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replace caps on motherboard issue

Discussion in 'Electronic Basics' started by robm, Nov 7, 2005.

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  1. robm

    robm Guest

    i have a dead mobo that has domed caps, not worth sending off but worth an
    attempt at repair
    i am amateur solderer i have couple different weller irons 15w 25w 60w

    I read somewhere about cutting the caps in half and exposing the two cap
    posts then soldering new caps to the posts instead of trying to completely
    remove and replace the caps

    Is this a bad idea ?
    is there a good reason not to repair this way ?

  2. w_tom

    w_tom Guest

    With a solder sucker, you need not do that. The spring
    loaded solder sucker will pull solder out of the hole so that
    a new cap can be properly mounted. It is far better - more
    reliable - to install new caps properly.
  3. I haven't done it as you describe, but I can understand why someone
    may have come up with that method after destroying an otherwise good
    board trying to get the wires out with just an iron. The problem is
    that these boards are at least 4 layers, with the caps soldered to
    vias that connect to big pours of copper in the inner layers. By the
    time you get enough heat into one end of the lead to melt solder all
    the way through the via, you may well remove the outer pads.

    I recently replaced all the large electrolytics on a mother board just
    as you describe. I ended up preheating the board with a hot air gun
    till it was at about the boiling point of water (put a drop of water
    on the board near the joint being worked on and wait for it to start
    to bubble), before applying heat to both leads with an iron heating a
    big blob of solder. Harder than removing the caps was the process of
    cleaning the holes well enough to insert the new caps. This also
    needed the preheat and a length of stranded wire I used as a solder
    wick. Soldering the new caps in required preheat, also. Using low
    temperature solder (63%tin, 37 % lead or 62% tin, 36% lead, 2% silver)
    is very helpful.

    By the way, to make this process worth while, be sure you have a good
    grade of replacement caps on hand, or the new ones may not last long.
    I used the 105 degree rated, high ripple current, low ESR FM series
    by Panasonic, from Digikey. I was also able to put slightly larger
    value caps into the same space.

    The ripple voltage on the board was way down after the replacements
    were installed. The board was very unstable before the replacements,
    but has been running perfectly since.
  4. I've never had any problems just heating one lead at a time while applying
    force on the cap on the other side of the board. When the solder gets loose,
    the other pin bends, and most of the heated pin comes out of the hole.
    Reverse the process on the other pin, bending the other direction. Usually
    a couple of iterations gets the cap off (probably not a recommended
    technique for a board you are worried about destroying, but it worked
    for me on a couple of old boards).

    Once the cap is out, you can use a solder sucker fairly easily on the
    holes one at a time to get enough room to put in the new cap.
    --email: icbm: Delray Beach, FL |
    <URL:> Free Software and Politics <<==+
  5. w_tom

    w_tom Guest

    John has demonstrated another technique that professional
    use - and that most anyone can do. Another factor in removing
    those leads is the quality of the soldering iron. Best irons
    will increase the heat as necessary when the load - the size
    of the solder joint - becomes larger. Just another trick that
    makes removing solder in a hole easier with less damage to the

    Also pay attention to the numbers and specs that John has
    provided for capacitors.
  6. Lord Garth

    Lord Garth Guest

    Least of which is the diameter of the replacement cap! Some boards
    do not have clearance.
  7. Pooh Bear

    Pooh Bear Guest

    It's quite a good idea.

    If the component is known to be defective then it matters most that the pcb
    isn't damaged.

    I often chop components in half so as to make removal easier and hence avoid
    collatteral pcb damage. I haven't done it with electrolytics though. Be careful
    about the electrolyte they contain. It may be slighly toxic.

    Was it an Abit mobo btw ? I need to recap my own Abit KT7-RAID soon.

    See this link

  8. robm

    robm Guest

    I wish it was Abit kt7

    it is a ECS k7s5a, i have a couple and they run fine for what they are doing
    i tried recaping one once by removing the CAP but i think i damaged the
    trace trying to get solder out of hole so now a second board has problem and
    i'd like to not damage the board

    it is using the famed luxon bad caps

    i have nichicons from mouser to replace
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