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Motherboard repair

Discussion in 'Electronic Repair' started by Mr. Land, Mar 22, 2005.

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  1. Mr. Land

    Mr. Land Guest

    Hi,

    I have an old AOpen AX6BC motherboard I've been running in a server for
    a while. The other night it blew out. There was a distinct burning
    electronic smell.

    After taking it out of the system and giving it a close inspection, I
    found one of the small surface mount transistors had burned out
    (charred, case cracked.)

    While I know there's a very good chance something else is wrong, I
    still think it's worth replacing that blown transistor to see if that
    fixes it before I toss the board in the trash.

    Problem is, I can't seem to find a replacement. The package is a small
    rectangle with 3 leads on one side and the collector plate on the
    other. I believe the case type is "SOT 89" There are only 4
    alphanumerics printed on the case, oriented as follows:

    +---+
    |D K|
    | |
    |0 J|
    +---+


    ....any chance someone could help me find a substitute?

    Thanks.
     
  2. James Sweet

    James Sweet Guest

    Chances are it's an N-channel power MOSFET, look for the regulator
    controller chip nearby and look up an app note, it should have a reference
    design with suitable mosfets.
     
  3. Rifleman

    Rifleman Guest


    It seems that the AX6BC is equipped with smt mosfet and the AX6B with
    ordinary hole mounted with heatsinks. On mine AX6B one of the mosfet
    transistors were burned but those were CEP6030L with TO220 package but for
    AX6BC I think they use D2-pak. Replaced it with another CEP6030L and it is
    still running.
    RC5051 is used as programmable synchronous buck controller. You might be
    able to check the drive to those two mosfet transistors. Download the
    application note AN-53
    http://www.fairchildsemi.com/an/AN/AN-53.pdf
    interesting reading.


    Bjorn
     
  4. Franc Zabkar

    Franc Zabkar Guest

    Can you locate the part in this photo?
    http://global.aopen.com.tw//Products/images/MB/products/ax6bc-b.jpg

    Is it in the top LH corner? If so, then check the nearby capacitors
    for signs of leakage or swelling. What is the part number and logo on
    the IC to the left of the lower 4 caps?


    - Franc Zabkar
     
  5. Guest

    You can buy a complete board on eBay for $15. How much is this worth to
    repair? I know, it is the principle of the thing but your time has
    value too.
    GG
     
  6. James Sweet

    James Sweet Guest

    What about the educational value and the satisfaction of repairing
    something? As others have pointed out in the past, this isn't
    sci.electronics.replace.
     
  7. Guest

    I've been getting satisfaction for many years repairing.electronics.
    Also, have done some design as well but when you're talking $15 to
    replace something or spend several hours working on multi-layer boards
    that may have internal burned traces and other bad components AND no
    prints, why bother? My time is worth more than $7.50 an hour. I assume
    yours is as well. Its not like you're restoring a collectible item that
    merits the TLC. I'm done
    GG
     
  8. Mr. Land

    Mr. Land Guest

    Hi,

    Thank you very much for your response. Unfortunately, even though I've
    dabbled in electronics in the past, this is sounding like I'm in a bit
    over my head...I don't even know what "look up an app note" refers to.

    But thanks for trying to help me.

    Cheers.
     
  9. Mr. Land

    Mr. Land Guest

    I went to read your reference...hmm, looks good, but way over my
    head!!!!

    I did manage to notice in their sample circuit the two mosfets in
    series, and since as I stated originally there is a second device
    (which is OK visually, at least) with the same part number, this might
    indeed be close to what I have.

    FWIW, since I had nothing to lose, I took a wild (in retrospect) guess
    that it might be an NPN transistor and tack-soldered one I had lying
    around. The BIOS booted! But as soon as it tries to read from the
    boot drive, it either states there's a missing file, or it can't read
    it at all. So I guess the NPN was a bad guess!

    Also in retrospect I'm pretty lucky I didn't blow the whole thing up!!!

    Thanks a lot for your help!!!
     
  10. Mr. Land

    Mr. Land Guest

    Actually, thanks, I did indeed acquire a replacement on eBay.

