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Repair ATX power supply

Discussion in 'Electronic Repair' started by Skeleton Man, Nov 24, 2007.

  1. Skeleton Man

    Skeleton Man Guest

    Hi All,

    I was wondering does anyone here have a good guide for how to go about
    repairing an ATX computer power supply ?

    I've never done anything like this before, so any help is appreciated..

    Chris
     
  2. CJT

    CJT Guest

    Generally they're cheaper to replace than to repair.
     
  3. PeterD

    PeterD Guest

    Way I do it is to go to the computer store, (or Staples, or Office
    Depot, or Best Buy, or...) and say: got a power supply?

    Since you have not said what was wrong with the supply, I'm sure that
    will work well for you too.
     
  4. Skeleton Man

    Skeleton Man Guest

    Generally they're cheaper to replace than to repair.

    I have already replaced it, but I want to repair the old one if I can as an
    exercise.. for all I know it could be a 5c resistor or something that needs
    to be replaced..

    Chris
     
  5. Skeleton Man

    Skeleton Man Guest

    Way I do it is to go to the computer store, (or Staples, or Office
    I have replaced the power supply, and now I want to repair the old one.. I
    shouldn't have to throw out a $50 peice of equipment because a 25c part is
    broken.. (spending a day or two finding the problem doesn't worry me)

    The problem is that it still supplies +5V standby, but refuses to turn on (I
    have a load attached and I do have the correct wires for PS_ON and ground).

    No fuses are blown, and nothing appears or smells obviously burnt.. I was
    told to check for open/high resistors near the large filter caps on the high
    side, so I did and they both read the correct ~220Kohms.. there's two
    diodes on the high side and both those are ok also.. what else should I test
    ?

    Chris
     
  6. James Sweet

    James Sweet Guest

    These are rarely repaired, however often the problem is bad electrolytic
    capacitors. Occasionally one will grenade in such a way as to cook virtually
    every semiconductor in it, but often they just refuse to start up. Check the
    standby power supply, if that's ok then try to figure out why the main one
    won't start up.
     
  7. Skeleton Man

    Skeleton Man Guest

    These are rarely repaired, however often the problem is bad electrolytic
    I think you may be onto something.. I took a closer look and several
    capacitors appear to be bulging a bit at the top.. one does have brown
    stuff around the edge, but I figured this was rust or crap that collects
    like dust, etc.. (but none of the others have this).. all the capacitors
    in question are 1000uF - 2200uF, 10 - 16V.. the small ones look fine..

    As for standby power, yes I get +5VSB, it just refuses to turn on and
    deliver the rest.. is this common ? I'm guessing there is a seperate
    circuit for standby voltage ?

    Chris
     
  8. Long time ago I wrote the following scheme:


    <begin>
    I repaired several PC supplies for a hobby, but if your time is valuable,
    buy a new PS.

    First of all read the sci.repair.faq. Especially the parts on safety and
    SMPS.

    Most of the times the fault is found between the mains connection and the
    transformer(s).

    1. In the most simple cases only the fuse is blown. After replacing
    this fuse, connect the PS to the mains using the serial lightbulb trick.
    - If the bulb burns brightly, you know that the old fuse had a good
    reason to quit, so the case is not simple anymore. The first thing
    you have to do now is to find the short circuit. The most suspected
    components are the mains rectifier, the filter capacitors and one or
    more of the power transistors. Use eyes, nose and an ohmmeter
    to find scene of the crime. Remove and check the suspected
    components. Replace defective components except for the power
    transistors at this time. It makes no sense to continue until you fixed
    the short circuit
    - If the fuse is good but the PS still dead, you can start to check the
    voltages.
    2. Check the voltage between pin 3 and pin 9 of the ATX-mainboard connector.
    This should be 5V.
    - If not you have to check the voltages on the mains side. Otherwise it
    will be wise to check the voltages on the mains side as well (3-5).
    Then continue reading up to point 12, not to miss some explanation.
    Continue at 12.
    3. The AC-pins of the mains rectifier should show the mains AC voltage.
    - If not you may have an interrupted trace or mains filter.
    4. Between plus and minus of this rectifier you should find about 310V DC
    or 325V DC depending on your mains voltage. I call it the primary power
    voltage.
    - If not you may have a faulty mains rectifier.
    - If the voltage is much lower (analog meter) or jumping around (digital
    meter), the large filter capacitors (p.e. 470 muF, 200V) are also
    suspected.
    5. Both filter capacitors mentioned above are in serial. The midpoint should
    be at half the primary power voltage.
    - If not, the mains rectifier, the filter capacitors and the parallel
    resistors (parallel to the capacitors) may be defective. Another suspect
    is a third capacitor (p.e. 1muF, 250V) that leads from the midpoint to a
    transformer.

