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How to fix PCB short

Discussion in 'Troubleshooting and Repair' started by The Rectifier, Jul 5, 2012.

  1. The Rectifier

    The Rectifier

    3
    0
    Jul 5, 2012
    The story so far...

    What we have here is a PSU from a Gateway 900G LCD monitor. I have replaced some burned IC's and capacitors that are known to have problems. The problem that is occuring, which also occured before I put in the new parts, is that the PSU keeps clicking when it is plugged into everything, i.e. the logic board, CCFLs, power and a VGA cable connected.

    So poking and prodding and it seems to be a short on the board someplace. Attached is a photo of the approximate area of the short. What I did was take a toothpick and press in different areas. About anywhere in the area enclosed with the red line where I press the clicking stops and the CCFLs light up and there is a picture on the screen. It seems as thought the board needs to be flexed downward to make some connection and then it works.

    I have looked over the board with a 10x loupe looking for cold solder joints or something that would indicate a problem. I have reheated a bunch of the through hole solder joints hoping it was a simple cold solder joint and that has not worked, or I did not get to the right one. It only seems as there is the top and bottom to the board and not a multiple layer board so I don't know where a run would be broken. I also checked continuity between many point and did not find any problems but obviously did not do them all.

    So the question is what could be causing this short that pressing on it and flexing the board fixes it? How do I find where the problem is? Or should I just put a wedge in between the board and case, thus giving the board the correct amount of flextion for it to work, can't imagine this is the right thing to do, and put everything back together and call it done?

    Thanks for your help, I appreciate it.
     

    Attached Files:

  2. GreenGiant

    GreenGiant

    830
    6
    Feb 9, 2012
    its not necessarily a short

    though there could still be a cold solder joint that you have missed somewhere, it could be one of the pins of the caps, or one of the resistors could be bad and you flexing it causes them to work properly.

    Have you figured out what it clicking?

    to test this try using a continuity mode on a multimeter and check pretty much everything, then flex it, and check everything again, if that doesnt work then there is probably a bad part somewhere
     
  3. The Rectifier

    The Rectifier

    3
    0
    Jul 5, 2012
    Thanks for the reply,

    I don't know why it would be the capacitor legs as it happened before the recap and after, if that matters but I don't know.

    I have no idea what is clicking. All I can think, with admittedly limited knowledge, is some sort of semiconductor, either the inverter controller or the inverter IC's that I replaced.

    I will do some more point to point work.

    As there is a fair amount of SM components I think it would be inadvisable to reheat each one to see if that fixes it. What is your take on that?
     
  4. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    I would start by reflowing any through hole parts in that area, there might be one that isn't making contact with all the layer due to a break in the plating...

    Depends on your skills and available tools... If you hold the board steady you can reflow it section by section with a small hot air gun or rework station... Holding it steady will ensure that if there are components on both sides and/or near by parts won't be jolted out of place when moving the heat around... You could also get some no clean flux and give the board a once over spray before you do any heating to refresh all the joints...
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,214
    2,695
    Jan 21, 2010
    It sounds to me more like an intermittent open circuit than a short, however the "clicking" from power supplies generally indicates they're overloaded. (The sound comes from the transformer as it is briefly grossly overloaded and the magnetic field causes things to move slightly.)

    I agree with reflowing around the sensitive part of the board, but also look carefully at the board and see if there are any cracks in it that may have fractured a track. Holding the board up to a strong light may help. Even if you can't see the traces that are broken, a crack in the board bay be visible. Repair any traces that cross it (solder a wire over the track at that point).

    Viewing the board under high magnification can also be helpful.

    Another possibility is that there is some excess solder under a surface mount component that is causing a short. This can be really hard to see. It's also unlikely to be intermittent unless it is caused by solder paste that hasn't melted. That would only occur if the part was hand soldered. In a reflow oven etc, everything will get hot enough to melt solder. (I mention this point because I did it to myself after being a bit ham fisted in the application of solder paste the other day.)
     
  6. The Rectifier

    The Rectifier

    3
    0
    Jul 5, 2012
    Still not sure where the short or intermittent contact is.

    I did a lot of point to point to check for continuity and everything seemed to be ok. Went back over it with the 10x loupe and only found one spot where the solder may have came away from a disc capacitor leg but it was in the opposite corner of the board than the apparent problem. Stuck a light behind it and the traces seemed continuous without cracks in them.

    I have slightly better skills than tools and my tools are substandard yet I also reflowed many of the joints of the through hole and SMD in the area. That seemed to help but when I screwed it down it still did not work. So I reflowed more points and put some of those red paper washers you get with a PC motherboard to shim everything so there was no flex or torque on the board and it seems to be working continuously. I put it all back together and still works so only time will tell, but I think we got it fixed.

    Thanks again to all for your help, I couldn't have done it with out you.
     
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