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    Default Cable tester

    Just wondering if anyone's seen a circuit for a simple cable tester that finds where a break is in the cable ?
    Would be very handy for like guitar leads. Save chopping until you find the fault only to end up with a good cable 6" long



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    Just realized that prob wouldn't work because the cable is shielded.
    Damn !

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    it is possible to make one if you got the time and expertise , you need a frequency generator and swr bridge and a directional coupler with a resistance comparator and a good calculater most of the gootar leads are about 75 ohms or there abouts so sending a signal down a cable to a open circuit means that the resistance matches the 1/2 wave length of that appears as a short /or open so calculate the frequency (300 over the frequency in mhz =wavelength in meters )will give you the distance to the open so you can make it if you want to , the resistance stack is used to get the cable impedeance using the directional coupler , or you can just chop the cable into little pieces lol don

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    Quote Originally Posted by hinekadon View Post
    it is possible to make one if you got the time and expertise , you need a frequency generator and swr bridge and a directional coupler with a resistance comparator and a good calculater most of the gootar leads are about 75 ohms or there abouts so sending a signal down a cable to a open circuit means that the resistance matches the 1/2 wave length of that appears as a short /or open so calculate the frequency (300 over the frequency in mhz =wavelength in meters )will give you the distance to the open so you can make it if you want to , the resistance stack is used to get the cable impedeance using the directional coupler , or you can just chop the cable into little pieces lol don
    Could you say that again .... ha ha ha

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    or the anritzu sitemaster hahaha

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    Quote Originally Posted by hinekadon View Post
    or the anritzu sitemaster hahaha
    No thanks, my limit is $2 lol
    Damn, I've taken a metre off this cable already.

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    Default dumbguy

    there is one way thats easier short out to other end connect your meter to the other end an wobble the cable until you get a flick on the meter but only move 2"" of cable at a time hows that ? then chop it oops will happen as murphy sits in the background giggling at you regards don
    there is another way that i have used on industrial cables beware !!!!!
    i used a old neon lighting transformer connected to a variac these produce up to 12kv ac so connect the centers ot each side and sloooowly build up volts the cable will go bang in the area of the fault and make the smoke come out its fun but a little dangerous works every time tho cheers don
    Last edited by hinekadon; 03-09-18 at 08:57 PM. Reason: more

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    Quote Originally Posted by loopyloo View Post
    No thanks, my limit is $2 lol
    Damn, I've taken a metre off this cable already.
    Then start on the other side.
    Typically, and more commonly guitar leads break near the plugs. So I first grab the inner conductor with fine needle pliers (or what ever you have lying around) and try to pull carefully the copper out. If the break is less than 10-15 cm away it should come out fairly easily. If not then chop chop.... and at some stage you will just have to make a new one with a few bucks worth of cable and re-using the plugs.
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    Quote Originally Posted by hinekadon View Post
    there is one way thats easier short out to other end connect your meter to the other end an wobble the cable until you get a flick on the meter but only move 2"" of cable at a time hows that ? then chop it oops will happen as murphy sits in the background giggling at you regards don
    there is another way that i have used on industrial cables beware !!!!!
    i used a old neon lighting transformer connected to a variac these produce up to 12kv ac so connect the centers ot each side and sloooowly build up volts the cable will go bang in the area of the fault and make the smoke come out its fun but a little dangerous works every time tho cheers don
    Yeah your first method works ok but only if the break can be made to touch while wriggling it around.
    Don't think I'd like to use your second method on an audio cable lol

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    I just remembered one of my meters is this one :

    The little thing sticking out on the top of the meter will pick up ac volts live, like near field.
    I just checked it with a 15vac power pack and it senses fine, even senses a single wire stuck into the end of the plug, so it will find where a break is....but no good for the guitar lead cos of the shield.

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    My method for microphone cables is to connect them to a mixer and turn on phantom power.
    Monitor the channel via headphones.
    Work your way along the cable and the faulty area will give loud crackles.
    Guitar Leads are a little harder.

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    you need to build yourself a TDR. Actually, probably not such difficult thing these days.