    Explanation:
    ATX-PS's usually has three power transistors at the mains side. One
    connected to a small transformer, the other two connected to a larger
    transformer. You can recognize the pair of transistors best by finding the
    emitter of one of them connected to the collector of the other.
    First you have to deal with the one transistor and the small transformer.
    (Go to 8 if you removed this transistor already.)

    6. Check the voltage on the collector of the transistor.
    - If this voltage is zero or very low there may be an interruption
    between the collector and the primary power voltage.
    - If this voltage is below the primary power voltage or jumping, there
    seems to be switching activity. You can check this with an AC voltmeter
    on a secondary coil of the transformer. The reading will not be correct,
    but if you find an AC-voltage you have to continue checking the
    secondary rectifier and regulator.
    - If this voltage is the primary power voltage the transistor does not
    conduct.
    7. Check the voltage on the base of the transistor.
    - If this voltage <0.6V the startup resistor may be defective.
    Otherwise the transistor may be gone (most likely.)
    8. Disconnect the PS from the mains and take the safety precautions to
    discharge the capacitors.
    9. Remove the suspected transistor and check it with an ohmmeter or a
    transistorchecker. Most of the times you will have to provide a new
    transistor. (Beware! Even a transistor that looks good under test conditions
    may malfuntion in the actual circuit.)
    This is also the time to remove, check and replace other fried, exploded or
    discolored components near the transistor/transformer combination.
    10. Re-power the PS using the serial light bulb.
    - If the lamp is burning brightly you have a short circuit in your PS.
    Most likely your (new) power transistor is conducting due to too high
    a continuous base-current. You have to dive deeper into this part of the
    circuit until you find the couse of this problem.
    - If the lamp is dim or not burning at all you can re-check the voltage
    between pin 3 and pin 9 of the mainbord connector. Finding 5V you can
    continue, otherwise you have to restart your investigation of the small
    transformer/one power transistor part of the circuit.(Point 6.) It makes
    no sense to continue until this part of the PS functions correctly.
    11. Disconnect the PS from the mains when you are done so far.

    Explanation:
    For the next part of the repair procedure you have to provide some load to
    the PS. This is simply because of some PS's will not function well without
    load. You may use an (old) main board. Someone ever told me he uses
    12V car bulbs, one on the +5V and one on the +12V. I prefer a huge and
    heavy old harddrive. Those old basalt blocks (we use to strengthen our
    dikes) consume a lot of energy. The one I use, provided enough load to all
    the PS's I ever repaired.

    12. Replace the power transistors you may have removed earlier.
    Reconnect the PS to the mains using the serial light bulb. Check the
    voltage between pin 3 and pin 9 of the main board connector. Connect pin 14
    of the main board connector to pin 13. This will switch on the main part of
    the PS, the part with the two power transistors and the large trafo.
    - If your load start to work, check the voltages of the several power
    connectors. When they have the correct values your PS is on air again.
    Check it out by removing the serial light bulb.
    - If (even after removing the serial light bulb) some but not all of the
    values are correct, you have a problem. You have to investigate the
    failing voltages from the secondary coil of the transformer till the
    connector. Quite a challenge.
    - If the light bulb is burning brightly you have a short circuit. Most
    likely your power transistors are gone so you have to check (and almost
    sure replace) them and their surrounding components, especially the start
    resistors. Pay also special attention to the freewheel diodes (between
    the collector and the emitter of the power transistors.) Don't forget to
    disconnect the PS and to discharge the filtercapacitors first! When you
    are done, restart at 12.
    - If if your lamp is dim or dark but your load does not work you may have
    defective or blocking power transistors. A fault on the secondary side of
    the transformer is another possibility.
    13. Search for switching activity on the secondary coils of the transformer
    using an AC meter.
    - If you don't find AC-voltage you have to check the voltages on the
    power transistors.
    - If you find an AC-voltage you most likely have a defective rectifier,
    filter capacitor or regulator at the secundary side. Disconnect from de
    mains, discharge the filter capacitors and try to find the failing
    components with an ohmmeter. You will have to remove the rectifiers
    from the board prior to testing because of the secundary coils have
    only few windings of thick wire so they are the shortest shortcuts as
    far as your ohmmeter concerns. Another trick is to use a controllable
    power supply. Connect it to the point where the removed rectifier was
    connected to its filter capacitor. Beware of the polarity! Power on both
    PS's and rise the voltage of your controllable PS to the level of normal
    operating of your defective PS. The regulator that sucks to much current
    with respect to the light load will be the main suspect.
    You have to go deeper into the circuit of this regulator if you want to
    repair it. Another challenge.
    14. Check the voltages on the power transistors. The collector of one of
    them should be at the primary power voltage, the emitter of the other
    should be at the common. The remaining collector and emitter are tied
    together and should be at half the primary power voltage.
    - If you can't find the primary power voltage at a collector you have an
    interruption. Maybe a bad soldering or the like.
    - If no emitter is connected to common you also have an interruption.
    - If the tied collector-emitter is not at half the primary power voltage
    you most likely have defective power transistors. (In my experience they
    always die together.) Disconnect, discharge and remove, check and
    replace the power transistors and their surrounding components. Restart
    at 12.
    - If the tied collector-emitter is at half the primary power voltage you
    can check the base-emitter voltage of the power transistors. If they are
    <0.6V you may be lucky and find only defective startresistor(s) and/or
    other base circuit components. But most of the times a defective base
    circuit will kill its transistor which in turn will kill its neighbour.
    So you will have to replace the whole bunch.

