USB fuse ?

Discussion in 'Abit' started by Eeyore, Dec 4, 2008.

  1. Eeyore

    Eeyore Guest

    I have a (several actually) KT7A mobos including RAIDS in a few PCs.
    Including one running a 2400XP CPU ! That FLIES and I picked up the
    2600XP-M CPU too which has much the same settings but a larger cache
    that gives it the extra speed.

    Anyway, a while back, my scanner fell of the table it was on and landed
    on its backside. It would no longer work, so I though I must have killed
    it, even though there were volts present on all the USB connector pins.

    Later, trying another USB device, the power LED would 'blip' on for a
    second and then go out.

    I twigged I'd probably blown a protective fuse/fusible resistor in the
    USB 5V supply most likely. Probably gone high resistance and could
    charge a capacitor on the mobo enough just to get that 'blip' on the
    power LED.

    I'm good with soldering and electronics generally but I don't know how
    to locate the part. Does anyone have any clues ?

    Eeyore, Dec 4, 2008
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  2. Eeyore

    Paul Guest

    look for a Polyfuse.

    They're typically green in color, with a "dimple" on
    either end of them. I see three in a picture of the KT7A,
    up near the I/O connectors. They can be used to protect
    PS/2 mouse, PS/2 keyboard, and USB devices.

    Don't reach for the soldering iron just yet.
    Use a multimeter, and check to see whether
    +5V is available on both sides of the fuse or
    not. Both before and after plugging in a USB

    A Polyfuse resets itself, after the overload
    is removed.

    Paul, Dec 4, 2008
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  3. Eeyore

    Eeyore Guest

    Ah, I've only seen them in Orange.

    Yup. You didn't see the PCB legend did you ?

    So it's not a Polyfuse then.

    Eeyore, Dec 5, 2008
  4. Eeyore

    Paul Guest

    When I suggest the multimeter, I'm suggesting to you
    that you observe the voltage on either side of the
    fuse. Chances are the fuse is not at fault. What
    I cannot tell you, is if there is any active device
    upstream of that point. If the USB header is fed from
    +5VSB and the +5VSB winked out, the computer would
    reset or power off. And you don't mention that in
    your symptoms. The computer would similarly crash,
    if the raw +5V was dying (but that rail can offer
    many more amperes, and can burn whatever gets in the
    way). That leaves something like a "power bug" in the
    path, an eight pin chip which monitors the current
    flow closely. It has MOSFETs inside to cut off the
    current flow. But if a Polyfuse is present, there is
    no need for a power bug. A power bug might be seen
    in a laptop for example. I haven't seen any in
    desktop motherboards.

    I guess what I'm saying is, I don't have any real good
    suggestions for a match between your symptoms and a cause.
    But you can check the fuse, by using your ohmmeter across
    the Polyfuse, and verifying it reads zero ohms. (Do that
    with all power off in the computer.) Then, when the computer
    is running, observed the +5V levels on either side of the
    fuse. For example, if you plug in a USB device, and
    both sides of the fuse no longer have 5V on them, then
    something upstream has switched off the power. And that
    suggests there is some other kind of power management

    (Clip the ground lead of your multimeter, to an I/O screw
    on the back panel of the computer. Then, you'll only
    need one hand to probe the ends of the Polyfuse, while
    the computer is running.)

    If the Polyfuse has 5V on one side, and nothing on the
    other, just after the USB device is plugged in. then
    it has opened for some reason. It is going to be
    pretty hard to put an ammeter in series with the
    circuit, to measure the actual current flow, and
    figure out whether the Polyfuse is tripping at
    a lower than normal current.

    I'm also not sure whether there are any special
    soldering instructions for Polyfuses, and whether
    hand soldering for them is recommended. If it was
    my motherboard, I'd want to be absolutely sure
    the thing was bad, before attempting a repair.

    Paul, Dec 5, 2008
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