Cable testing with multimeter...how do you do it?

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Barnzy, Feb 19, 2012.

  1. Barnzy

    Barnzy Member

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    Sorry for what may be a silly question, but can someone kindly explain how to test a cable using a multimeter? Or can someone refer me to an article that explains this process. I'm pretty weak in electrical theory....
    Barnzy
     
  2. fumbler

    fumbler Member

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    Set it to read resistance (Ω's). Start with the highest Ω setting on your meter. With the probes not touching anything it should read infinite. Touch the probes to each other and it should read zero.

    Touch the probes to the tips of the two ends of the cable. Should read zero. Otherwise the hot signal conductor in the center of your cable is broken (or there's a bad solder joint to it in one of the plugs).

    Touch the probes to the shafts of the two ends of the cable. Should read zero. Otherwise the braided shield layer in your cable is broken (or there's a bad solder joint to it in one of the plugs).

    Touch the probes to one tip and one shaft. Should read infinite. Otherwise you have a short between the conductor and shield in your cable (or a solder joint that is shorting them in one of the plugs.)


    Over the years, I've had MANY cables die because of broken conductors (or more likely broken insulator sheath that causes a short from shield to conductor) within a foot of the plug. You can tell when it intermittently cuts out when you wiggle it right at the broken spot. I just cut 'em off and solder on a new plug. So now I have a bunch of working 19 foot cables. I almost never see a solder joint in a plug go bad, though.
     
    Last edited: Feb 19, 2012
  3. Barnzy

    Barnzy Member

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    Thanks for taking the time Fumbler! I really appreciate the lesson. This is why TGP gets a good name!
    Barnzy
     
  4. walterw

    walterw Gold Supporting Member

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    +1, but i see plenty of center conductors get broken right at the solder joint to the plug.

    the other quick test is to plug the cable into a hot amp, hold the free end without touching the plug body, touch your finger to the tip (inducing all kinds of racket) then wiggle and shake the cable. you might hear a little extra sound ("handling noise"), but the primary buzzing noise shouldn't cut in and out while you're doing that. this confirms that the center conductor is good throughout the cable.

    then, grab the plug body at the same time (so your fingers touch both the tip and the sleeve); it should get way quieter, as you're now partially shorting the hot and ground together; shake the cable again to listen for any significant noise. this confirms that the shield is good throughout the cable.
     
  5. Barnzy

    Barnzy Member

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    Thank you also Walter...now I'm ready to be a "cable repair guy."
    Cheers,
    Barnzy
     
  6. m-m-m

    m-m-m Member

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    wow - awesome info here. I've never attempted to check a cord with a multimeter, but now that I see it all mapped out it makes perfect sense.
     
  7. nrandall85

    nrandall85 Supporting Member

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    I like this way better than the multimeter. Unless you're good at using chopsticks, it's hard to perform a "wiggle test" while holding multimeter probes.
     
  8. Sensible Musician

    Sensible Musician Member

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    a proper cable tester does everything in fumbler's and walter's methods. a good one is less than $50. the problem with just meter is that generally the cable is going to be static on your bench. the problem with aural testing is that it doesn't catch resistance between hot and ground. a proper tester catches all. i use the cheap behringer one, which is great.

    you can still measure everything with a meter and check for mechanical problems if you simply use clips on your meter and rig up e.g. a pickguard scrap with two jacks. i wouldn't put a cable into service that hadn't been tested electronically and mechanically
     
  9. walterw

    walterw Gold Supporting Member

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    that's not quite true; any loss from hot to ground will be heard as a dulling and quieting of the noise.

    actual cable testers are still absolutely a good idea.
     
  10. Rockledge

    Rockledge Member

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    Anymore most meters have a tone that sounds to show continuity. It is a sole position on the dial. When you touch the leads together you get a tone. The same when you touch tip to tip , ring to ring, sleeve to sleeve. If you touch the tip or sleeve on both ends of the cable at the same time and don't hear a tone, the cable is open somewhere.
     
  11. walterw

    walterw Gold Supporting Member

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    sensible's point was that it's hard to hold the meter leads on the ends to get that continuity test tone and then shake the whole cable.
     
  12. Rockledge

    Rockledge Member

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    That's what God made gator clips for.
     
  13. Barnzy

    Barnzy Member

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    In true TGP fashion, I need to ask...who makes a quality cable tester? (...features and functions considered.) This is a tool that I would appreciate owning....
    Barnzy
     
  14. GtrDr

    GtrDr Member

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    Use that meter to check a cabinets resistance before you plug it into a head. This also checks the speaker cable. I have grabbed cabs that said 8 ohms on the side only to find it was rewired (Or speaker replaced) & is now a 4 ohm cab. I keep a multi meter on the top of an amp, reminds me to double check. Makes for happy amps:)
     
  15. GtrDr

    GtrDr Member

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    Just get an ohm meter. Most have a setting that gives you an audible continuity conformation. Check the ground, check the center conductor, and finally attach to the ground on one side & the center conductor on the other side to make sure you don't have a short.
    Everyone should have one, you don't need anything expensive. Learn how to make sure your gear is grounded so you don't get zapped when you touch a mic stand. I also use it to track down a hum or noise. All the info is on the net. Could save you a trip to the shop.
     

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