Is there a trick to speakers, or did I just get a dud?

Discussion in 'General Electronics Chat' started by s_mack, Oct 20, 2015.

  1. s_mack

    Thread Starter Member

    Dec 17, 2011
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    Sorry, I have near-zero experience with speakers other than listening to them.

    I bought a "thingy". It came with a little 8ohm speaker. In fact, I found the exact same one here: http://www.electrodragon.com/product/0-5w-8-ohm-mini-speaker/#prettyPhoto

    Here's a picture

    [​IMG]

    Mine came with a twisted pair pre-soldered to the blobs of solder south of the green tape(?). One of the blobs was lifted so bare PCB was under. As far as I can tell, the entire slab of metal to the right/left of the green tape is physically the same entity as where the blobs were soldered to. Is that incorrect? My meter sure thinks it is! I removed the wires and soldered them solidly to those slabs but no sound comes out of my "thingy".

    OK, so maybe "thingy" is busted, right? I took my PC speakers and grabbed some aligator clips and attached it to the the 3.5mm plug and hooked it up and it worked fine, ruling out bad "thingy". So I'm back to the speaker. Is the speaker junked or am I mistaking how to solder it somehow?

    One thing... before doing my "fix" my meter indicated that there was an intermittent short between - and + due to the lifted blob flapping around. So it is more than possible that it shorted while it was in "use". Does shorting a speaker like that permanently break it?

    I never heard even a slight pop out of it.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Put your meter on Ohms and measure the resistance of the speaker coil, it should be about 8 ohms, if its open circuit then its damaged.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If the coil measures ok, then glue the flapping part to something non-conductive and solid.
     
  4. s_mack

    Thread Starter Member

    Dec 17, 2011
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    By "coil" are we just talking between + and -?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    yes, measure the resistance by putting one lead of your meter on one blob of solder, and the other lead of your meter on the other silver blob of solder.
     
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  6. MrSoftware

    Active Member

    Oct 29, 2013
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    A speaker is pretty simple, electrically it's just a coil. Those two solder balls represent the ends of the wire that the coil is wound with. If all is well, you should get in the ballpark of 8ohm resistance across those two solder balls. If you get an open circuit then the wire is broken somewhere. If you get a short then the wires might be touching the metal speaker frame, or each other, somewhere that they shouldn't be.

    That speaker is so small that repairing anything beyond the pads where you connect the wires is probably not practical.
     
  7. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    Open circuit. OK, thanks!

    I have 10 more on the way by slow boat from China. Hopefully one of them works.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    you can also look for the thin wires coming from the back of the paper cone - these should have been soldered to the little section of the PCB. If it is broken off of the PCB, you can try to re-attach the broken wire to the PCB or to a thicker wire. It should work fine once repaired.
     
  9. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
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    It is mylar, rather than paper... only way I can see to check that is to cut the mylar. I'd assume that would destroy the speaker?
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The speaker wires will be glued to the black paper diagram with flying leads from the blobs of solder, it is possible to repair these leads, but it needs a gentle hand and soldering iron, otherwise bin it!
     
    Last edited: Oct 20, 2015
  11. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    While you are waiting, keep in mind that you won't get much (quality or volume) sound out of it until it is installed in some kind of enclosure.
     
  12. s_mack

    Thread Starter Member

    Dec 17, 2011
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    At $0.44 each... I'm not too terribly concerned about it so I cut the mylar off just to see what you were talking about. But by the time I located the wires, I severed them lol. Now I know what to look for next time :)
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    8 Ohms is the AC impedance - the DC resistance will be somewhat lower.

    But some cheap meters can't resolve the difference anyway - a *REALLY* cheap meter might not even notice any resistance.
     
  14. s_mack

    Thread Starter Member

    Dec 17, 2011
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    Mine isn't a Fluke, but it isn't a $5 special from the Dollar Store either.
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    In days of old - 3 Ohm speakers were common, 4 Ohm speakers aren't exactly rare - some of the "not too bad" meters have trouble with those.

    Unless you're rich, a DMM probably doesn't have a zero Ohms adjust - then you can't null out the resistance of the test leads, that can amount to some fraction of an Ohm.

    Unless you've spent laboratory instrument prices, speaker resistances are approaching the margin of diminishing accuracy.

    Most people get by - but you have to bear these things in mind.
     
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Thank You!
    Now I can tell my mom that I'm rich. I have proof, I own a DMM with zero Ohm adjust!
     
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  17. Cjuried

    New Member

    Aug 31, 2015
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    Using a common DMM set to resistance will get you close. Use a LCR Bridge/Impedance meter, if you wish to document the "true" measurement of the speakers Impedance, to include inductive and capacitive reactance.
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    The traditional way to measure low resistances, is to pass a calibrated constant current through them and measure the volt drop.

    With speakers, you need pretty low current to avoid slamming the cone, but with careful design you can get it in the same ball park as the test current from a DMM Ohms range.
     
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