open wire on a cable

Discussion in 'General Electronics Chat' started by jdmech, Jan 26, 2010.

  1. jdmech

    Thread Starter New Member

    Jan 21, 2010
    I have a 50 ft cable with 15 pins from end to end. I have a break in one of the wires inside of the cable. Is there a way to pinpoint a break? can i use this sensitive audio detector to pin point a break? not a short.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. jdmech

    Thread Starter New Member

    Jan 21, 2010
    Hey thanks for the reply, and that sure looks like an expensive piece of tester, anyway, this cable goes on a forklift, from the cab to the forks. It controls the forks. I did the pin to pin and know that its bad and will be replaced. But its a simple repair if I can pinpiont the break.
  4. boriz

    New Member

    Jul 16, 2009
    First thing I do (and it has worked for me in the past) is waggle each inch of wire to see if it effects the signal.

    Failing that, something like this might work:

    Send an audio signal (say 1KHz) down each wire in turn, with one pole of the signal connected, through a long wire, to the Source of a MOSFET. Use a short wire, couple of inches, connected to the MOSFET Gate wrapped loosely around the cable you are testing and slide this loop along the cable. The audio signal might need to be voltage amplified first, maybe not. But even a tiny variation in the electric field around the Gate wire should be seen as varying current on the MOSFET Drain. Feed this Drain signal into an audio amplifier and you should be able to hear the 1KHz signal, but only where it’s getting through the conductor. Maybe.

    Some Biasing of the MOSFET should increase sensitivity.
  5. jdmech

    Thread Starter New Member

    Jan 21, 2010
    Hey boriz , thanks for the reply, well I did the wiggle and waggle thing and that didn't help but what your describing sounds promissing but maybe a diagram would held coz you lost me at MOSFET, but I already butchered this cable it took about 5 small cut and one long one. Military couldn't wait and the part is a long way from here, so maybe for future can you show me what your talking about?
  6. Bernard


    Aug 7, 2008
    Cable breaks in long multi pair cables is a real time & $ waster. Our cables , 1320 ft long, 13 pair, had plugs on each end. I used a rotary switch to isolate the suspected pair, While grounding all others. Two steps, first used a capasitance bridge to pinpoint break to within about 25 ft, then used a signal on broken wire,1k Hz maybe, and a high gain audio amplifier with phones, for the last inch. The signal would pulsate while running down the cable untill passing the break when the signal would become steady & maybe reduced volome. Still use amplifier for finding bad Christmas tree lights [in flag in Am Legion].
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    A telephone engineers type pair tracer can be used for this, basically what Bernard describes above.
    It's just an audio oscillator or tone pulse generator, plus a sensitive amplifier probe.

    Connect the tone generator to one end of the broken wire and one good wire. connect all the others together to minimise capacitive cross coupling.

    The probe should not pick up much leakage before the break, as the signal is balanced within the cable. Past the break, it should be much stronger as it's now unbalanced.

    Using the pulse type tone sets (aka 'Knocker'), the setup is so sensitive you can hear the signal build and null along each twist of a wire pair wthin a 50- or 100 pair cable.
  8. someonesdad

    Senior Member

    Jul 7, 2009
    You can do TDR with an oscilloscope and pulse generator. Or, a suitable generator can be made from one IC. I've used it to look at LAN cable.
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008

    You can BUILD a TDR tester with a single 555 timer chip and a differentiator! Just add one el-cheapo oscilloscope. Voila.

  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Capacitance meter, one probe on the ground wire or shield. That cable probably has 25pF per foot or so, so it wil be very obvious when one wire reads much lower capacitance than the other wires...