    But when I look at that board, all the parts on it, it just seemed like
    such a waste to just toss it.

    Thank you for your reply.
     
  11. Mr. Land

    Mr. Land Guest

    Yes, I can see it! This is cool!

    I had to load your picture into a graphics editor and zoom in a bit.
    But here's where the failed part is: to the right of the PCI sockets
    there is one large, square IC that appears to have some kind of
    orange-colored mark on its upper left hand corner. If you sight along
    the top edge of that IC, and move your gaze directly right, you'll come
    to 2 electrolytic capacitors standing next to each other; just below
    those capacitors is a row of small surface mount components, some
    caps, some resistors I think. If you sight along this row and continue
    moving your eyes to the right, you'll come to a white dot. That is the
    metal (heat sink?) tab of the device that blew out. The body of the
    device is above that, and the 3 connection leads are just above that.
    If you go back to the 2 electrolytic capacitors, and look directly
    above the left one, you'll see the other device that matches the one
    that burned up.

    Hey, thanks so much for taking the time to post this picture!!
     
  12. James Sweet

    James Sweet Guest

    If you got that close then it's almost certain that replacing that one
    component will fix it. Try an N-channel mosfet.
     
  13. Mr. Land

    Mr. Land Guest

    I can't explain or justify my desire to fix this board.

    All I can say is that when I go to drop it in the trash barrel, I look
    at all the components on it and the work that went into making it (ok,
    well, my board and 99,999 others just like it) and I think that I may
    be chucking it all because of 1 little component, I can't bring myself
    to do it.

    You're right, I guess that's pretty silly. But I also think that if I
    manage to fix it, I'll feel good about accomplishing that, even if the
    $$$ doesn't weigh out.

    Thank you for taking the time to reply.
     
  14. Mr. Land

    Mr. Land Guest

    Where would an ordinary person like myself acquire one of these
    CEP6030L's in the D2 package?

    Thank you!
     
  15. James Sweet

    James Sweet Guest

    FDP6030L from www.digikey.com should do the trick for you, might even be
    able to get a free sample from Fairchild.
     
  16. Franc Zabkar

    Franc Zabkar Guest

    The advice you've received so far is for the MOSFETs to the left of
    the CPU slot. Clearly you have some other fault near the southbridge.
    Does your component have a circuit reference, eg Qnn (transistor nn)
    or Dnn (diode nn)? Judging by the copper tracks, it appears that your
    transistor (?) is driven by the IC below the second capacitor. If I'm
    right, and if you can identify this IC, then its application circuit
    may reveal the function of the transistor.


    - Franc Zabkar
     
  17. Mr. Land

    Mr. Land Guest

    Yes, there is a Qnn number near the blown component. Unfortunately,
    right now I'm at work, and the board is home, but I will post that
    number later tonight.

    Also, yes, while I was tacking in the NPN xstr, I did notice that the
    foil trace for (what I thought was) the base lead went down to the
    small IC below and to the left. That IC is an LM324 (I remember it
    because I recognized the number.)

    Thanks again, folks!
     
  18. Mr. Land

    Mr. Land Guest

    Thanks. Looks like the FDP6030L is a TO-220 package, while the
    FDB6030L seems to be the smaller "D" package which I believe was on the
    board originally.

    Unfortunately, it seems the minimum quantity for the FDP part is 45,
    for a total of just under $40. Think I'm heading for a dead end...

    Thank you.
     
  19. Mr. Land

    Mr. Land Guest

    Spoke too soon, found one at Mouser (in the right package) for $1.50.
    Thanks!
     
  20. Franc Zabkar

    Franc Zabkar Guest

    The "nn" is not important, it's the "Q" that tells us that the
    component is indeed a transistor of some kind. A "diode test" of the
    good part using your DMM should identify whether this is a BJT or a
    MOSFET.
    An LM324 is a quad opamp. I have no idea what its function would be on
    this board. OTOH, an LM3524, a PWM regulator, might make sense if
    there was a coil nearby, but I don't see any.

    - Franc Zabkar
     
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