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Whole group of LEDs stopped working.

Discussion in 'Troubleshooting and Repair' started by skazclaw, Sep 5, 2013.

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  1. skazclaw

    skazclaw

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    Sep 5, 2013
    This is a radar speed sign, it detects your speed and then flashes the speed you're going. This board is attached to as trailer, so it does get shaken a bit when driven from location to location, but it is designed for that. No funny smells, I can't see any burnt components.

    It's made up of a bunch of groups of LEDs, each group has around 21 LEDs.

    I assume that the LEDs themselves are ok, since the chance of all the LEDs in one group failing would be very low.


    Does anyone have any idea what the problem may be? I attached some pics, but they aren't very good.

    VID: http://tinypic.com/r/2aand3k/5
    http://share.pho.to/3eA7I

    Thank you experts!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What is used for multiplexing? I would look at that first.
     
  3. skazclaw

    skazclaw

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    Sep 5, 2013
    I'm not sure what that even is.

    This is a very similar product.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, first understand that this process is going to be a bit like what would follow from this conversation:

    Person 1: "My boat doesn't work. What's wrong?"

    Person 2: "Is the engine running?"

    Person 3: "What's an engine?"

    OK, now consider that rather than a tinnie with an outboard, you have a cruise ship...

    The first thing you're going to have to do is look carefully at both sides of the board, trying to locate an obvious fault. This might be a component that looks melted or burnt, has a hole in it (where clearly it shouldn't), is cracked, or has a smudge of whitish looking residue.

    Search for them, then take sharp images of the locations with the suspected fault clearly visible and we can decide if that's normal or an actual fault.

    Start by looking at devices with 8 or more pins on the main board. I suspect the fault is with something fairly significant.

    When you take photo it is vitally important NOT to use flash. I recommend doing them outdoors in shade. The images need to be sharp and clear so that we can easily read the markings on the components.
     
  5. skazclaw

    skazclaw

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    Sep 5, 2013

    Thanks for the detailed response, I hope you can excuse my ignorance.

    I will check the PCB and components once again.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Ignorance is fine (I'm ignorant about a lot of things too).

    Just don't expect a simple and trivial diagnosis and fix. Sometimes the diagnosis is simple, and sometimes the fix is simple. But often one or the other requires some skill...

    We'll be glad to help if we can though.
     
  7. elebish

    elebish

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    Aug 16, 2013
    Check for an open land on the circuit board or a bad connector. Ed.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    If the LEDs are in series, as I would suspect, a single defect LED will cause the whole string to be off. compare this to christmas tree lights, where a single defect can casue the whole string to be off - unless special measures have been taken to close the circuit in case of a defect lamp or LED.

    One way of looking at this (there are many ways) is to measure the voltage across the string of LEDs (group of LEDs) If you measure no voltage, the fault is elsewhere. If you measure a voltage high enough to drive the LEDs, then the falure is with the LEDs. How much voltage you should expect depends on the type of LED and the way they are connected.
    Is one group still working? Zen you can use that group's voltage for reference.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Take a look at his video. The problem seems more complex than just some LEDs not working. It seems to be randomly flashing parts of the display at differing brightnesses.

    This is why I suggested looking at whatever is responsible for multiplexing the display (an assumption that the various "segments" are multiplexed on my part)
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You're right, the video gives a different impression than the text.
     
  11. skazclaw

    skazclaw

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    Sep 5, 2013
    The video is really bad by the way, I think the brightness difference is because of the camera.

    We looked at the whole pcb for bad soldering joints but couldn't come across anything. Near the group of LEDS that were not working I tapped the PCB and now it seems to work, but it's only a matter of time before it stops again from vibration.

    Thanks all, much appreciated.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    So you're saying that it looks different from how it appears in the video.

    sheesh.

    Given that "tapping" the board fixed the problem, I would be looking for poorly, or even unsoldered joints OR mechanical damage causing a trace to break near a pad that has become detached.

    Try resoldering all the connections (just touch them with a hot iron and maybe add a tiny bit of solder.

    The latter problem will require you look at the board under high magnification, and even then it won't be easy.
     
    Last edited: Oct 3, 2013
  13. elebish

    elebish

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    Aug 16, 2013
    If the leds are hooked together in series, you may have one led bad. If that is the case, when the leds don't lite when power is applied, read across each individual led with a voltmeter and the led that is bad will read source voltage. Verify that by shorting out that led and the rest should lite. You may also have a hairline crack in the board. The way to find that is to turn out the lights (work in the dark) and wiggle the board and look for arcing across the cracked land. Another method is to connect a piece of wire from the neg terminal of the first led in the circuit to the positive terminal of the next led and then follow that procedure for all the leds in the circuit until they all lite. The crack will be somewhere between those 2 points last jumpered. Also, follow the advice of pevious respondants. Ed.
     
    Last edited: Oct 3, 2013
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