    Of course, this story does not cover all possible faults of PC-power
    supplies, but I only once failed to repair a PS using this scheme.
    <end>


    Some Czech, Pavouk, drew a schematic of a common ATX supply and wrote an
    explanation of it's inner workings.
    http://www.pavouk.org/hw/en_atxps.html
    which reading I strongly recommend.

    I do not repair them anymore this days as most of the times the power
    transistors are gone and new ones are more expensive then a new power
    supply.


    petrus bitbyter
     
  9. Skeleton Man

    Skeleton Man Guest

    Long time ago I wrote the following scheme:

    That looks excellent, I'll have a read through and see if I can find the
    problem..

    A quick question about transformers... mine has 4 of them in total, a small
    one near the big capacitors, then two more small ones and a really huge one
    (like 4x the size of the rest) near the power transistors.. I was comparing
    model numbers with a photo of a simmilar supply, and they both seem to have
    the following numbers in their model:

    Smallest transformer: 16 (WIN-16LA)
    Small transformer: 19 (WIN-19L)
    Really big transformer: 35 (WIN-35P)

    Are these numbers for one of the voltages or what ?

    Here's the pic I was comparing with - different brand, but the layout of
    components is almost identical to mine:

    http://www.oakpc.com/mimg/eart/images/041221oak0235.JPG

    Regards,
    Chris
     
  10. Franc Zabkar

    Franc Zabkar Guest

  11. Skeleton Man

    Skeleton Man Guest

    Long time ago I wrote the following scheme:

    I just tried the supply again (had been sitting a while since it first
    stopped working) and this time the fuse blew..

    The rectifier appears ok, but the two power transistors (2SC2625) are giving
    zero resistance between any of the pins.. does this mean they're fried ?

    Chris
     
  12. Skeleton Man

    Skeleton Man Guest

    Long time ago I wrote the following scheme:

    Further to my prev message, I tried testing power transistors on diode
    setting, and nomatter what combination of pins/tab I connect to, it always
    reads zero volts..

    Chris
     
  13. CJT

    CJT Guest

    Since you also said there aren't any burn marks or bad smells, my guess
    is the caps are bad. That could certainly account for the symptom.

    If it's the right vintage, you may be a victim of this:

    http://www.siliconchip.com.au/cms/A_30328/article.html
     
  14. Eeyore

    Eeyore Guest

    Do you have any understanding of basic circuit theory ? If you don't even have
    that then forget it.

    Graham
     
  15. James Sweet

    James Sweet Guest

    If the caps are bulging, they're bad. It was a real common problem for a few
    years.
     
  16. James Sweet

    James Sweet Guest

    They may have shorted from failing capacitors, but more likely they just
    appear shorted because of other things in the circuit. You usually have to
    remove them from the circuit for testing.
     
  17. Eeyore

    Eeyore Guest

    Replace ALL the larger caps even if they aren't bulging. Looks like you have a
    PSU made with 'bad caps'.

    http://www.badcaps.net/pages.php?vid=5

    NB - the caps on the secondary side MUST be 'low ESR' types made for smps use.

    Graham
     
  18. Eeyore

    Eeyore Guest

    In or out of circuit ?

    Graham
     
  19. Skeleton Man

    Skeleton Man Guest

    They may have shorted from failing capacitors, but more likely they just
    I did remove both of them from the circuit for testing..

    Chris
     
  20. Eeyore

    Eeyore Guest

    And you get '0' on a diode test between any combination of pins ?

    They're stuffed in that case.

    Graham
     
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