How to find open and shorts on long mic XLR cable?

Discussion in 'General Electronics Chat' started by Cal Rico, Dec 19, 2014.

  1. Cal Rico

    Thread Starter New Member

    May 1, 2012
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    I need to find the location of open and shorts on Long stage Mic cables XLR. I have noted the DIY TDR tester. only if requires a scope and this I am doing on location. Isn't there a way to use a LCR meter, or the capacitance function on a DMM? If I already know both capacitance and resistance per meter, can't I somehow calculate the distance?
     
  2. RichardO

    Well-Known Member

    May 4, 2013
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    I like the capacitance idea. I have never tried it but it sounds like it might work. It will, of course, only work on an open cable. A shorted cable will act like a shorted cap -- no capacitance. You may find that the capacitance varies a lot from cable to cable making the measurements too inaccurate to be useful.

    I don't think that resistance will tell you the length of a shorted cable since you have no way of knowing the resistance of the short.
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    We made a test rig that used a balanced bridge configuration. The individual wires of the cable made up two legs of the bridge elements, a control making up the other two allowed balancing. Moving along the length of the cable and flexing it would result in the bridge becoming unbalanced whenever a bad spot was flexed.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    If you can generate a 1 MHz oscillator, connect to one pin of the cable. Then use an old fashioned AM radio and walk along until the signal (tuned to 1MHz) disappears.

    Repeat with each pin.

    Since you are doing one pin at a time, and not grounding the sheath, it will hopefully work. No promises.
     
  5. #12

    Expert

    Nov 30, 2010
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  6. ian field

    Distinguished Member

    Oct 27, 2012
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    With coaxial cable of known termination impedance, you can measure the exact distance to a short or open with Time Domain Reflectometry - I've no idea what, if any, other cable types it can be used on.
     
  7. RichardO

    Well-Known Member

    May 4, 2013
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    TDR also works with twisted pairs -- just a different impedance.

    I have some thoughts how to do a crude TDR with some very fast voltage comparators, a fast ramp generator, a set/reset flip flop, a sample and hold, and a panel meter and _no_ microcontroller. I have not fleshed it out yet...

    Or maybe something like this would work:
    http://www.testequipmentdepot.com/p...TT&ref=gbase&gclid=CLTo55PV1cICFcayMgodSR0AmA
     
  8. RichardO

    Well-Known Member

    May 4, 2013
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    I am sure that there are simpler and more clever ideas on the Web. But...
    Here is my simulation of a TDR circuit. It may or may not work in the real world. ;)

    TDR_analog.png
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    A really narrow pulse is a good starting point - Zetex (was Ferranti) produced the ZTX415 avalanche transistor, and various appnotes.

    The 2N2369 is also a good choice - look for Jim Williams appnotes, some of them deal with TDR and related subjects.
     
  10. RichardO

    Well-Known Member

    May 4, 2013
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    I think that the circuit simulation I supplied is barely fast enough to do the job. Since I have not built it, and don't intend to, I can't be sure.

    I have built avalanche pulsers. A friend tested one on his very fast scope as having a falltime of about 350 ps. This is questionable since the attenuator used may (probably) is having a peaking effect, making the transition appear faster than it really is.

    I first built the prototype for this pulser on a solderless breadboard using a surface mount 2N2369 transistor soldered to a SIP plug. I got a reasonably fast falltime -- on the order of 1 to 2 ns. Not too bad...

    I then built the same circuit onto a BNC connector keeping the stray inductances and capaitances small. To my surprise, the falltime was _slower_ than the breadboard! I changed every part in the circuit including the cap and transistor with no improvement. At this point I could not see what was different.

    Then... When I was looking for another part in my inventory, I happened on some 2N2369's. I then remembered that the first prototype had been built from that batch. When I put one of these in the circuit built on the BNC, it was as fast as the original circuit. I then took some of the "new" batch of 2N2369's and plugged them into the breadboard and they were all slow.

    There was 100% consistency -- old batch fast, new batch slow. The 2N2369's were made by different manufactures which may explain some of the difference. Of course, since the avalanche characteristics are not specified for the 2N2369 I have nothing to complain about. Jim Williams says that he had to hand select his transistors to get the fastest ones.
     
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