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Xenon Strobe repair

Discussion in 'General Electronics Discussion' started by animateme, Jan 24, 2013.

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  1. animateme

    animateme

    6
    0
    May 27, 2011
    Hello,
    I have an old xenon strobe (disco light) that had been stored for a very long time and when I tried to turn it on recently, it no longer worked. I first assumed it was the xenon tube bulb so I replaced it but it still does not work.

    I opened it up (circuit was inside wood box) to try and measure voltage at different points to see if I could determine any current interruptions but as soon as I plugged it in, there was a point on the back of the circuit that lighted up (burning) and even some smoke came out. I immediately unplugged it.

    Searching information on this I found this topic: http://www.aaroncake.net/Circuits/strobe2.asp , and it seems like my circuit is very similar. The burning came from one of the terminals on what seems to be the 4KV Trigger Transformer (see red arrow on attached photos).

    I am no electronics expert (I do own and can operate a multimeter), and that is where I would greatly appreciate any support on this forum, is there a way that I can troubleshoot this circuit to get it working again? Any suggestions or ideas?

    Thanks on advance for any comments.

    [​IMG]
    http://www.freeimagehosting.net/3t4m3

    [​IMG]
    http://www.freeimagehosting.net/krfks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
    2,718
    Jan 21, 2010
    My first suggestion is to be careful. There will certainly be high voltages there that could give you a nasty shock, and perhaps even kill you (do I see a lead going off to a mains plug?)
     
  3. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Odd place for a burn - especially with no visible damage to the transformer. If there was a short in the windings the wire would be the first thing to light up. The point of the burn is also away from the transofrmer lead - its between that point and the copper trace to the right.

    Could the burn have been caused by arcing between those 2 points? It does appear to be at the point of shortest distance (and its also where you saw the light show).

    In any case - what steve said. That's a bad and dangerous circuit and I wouldnt mess with it. If it doesn't fry you it will burn you down.

    What you could do is salvage the parts and make a safer version using LEDs. They are just as bright (if not brighter) these days and it would make a really fun DIY project since its pretty easy to make :)
     
    Last edited: Jan 25, 2013
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
    2,718
    Jan 21, 2010
    I'd say it's a certainty.

    The trigger pulse may be a couple of kV. It can jump some distance. Clearly it has in this case, and the conductive path has sustained a discharge which has heated then charred the board.

    I would replace the trace with a flying lead.

    edit: it's also possible that when this path became highly conductive that the discharge current was high enough to fry the trigger transformer.
     
  5. animateme

    animateme

    6
    0
    May 27, 2011
    --- Thanks for the warning. I was not aware of this risk. Yes, there is a cable going to a mains plug.
     
  6. animateme

    animateme

    6
    0
    May 27, 2011
    --- It does seem like there was arcing as the burn is right between the 2 points. However, the strange thing is that this circuit had been working before without any problems so why would it suddenly burn like this at this point? ...would the fact that I plugged it in without the Xenon Tube Bulb in place have anything to do? That's the only difference in circumstances that I can think of that was present when this occurred. I had never tried plugging the circuit in without the Bulb. ---???
    Thanks for the suggestion on the LED version.
     
  7. animateme

    animateme

    6
    0
    May 27, 2011
    --- Thanks for the response. Again, why would this arcing happen suddenly if the circuit worked fine before?

    Can you please explain what you mean by "replace the trace with a flying lead" (I am not familiar with the terms)
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
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    Jan 21, 2010
    DO NOT try to measure voltages or do anything that involves getting fingers near the board while it is plugged in (even if it is turned off)

    I'm not sure if the capacitors will self-discharge, and it's safe to assume they won't. Even if the unit is unplugged, it may remain very bitey for several hours.

    Use a multimeter to measure the voltage across the big capacitor. Make sure it's on a 1000V voltage range (or similar) and make the measurement while holding both probes in the one hand (if you slip, you'll get a shock through your fingers that may be very uncomfortable. A shock across both arms has the potential to make your relatives very unhappy)

    Once the voltage gets below about 50 volts, you're probably OK to work on the board with extreme care. I would wait until the voltage is under 12V, preferably closer to zero. There is still energy stored that can surprise you.

    Look for threads on safely discharging capacitors if the capacitors are not discharging fairly rapidly by themselves.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
    2,718
    Jan 21, 2010
    It takes time to damage the board so much it's conductive. Maybe it started doing this from new, maybe a a particularly humid day started it. The arcing itself probably would not stop the circuit operating.

    The copper trace on the printed circuit board is too close to other conductors. I would physically disconnect the wires from it, and leave them on the top side of the board. Then I would connect some very well insulated wire between these two points, ensuring it does not pass too close to anything. Some heatshrink over the soldered joints would probably be ineffective at insulation, but I's do it anyway.
     
  10. animateme

    animateme

    6
    0
    May 27, 2011
    Hello again,

    I scraped away the carbon area between the lines and even put some nail polish on it (I read somewhere that this was a good insulator - ???). I placed the xenon tube back in and powered up. The arcing burn is no longer there! ...however, no strobe flash.

    So, I got rid of that issue, now I just need to get the circuit to flash that tube again.

    I read somewhere that the electrolytics could be dead due to being inactively stored for such a long time, correct? If this is the case, would you recommend that I try replacing these? I did try measuring the voltage at the capacitors, as recommended, just right after unplugging the circuit from power, but I got a zero reading.

    The current electrolytic caps on the circuit read: 22 uF, 250 WVDC

    As replacement for the electrolytic capacitors, I found these online from a local store:
    [​IMG]
    Electrolytic Capacitors
    The specs are:
    - Radial
    - Aluminum
    - 22 uF, 250 Volts
    - 13 x 29 mm.

    I assume that these will do the job, right? Or, is there anything else that I should consider when looking for the replacement?

    Any other suggestions to troubleshoot the circuit before replacing parts?

    Thanks!
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
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    Jan 21, 2010
    I doubt it's the capacitor. And those capacitors may work, but they're not designed for huge currents, so they may not work for long.

    As I said above, I suspect that the trigger transformer has been destroyed.

    See if there is a path between the trigger output and either/both the other terminals of the trigger transformer.
     
  12. animateme

    animateme

    6
    0
    May 27, 2011
    Thank you Steve.
    Can you please detail how it is that I should check if there is a path between the trigger output and either/both the other terminals of the trigger transformer as you say? Is this using the multimeter?

    First of all, this trigger transformer is the one marked with the arrow, correct?
    [​IMG]
    http://www.freeimagehosting.net/3t4m3

    If this is the trigger transformer, I see that it has 4 terminals. How can I tell which terminals I should be checking for the path? Can I check this without the need to remove the trigger transformer from the board?

    Thank you.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,270
    2,718
    Jan 21, 2010
    Yes, that's the trigger transformer.

    Typically trigger transformers have 3 terminals (2 of the windings are connected internally, but 4 connections is also reasonable.

    You may be able to see that one winding is made up of thicker wire than the other. This is the primary winding and it is almost certainly intact.

    The other winding has many more turns and is the secondary. One of those will be connected to the lead which goes to the trigger lead of the flash tube. The other end is most likely connected to one of the connections of the primary.

    Since the trigger lead has no low voltage DC connection to anything other than the transformer, the easiest test is to get a multimeter on a x1000 (or 20k) range and read the resistance between the trigger lead and all others.

    You should get a resistance between the trigger output and at least one other lead (possibly all three others) if the transformer is not open.

    See what you get an report back.

    This doesn't guarantee the transformer is OK, but it rules out one major fault in it.
     
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