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    My cheats way is to use a pin, one buzzer lead to the tip of one plug and the other to the pin. Push it through the insulation into the inner (sheilded) wire. If you have continuity there, move along a bit, and try again. Seriously, unless you are repairing high end cables, it would be less stressful, just to buy a new lead. Typically, if a cable fails in service, there's a good chance it's going to fail again, unless, of course the plugs are stuffed, that, then is a) obvious and b) an easy fix. I've given up wasting my time repairing guitar leads over the years, If it's noisy, it's history!
    I'm out of my mind, but feel free to leave a message...

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    Quote Originally Posted by nomeat View Post
    Then start on the other side.
    Typically, and more commonly guitar leads break near the plugs. So I first grab the inner conductor with fine needle pliers (or what ever you have lying around) and try to pull carefully the copper out. If the break is less than 10-15 cm away it should come out fairly easily. If not then chop chop.... and at some stage you will just have to make a new one with a few bucks worth of cable and re-using the plugs.
    Um that was 1/2 metre off each end.

    Yes , some of my cables are high end.
    If the break is near one end, that works out ok.
    Without any way of testing, the only way to fix them is by elimination, half by half lessens the impact.
    In other words, cut the cable in half, and you will end up with one good cable, but it had better be at least 10 meters long to begin with.

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    I have a cable tester that injects a signal onto the conductor to be traced with a separate receiver. It's a proximity receiver so contact is not required except as the final confirmation that you've found the right conductor.
    IE inject signal on 1 core of a multicore cable, that cable can be traced for a considerable distance and the particular conductor confirmed at the remote end but not sure it would work so well on on the core in a well shielded audio (coaxial) cable.

    It's intended for testing ethernet type cables.

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    have used this method but its very limited on coax sometimes 6/7 meters don

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    Very interesting.
    I found these two articles on TDR's.



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    Default time delay reflectometer

    Quote Originally Posted by hinekadon View Post
    it is possible to make one if you got the time and expertise , you need a frequency generator and swr bridge and a directional coupler with a resistance comparator and a good calculater most of the gootar leads are about 75 ohms or there abouts so sending a signal down a cable to a open circuit means that the resistance matches the 1/2 wave length of that appears as a short /or open so calculate the frequency (300 over the frequency in mhz =wavelength in meters )will give you the distance to the open so you can make it if you want to , the resistance stack is used to get the cable impedeance using the directional coupler , or you can just chop the cable into little pieces lol don
    this is a tdr ???? not a new idea but not very accurate unles you use a resistance comparator to get the impedence of the cable so whats new in the post

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    Yep, what you want is called a TDR. (but I can see you've already discovered that).
    You can buy them surplus for as cheap as $20. That's how much I've paid for each of mine.
    I do often see them at hamfests. Most people completely overlook them and have no idea what they are or how to use them.
    I'll buy them if they're cheap enough. There are a couple of ex PMG/Telecom/Telstra untis getting around. They have a simple common fault.
    So a lot of units you see for sale aren't working, but aren't hard to fix if you know the issue.

    You can of course just make a TDR. They are really easy to make.
    hinekadon's description is overly and unnecessarily complex. Bob is correct, they're not hard to make.

    The simplest is a relay with an oscilloscope.

    The relay is set up as a multi-vibrator so it just buzzes. DC is fed directly to the cable or coax via a T piece sending pulses down the cable while looking for the echo's of those pulses.
    A 555 time or proper crystal based pulser would of course be a lot better. I've got an old lab single generator for such jobs. I just dial up the pulse width and duration I want.
    Set the trigger on the CRO to the falling edge of the DC pulse and wait for the echo. The impedance of the cable is completely irrelevant.
    If you know the velocity factor of the cable, then you can calculate the distance to the fault. Else you know the time base on the scope. If the DTF is known,then the velocity factory is calculated.


    With respect to my real TDR units. They're incredibly versatile units. They were designed to measure phone lines. But they as easily measure coax, wire fences, pipes, and pretty much anything that will conduct electricity. I can measure anything from about 1m long out to about 20km.
    I can see joints in the cable and even bends. I can see if a fault is open circuit or short circuit and how well the impedance is matched at any point along the cable regardless of what the impedance actually is.
    Last edited by trash; 08-09-18 at 04:37 PM.
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    We used to use them for finding faults in coax and wire pairs in aircraft. Work better on coax though. And it sure beat pulling off panels and checking at each connection, to find the damaged segment. The other thing is you can check the quality of cable connections pretty accurately, once you learn to read it properly.

    What is nice is optical TDRs. We played about with them a bit at tech, when I did the fiber termination course.

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