prolightdesigner Posted February 2, 2009 Share Posted February 2, 2009 Hi all, I'm not sure if this is the proper place to post this, but here it goes. I've been making a batch of XLR cable intended for use with DMX. After making my new cables I tested each one with a multimeter and detected that one of them has a short somewhere in it. I removed the XLR connectors from each end to re-test and the cable still shorts (red shorts to ground). Unfortunately, the faulty cable is 30m long, and I was wondering if anyone knows of a method of finding internal faults in a cable without chopping it up. The exterior of the cable looks undamaged, if that helps. Thanks,-Jeff Link to comment Share on other sites More sharing options...
Tom Baldwin Posted February 2, 2009 Share Posted February 2, 2009 You need a Time Domain Reflectometer (TDR)This will get you very close to the location of the fault(s). They're expensive, though (many, many, many) times the value of the cable, so unless you have a handy contact at an engineering firm or university, chopping the cable is probably the cheaper option. Is it a dead short, or is there some resistance? If there's several ohms of resistance, you could try driving an audio signal down the cable, and using a sensitive coil to trace the signal as far as the short, perhaps. A bit hit and miss, but potentially cheaper than a TDR. Link to comment Share on other sites More sharing options...
HobitLight Posted February 2, 2009 Share Posted February 2, 2009 I'm not sure about DMX, but for XLR (sound) I've been taught to plug it into a sound desk and shake the cable all the way along. If you hear a crackle when shaking it, that's where the fault is. I'm not sure if this would apply to DMX cable but the same principles may apply. :** laughs out loud **: Link to comment Share on other sites More sharing options...
Just Some Bloke Posted February 2, 2009 Share Posted February 2, 2009 Well the old fashioned way is to cut the cable in half and test each half then keep doing the same with the dodgy half until the cable gets too short at which point you throw away the bad bit. You end up with a 15m cable, a 7.5m cable and a 3.75m cable, but at least they all work! Still, you proabbly knew that! :** laughs out loud **: Link to comment Share on other sites More sharing options...
daifuse Posted February 2, 2009 Share Posted February 2, 2009 Telephone engineers use a form of Wheatstone bridge and measure the resistance (v.low) to the short cct. from each end. :** laughs out loud **: Dave Link to comment Share on other sites More sharing options...
prolightdesigner Posted February 2, 2009 Author Share Posted February 2, 2009 Is it a dead short, or is there some resistance? If there's several ohms of resistance, you could try driving an audio signal down the cable, and using a sensitive coil to trace the signal as far as the short, perhaps. A bit hit and miss, but potentially cheaper than a TDR. Yes, it seems to be a dead short, but maybe some shaking can produce some activity. I'm not sure about DMX, but for XLR (sound) I've been taught to plug it into a sound desk and shake the cable all the way along. If you hear a crackle when shaking it, that's where the fault is. I'm not sure if this would apply to DMX cable but the same principles may apply. :** laughs out loud **: That might work, so I'll start with that. I'd do it with a mic and a mixer before I do that with a console and one of my movers! I know that there are some differences between mic cables (XLR) and dedicated lighting cables, but I'm trying not to start a "DMX is a protocol, XLR is a cable" debate :** laughs out loud **: . Thanks for the advice!-Jeff Link to comment Share on other sites More sharing options...
paulears Posted February 2, 2009 Share Posted February 2, 2009 Is the short between core and core, or core and screen? I'd also do an audio test - and the other thing to do is to squeeze it all along its length - sometimes you will be able to feel a difference in the area where it has failed. The other thing to consider is that cables rarely short along their length without mechanical damage. A short on a decent piece of cable means that they could have failed by overheating -maybe the cable was carrying DC at some point and the conductors overheated? I've never seen it in practice, but I did use a length of mic cable to power a transmitter once, and the cable failed - one core melted and the conductors parted - was a real mess inside, but a short could have been possible? Link to comment Share on other sites More sharing options...
grizzly Posted February 2, 2009 Share Posted February 2, 2009 ... "DMX is a protocol, XLR is a cable" debate :** laughs out loud **: . XLR is a connector, not a cable :** laughs out loud **: Link to comment Share on other sites More sharing options...
Biskit Posted February 2, 2009 Share Posted February 2, 2009 Telephone engineers use a form of Wheatstone bridge and measure the resistance (v.low) to the short cct. from each end. :** laughs out loud **: Dave Hmm I actually have a genuine ex-GPO Wheatstone Bridge in a lovely french-polished box. Unfortunately I need a decent galvanometer to go with it. Incredibly accurate piece of kit though when I did have it working... as long as the calibration still holds (can't see how it could have altered much even in 50 years). I'd forgotten all about the thing... might have to dig it out for a play if I'm ever bored... Ben. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.