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Bad Capacitor?

Discussion in 'Troubleshooting and Repair' started by DiodeDave, Sep 11, 2015.

  1. DiodeDave

    DiodeDave

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    Dec 7, 2011
    I have a small board with several relays caps, transistors etc.

    The board sustained an impact and stopped working.

    I've eliminated the power supply and cables as the problem. The signs of the impact are minimal except for a 1uf cap. It was bent and when looking at it closely, there is a crease in the side near the base. One of the transistors was bent slightly but the leads were intact.

    While still on the board, I put my ohm meter across the leads. The reading went from some value to infinity in 3 seconds.

    Is it possible the crease has changed the value of the cap and is the cause of the board not working?
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    A capacitor 'should' change resistance when you measure it.
    Why?
    The Multi-meter applies a voltage to the part you are testing. A capacitor will actually 'charge' while being tested.
    When a capacitor has no charge, current flows freely as the charge accumulates. As the charge builds up, so does the effective resistance. The time it takes to charge or discharge is well-known and is a function of the resistance and capacitance values.

    At first, is sounds fine, can you share any pics of the board, and tell us the value of the capacitor?
     
  3. DiodeDave

    DiodeDave

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    Dec 7, 2011
    The capacitor is 1uf 100v(rev).

    After the impact, the capacitor was on it's side. The arrow points to the crease in the side.
     

    Attached Files:

  4. Gryd3

    Gryd3

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    Jun 25, 2014
    It still 'sounds' fine, but to be sure you would need to remove it from the board and test it again. The only thing the multimeter can do though is detect if it's shorted, or open circuit. (both failure conditions) It will tell you little about leakage, or current capacitance. If it's damaged, the value could possibly change.

    That said, I'd still like to see the rest of the board. Damage may be else-where, and seeing how that cap is used will tell us what is acceptable or not.
     
    Martaine2005 likes this.
  5. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Are those relays mechanical, or reed relays?
    I would think they'd be mre suceptible to impact damage.
    Your 1 ufd 100v cap isn't all that expensive to replace if it concerns you, and would eliminate that possibility.
     
  6. DiodeDave

    DiodeDave

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    Dec 7, 2011
    Here are pictures of board. It is connected to a 24v controller which operates my 1/8 scale locomotive. The purpose of the board is to provide the battery voltage and error codes. When connected, the display is blank.

    The cap in question is to the left of the handwritten "T" at the top of the picture. The transistor to the left was bent over as well. All of the leads look to be intact.
     

    Attached Files:

  7. dorke

    dorke

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    Jun 20, 2015
    First thing ,the board is full with "solder splashes",probably causing a short circuit on the board.
    They should be removed gently(with a small screwdriver and an old tooth brush).
    remove all of them then test again.
    See the bigger ones in the photo( in red). board front.jpg
     
  8. DiodeDave

    DiodeDave

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    Dec 7, 2011
    Thanks Dorke

    I'll remove them the best I can. The entire board is covered with some type of varnish. I think you would agree, the thing looks rather rough.

    The board did work prior to the collision.

    Your arrows did point to a possible cause which I didn't see. Look at the 3rd lead under your arrow (Under the IC). I can't photo it, but the solder is scraped and I can see the copper where the lead is supposed to connect. This may be the site of an additional impact.

    I won't be able to test the board for a while. The location is deep inside my locomotive and it will take some time to access. I have several projects taking time.

    Thanks for your input.

    Dave
     
  9. dorke

    dorke

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    Jun 20, 2015
    The varnish is not a problem,don't scrap it.
    Just remove the splashes gently ,and clean with alcohol and a cotton swab-stick.

    If you think any solder-point needs re-soldering ,solder it well.

    Check for shorts on the entire board between adjacent solder-points.

    I would replace that "smashed" cap as well.
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    That is a definite .... if it has been folded over as you said @DiodeDave, then there's a high probability that
    one or both of the leads have been partly disconnected inside the cap


    Dave
     
  11. Old Steve

    Old Steve

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    Jul 23, 2015
    I think I see a small ring indicating a cracked solder joint around one pin below the IC, the right-hand pin of the two close to the scuff mark. Could be the problem, along with that cap.
    See my circle in the pic below.

    Also, as indicated, definitely replace that cap. The way that they're constructed, it's very likely to have been affected by that dent.

    If you have a magnifying glass, grab it and carefully inspect the joint I indicated, (and the rest), for potential fine cracks. (Or just re-solder it, to be sure.) Check the tracks in that vicinity too.

    board front.jpg
     
    Last edited: Sep 14, 2015
    davenn likes this.
  12. Old Steve

    Old Steve

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    Jul 23, 2015
    How did you go with this, Dave? Get it sorted out?
    I've been wondering if that was a crack around that pin.

    It's always good to give this sort of topic closure, for the benefit of those who are following.
     
  13. DiodeDave

    DiodeDave

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    Dec 7, 2011
    To quote myself in message #33, "I won't be able to test the board for a while. The location is deep inside my locomotive and it will take some time to access. I have several projects taking time.

    Thanks for your input."

    To access a cable for the board, I have to disassemble most of the workings of my locomotive. This is prime train running season so, as long as the locomotive works, I'll work on the "fine tuning" when time permits. Thanks to the input on other threads, my loco works very well.
     
  14. Old Steve

    Old Steve

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    Jul 23, 2015
    I missed that bit. No problem.
     
  15. davenn

    davenn Moderator

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    probably cuz there is no message #33 hahaha the thread only goes up to #15

    but looking way back through his posts I did find the comment in post #8


    D
     
    Old Steve likes this.
  16. Gryd3

    Gryd3

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    Jun 25, 2014
    He's only at post 34. I think he used his personal post count instead of the post # in this thread ;)
     
    Old Steve likes this.
  17. davenn

    davenn Moderator

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    Sep 5, 2009
    possibly, which means nothing to anyone else

    not to worry, all is good :)
     
  18. DiodeDave

    DiodeDave

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    Dec 7, 2011
    Huh?

    Oh well, I scrubbed the board with a tooth brush and picked off anything that didn't look friendly.

    I also put some heat on any area that looked like it might have had impact or a crack. I heated until I saw the solder melt.

    Nothing. I'll try a new cap when time allows.

    Thank you all for your interest.
     
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