Need help repairing SMPS

Discussion in 'The Projects Forum' started by redpencil, Aug 23, 2014.

  1. redpencil

    Thread Starter New Member

    Aug 23, 2014
    4
    1
    Hello,

    I am trying to fix a switched mode power supply on a piece of audio equipment. This would be my first time doing so (I have not worked with AC before) and I'm not sure what to look for. The issue is that there is no power coming from the output 12V pins and the power supply makes a clicking sound every second coming from the control circuitry (which is a package containing SCC6210A, 2 electrolytics and some other parts).

    This happened one day when powering on. The only other symptom leading up to the failure that I have noticed is that when plugging in the power cable, I would hear a click/spark. Many power supplies do that when plugging in, except my other working unit does not do that.

    The equipment I have is a multimeter, soldering station, a device to test capacitance/ESR/diodes/transistors as well as another working SMPS (same model). I have already tested the fuse as well as each resistor and they are all within tolerance, so something else is causing this. There are also no visibly burned or damaged parts.

    Attached is a schematic of the SMPS.

    Any help would be greatly appreciated.
     
  2. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    Hi! First of all, are all the 12V out pins dead (the standby as well as the switched supply?)

    And have you checked the transistors (particularly Q103)?
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    The clicking is probably an indication that the power supply is in current limit and it periodically coming out of current limit in case the short is removed.

    If possible, disconnect the load from the power supply and check the load at the power supply connector to see whether it has a very low resistance of shot. If that looks ok, move on to checking the rectifiers on the power supply's secondary. If those are ok keep digging until you find a short.

    Yes, it could be something other than a short, but that is the most likely cause.
     
  4. to3metalcan

    Member

    Jul 20, 2014
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    I think that's called "hiccup mode," right, Dick?
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Test and replace the faulty electro caps.
     
  6. redpencil

    Thread Starter New Member

    Aug 23, 2014
    4
    1
    I tried testing my other working power supply on the load and it works ok, so the issue is not with the load. I'll try plugging in the faulty power supply again to rule that out completely.

    Upon further inspection, the clicking sound is actually coming from the transformer (T1 on the diagram).

    I managed to damage the IC on the ESR/diode/transistor testing kit I have, so that won't be possible atm (apparently having any residual charge on a cap, even in the mV fried it).

    I would like to do tests on a less expensive load, but I'm not sure what to hook it up with. There's no documentation on what loads each line is meant to drive.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If you can't test the electros, just replace all the small electros, especially any that are near something hot like a heatsink or power component.
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    it sounds like one or more of the small electrolytics near the switch mode chip are bad. probably the bootstrap caps in series across the large cap, they supply power for the smps chip to start up usually around 15 volts or so.
     
  9. redpencil

    Thread Starter New Member

    Aug 23, 2014
    4
    1
    I'll have a go at replacing the electrolytics. From what I've read here, it seems C11 would be the culprit.
    I have a nagging feeling that one of the thermistors failed, which is causing other components to fail from excessive inrush current (ex. the power supply died when the cord was plugged in). How likely is this scenario?
     
  10. DickCappels

    Moderator

    Aug 21, 2008
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    [ off topic reply: Some bodily function -hiccup , burp..]

    And don't forget the diodes connected to the secondary/ies. When they go short, its usually instant and VERY SHORT :)
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Hiccupping SMPS's rarely have badly blown power components.

    They short out, and that usually takes out a fuse or fusible resistor and makes the SMPS very dead.

    Hiccupping is normally caused by something in the feedback being out of spec, like bad electro caps. Or a faulty optocoupler etc. Or a short on the output rail (overload).
     
    absf likes this.
  12. redpencil

    Thread Starter New Member

    Aug 23, 2014
    4
    1
    I replaced all the electros and it still did not work.

    Turned out C6 was bad. I replaced it with a ceramic (it was originally an smd) and all works fine now.
     
    to3metalcan likes this.
  13. tom_s

    Member

    Jun 27, 2014
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    /me pats redpencil on the shoulder - well done, nice to see a happy outcome
     
  14. to3metalcan

    Member

    Jul 20, 2014
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    Well done! Break out the champagne--a repaired smps is an occasion for celebration!
     
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