY" monitor (For those unfamiliar with this long
    forgotten? technology - there isnt any raster scan, the deflection
    coils are driven directly by amplifiers that are driven via 2 separate
    D/A converters on the logic board, kind of like a CRO.

    After a bit of stuffing around, we discovered that he had a spare
    logic board of the same game, but made by a different manufacturer, so
    we decided to try it. All was not well, the CPU was running and we
    could see the game on the monitor, but the picture was shaking like
    mad in both directions. Just as I was sticking my head in the back of
    the machine to try and diagnose the fault, there was an almighty
    "BANG" like a gunshot, followed by a second "BANG" a few seconds
    later. Both were so loud our ears were ringing and sore for the next
    hour or so.

    Turned out the logic board had an onboard power supply providing +/-
    12v for the D/A converters, and the AC was fed directly from a power
    transformer to the board. Only problem was that one board had 4700uf
    16v caps on it, and the other had 25v caps on it, and one cabinet had
    a much lower secondary voltage coming from its transformer than the

    when swapped - BANG !

    I learned a very important lesson about the dangers of electrolytics
    that day, fortunately havent had a repeat.
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