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What happened to this motherboard?

Discussion in 'Electronic Repair' started by [email protected], May 14, 2007.

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  1. A while back I bought a used 500mhz computer on Ebay. (that was my
    big mistake). When I got it, it worked well, except for the floppy
    drive. I contacted the seller immediately and he sent me another
    floppy drive to replace the bad one. I replaced the drive and turned
    it on. It worked fine but since I wanted to add another harddrive (as
    soon as I got one), I left the cover off the case. That turned out to
    be a good thing, because I was sitting at my desk on the web when I
    noticed a hot burnt odor. Then I saw smoke. I looked at the computer
    and saw smoke coming off the motherboard. There are these donut
    looking things on the board. They look like a white lifesaver candy
    wound with 20 or 30 loops of a fairly thick enamelled wire (actually
    not all that thick, probably a 20 gauge). Oddly enough, the computer
    continued to work the whole time, and I know that because I saved
    whatever I was typing before I shut off the power. I found all the
    enamel burned off the wire on that coil and it was black. I was glad
    the cover was off the case so I could see what was happening.

    I carefully looked for anything that might create a short and found
    nothing. I got my fire estinguisher (just in case), and turned the
    computer back on. It booted up, and worked fine. I turned it on and
    off several more times, and it still worked, and I never saw more
    smoke. The seller would not replace it because it was 2 ot 3 weeks
    later by then.

    What could have caused this? What are those coild called and what is
    their purpose. It seems there are 2 or 3 of them on all motherboards.

    Needless to say, I do not use this computer. I dont trust it. I just
    took out all the drives, ram, and cards and put them on another
    motherboard. I know I can use that case again, but question the power
    supply. I definately will not use the motherboard again, and worry
    about the CPU.

    Anyone have any idea what caused this? I'm just curious more than
    anything else. I figure the MB is best going into the trash, and I
    have a faster CPU now too.


  2. Tim

    Tim Guest

    Those are probably inductors used to limit noise on the DC power lines.
    If they overheated, that usually indicates a fault on the motherboard.
    It takes a great deal of energy to burn those lines, so I would look for
    a damaged regulator or shorted capacitor on the board. The problem may
    be a short on one of the inside layers of the board as well, so you may
    have to remove the board and try to look through it. If one of the
    layers is a ground plane, you will not be able to see through the board.

    - Tim -
  3. David

    David Guest

    Actually the most of the inductors described are not noise filters at all,
    but part of a switching power supply regulator that converts either 5 or 3.3
    volt supplies to lower voltages such as 1.5 volts for the CPU. They often
    operate at high currents (perhaps as much as 10 - 20 amperes). There are
    also some boards that have regulators to generate higher voltages for other
    components on the board such as built in audio or RS232 ports. I have never
    seen one get hot enough to cause smoke without also something else wrong. My
    guess is that particular regulator was associated with something other than
    the main CPU/memory system or else the machine would not run. Do all of the
    I/O ports, built in peripherals, and add-on cards operate?

  4. Dave

    Dave Guest

    New motherboards with CPUs and often CPU fans can be had at retailers such
    as Frys (they also have a a web) for $80 to $100. I have built many PCs with
    these: P4s, AMD etc,
  5. James Sweet

    James Sweet Guest

    Something has shorted on the motherboard, it's not worth trying to repair
    one so old, but the rest of the components will be fine.
  6. catguy

    catguy Guest

    Look for the buldging electrolytic capacitors....Paul
  7. Jeroni Paul

    Jeroni Paul Guest

    I have seen old Pentiums run at the wrong core voltage, and they would
    work, maybe hotter or less stable. It could be a shorted regulator
    putting out 3.3V to the 1,5V line, not sure if a P III would hold that
    but it might work for some time.
  8. Jamie

    Jamie Guest

    bad caps.
  9. w9gb

    w9gb Guest

    This may be educational

    Personally, I would not have spent a dime for a 500 MHz PC, when Fry's area
    selling brand new Pentium 4 with software for $ 299 (last year models being
    replaced by new ones).

  10. Hi!
    If the coil got hot enough to smoke, the voltage may have been held down to
    a much lower value by virtue of the coil's resistance.

  11. AZ Nomad

    AZ Nomad Guest

    Many many motherboards from that era had bad caps. Did you examine the
    caps for bulging and/or leaking?
  12. Thanks for the link. You (and others) were correct. There are 3 caps
    right next to this coil and very close to some major power transistors
    (probably regulators). All 3 of those caps have bulged tops and there
    seems to be a brown stain next to one of them. Until looking at the
    pics on this website, I did not know what to look for. It's now
    obvious those 3 caps are fried. Oddly enough that motherboard still
    boots and runs. In fact I loaded windows and a bunch of hi-res photos
    and no problems. I think I am going to see if I can find three 1000uf
    caps and replace them. No sense trashing a working motherbd for $5
    worth of caps. I wont replace the others, jsut those three. That
    coil is charred but it still works. Since the windings dont touch I
    dont see where it matters if the enamel is burned off. I got a
    soldering iron so what the heck. Maybe I'll turn that into my Windows
    98 machine. I still like using 98 when XP pisses me off, which it
    does quite often.

    Just curious. What exactly do those coils do? Every motherbd has
    them, at least 2 of them. They are just a core with about 10
    windings. All I can figure is a choke of some sort.

  13. GPE

    GPE Guest

    The coils are an important part of a switching voltage regulator.... as are
    the capacitors.
    The caps are probably low ESR type.

    -- Ed
  14. Guest

    <snip older>

    That brown stain is the acid leaking out of the cap. If it's been
    there too long it will eat away the copper traces on the top side. We
    have lots of Sony Digital Betacam decks at work that suffer this fate.
    Many of the boards can be salvaged if discovered before any serious
    damage. One guy does almost full time changing through hole and
    surface mount caps. BTW he uses a Metcal soldering station with an
    STTC-138 tip for through hole and STTC-126 for the SMT parts.

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