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testing ATX power supply

Discussion in 'Electronic Repair' started by tempus fugit, Jan 7, 2004.

  1. tempus fugit

    tempus fugit Guest

    Hey all;

    A friend handed me her dead computer to see if I could get it up and
    running. When you turn it on, nothing happens. No lights, no fans, nothing.
    So, I figure it's either the power supply or the motherboard (at least).

    I want to test the PS first, before just replacing it. It is an ATX 300w
    supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
    and turned it on, but nothing happened. I then turned it on and shorted the
    green wire to ground, but still nothing. Can I assume the PS is dead, or is
    there something else I should be doing?

    Thanks
     
  2. That should do it assuming your AC power is present.

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: The email address in this message header may no longer work. To
    contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  3. You can get a faulty mains switch or power lead too, the quality isn't too
    great !
     
  4. tempus fugit

    tempus fugit Guest

    Thanks

    I've tried it with a known good power cord, so that's not it.

    Thanks for the tips
     
  5. You can not just short the +5 to ground through a resistor on an ATX
    supply. There is a line on the ATX connector to the motherboard that
    must receive a signal before the PS will supply full power. what you
    can do is disconnect the PS from the motherboard and jumper the green
    wire to a ground (black) wire. The PS should now come on and you should
    be able to read voltages across the outputs.

    It could be anything. On the ATX motherboards, there is a constant
    voltage applied to the motherboard. when you press the power switch,
    the motherboard will do a series of tests. If the tests fail, no power
    will be supplied to the motherboard. If the tests pass, then power will
    be supplied and the system powers up.

    There are many factors that will prevent a system from powering up
    nowdays... Some won't power up if there is no signal received from the
    CPU fan.

    Other things to check:
    make sure all memory is seated in the slots correctly
    make sure that the motherboard isn't grounded against the case
    make sure all connectors are plugged into the motherboard properly
    I have seen a hard drive prevent a system from powering up, so check
    connections there, too
    even a PCI (or AGP) card loose will prevent it from powering up
     
  6. Not on an ATX supply. The PS won't powerup unless the motherboard
    signals the everything is good first. You have to ground the green wire
    in the ATX connector.
     
  7.  
  8. Isn't that exactly what he said he did? :)

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: The email address in this message header may no longer work. To
    contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  9. Hooi,

    I ever wrote a reply on repairing PC power supplies. It follows below. IMHO
    it contains all you need (and more) to decide wether your power supply has
    gone or not.

    petrus

    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.
     
  10. tempus fugit

    tempus fugit Guest

    Geezzh!

    Thanks a lot - that will come in handy.
     
  11. Yea.. On the first pass, I missed where he said that he did short the
    green to ground..
     
  12. That was a very good source for information!

    <snip>
     
  13. Denny

    Denny Guest

    On more than one occasion, I've seen it be simply the 110/220 input
    voltage switch.

    I think sometimes they get moved partially out of position when the case
    is grasped just right [just wrong?] and pulled out from under a desk.

    Worth not overlooking.

    -Denny
     
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