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Basic questions about multimeter testing

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  • Basic questions about multimeter testing

    Hi Folks,

    I'm doing some testing on a pair of speakers that I built for a relative a LONG time ago that encountered some kind of catastrophic failure (guessing the power amp fried). The scanspeak woofers are definitely gone and I have already tracked down a used pair of replacements. I thought we also lost the tweeters as well but the first one is actually reading about 4.6ohms so I think may be ok. What's strange is that I seemed to be getting very reduced output from it. So this prompted me to disassemble the crossovers figuring maybe there was some issue there... these are, after all, about 20 somewhat years old.

    Pretty much all of the caps are reading high by about 10%. Not great but essentially in spec (albeit on the edge) for these axon caps -- not likely a huge issue. The resistors are all fine. I don't believe I can check the actual inductance of the inductors but I don't seem to be getting much resistance. These are solen air core inductors. All 3 of them have continuity and are reading about 0.2ish ohms of resistance but the reading doesn't seem to be that stable. For example the smallest one (0.25mh) settles at about 0 resistance. It seems highly unlikely that the inductors would fail in the first place but just hoping to make sure. I'm guessing this is a super simple question so apologies in advance!

    thank you!

    edit: nevermind on the resistance question for the inductors, I just found the spec page for them on wisconsin vendor's site and the resistance is lower than I expected. The 0.22 one is spec'd at 0.08ohms so mine would be even lower than that... probably ok. Leaving the post up just in case anyone has suggestions for other tests that I should run.

  • #2
    Build yourself an impedance test rig to use with REW or LIMP.

    Some meters have an issue with measuring very low resistances. Put a resistor in series with the inductor, maybe around 10 ohms or so, and then use the difference in measurement. This will also remove the resistance of the test leads.

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    • #3
      Originally posted by devnull View Post
      Build yourself an impedance test rig to use with REW or LIMP.

      Some meters have an issue with measuring very low resistances. Put a resistor in series with the inductor, maybe around 10 ohms or so, and then use the difference in measurement. This will also remove the resistance of the test leads.
      Great tip - had not thought of that. Thank you!

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      • #4
        Before measuring a resistance, especially one that is likely to be very low, simply short together at their tips the two test leads and note what that resistance value is, which is the leads' resistances, then subtract that value from whatever you're trying to measure. Any instability your experience with the measurement indicates you are not applying a constant pressure when measureing or the test leads have poor internal connections, like from crimped joints.
        Paul

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        • #5
          And if you are holding the leads with your two hands electrically in contact with the test leads, you are part of the circuit.

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