mervaka Posted November 27, 2007 Share Posted November 27, 2007 hey peeps, I just managed to kill one of the channels in the smaller (20m) of the two of my multicore cables last night, and after a quick look today, all three poles are shorting together. any ideas on how I can locate this fault along the cable? :/ Link to comment Share on other sites More sharing options...
J Pearce Posted November 27, 2007 Share Posted November 27, 2007 With a very expensive piece of equipment, whose name I forget, that works how far away the fault is. Link to comment Share on other sites More sharing options...
timtheenchanteruk Posted November 27, 2007 Share Posted November 27, 2007 have you checked the connectors at the ends? this is the most likely fault, failing that for all 3 to short together I would have thought it would be obviouse from some physical damage. Ps who moved the spell check?? Link to comment Share on other sites More sharing options...
cedd Posted November 27, 2007 Share Posted November 27, 2007 Time Domain Reflectometer Radar for cables! Sends a pulse down and looks for echoes. Great pieces of old kit if you can find one. I have one at work but I think Leeds is a bit far for you to come! You could try and use a telecomms tone and probe kit. Connect the tone to the line and go along the cable with the wand (probe) untl the tone stops. It MIGHT work, though a TDR is your best bet. Link to comment Share on other sites More sharing options...
Shez Posted November 27, 2007 Share Posted November 27, 2007 Cut the cable in half & test again. Cut whichever half has the fault in it in half again and test again. Keep going, cutting the faulty one in half until you have a bit that's only about an inch long, and there's your fault. Or did you mean a non-destructive test? Link to comment Share on other sites More sharing options...
Rob_Beech Posted November 27, 2007 Share Posted November 27, 2007 A bit excessive for a multicore don't you think Shez :( Link to comment Share on other sites More sharing options...
Wuddy Posted November 27, 2007 Share Posted November 27, 2007 Set your multi-meter to Ohms, measure the resistance across two of the faulted cores, divide result by two and this gives resistance to fault. From cable manufacturers data ascertian resistance per meter for cable and divide this into previous value obtained for resistance to fault. This will give the distance to the fault. Do it from both ends as a check. Link to comment Share on other sites More sharing options...
bruce Posted November 27, 2007 Share Posted November 27, 2007 Although good in theory, I doubt wuddy's method would be accurate in real life - it needs particularly good meters to measure low resistances accurately. And it assumes a "perfect short", with no resistance - you're actually measuring (2x resistance of cable + resistance of fault + resistance of connectors etc) Check the connectors. Check where the wires enter the connectors, Check the harness where the wires leave the outer jacket, and inspect the whole cable for physical damage. Then stick a meter on it, and wiggle all the bits, Look for the the meter "kicking" when you wiggle the damaged bit. And as Cedd says, a tone tester would perhaps do the job, although they're not always particularly useful with screened cable. or connect it to a high current source and look for smoke :( Link to comment Share on other sites More sharing options...
Wuddy Posted November 28, 2007 Share Posted November 28, 2007 And it assumes a "perfect short", with no resistance - There's another sort? Link to comment Share on other sites More sharing options...
mervaka Posted November 28, 2007 Author Share Posted November 28, 2007 I've already hacked the connectors off and checked them, no problems :( after a wiggle I can feel something just off the stagebox end.. I remember there being a big heavy 4x12" cab there where the cable was laying last night :'( oh dear.. its actually a thomann sssnake 8 way that I picked up the other day. with regard to TDRs, aren't most of them more used to working at longer ranges, like hundreds of meters for telecoms? the time taken for electricity to travel down less than 20 meters of cable and back again is pretty damn small! do TDRs exist which are geared up especially for audio cables? if not it should! wuddy: there might well be resistance in the fault, from a bad connection. I guess its wrong to call it a "short circuit" in those cases :D else your theory works flawlessly! I might try it anyway, and note the resistances between each pair of wires I measure. Link to comment Share on other sites More sharing options...
bruce Posted November 28, 2007 Share Posted November 28, 2007 [pedantic]ok - I guess a true "short circuit" would be zero resistance, but you know what I mean :( [/pedantic] TDRs - yes, they're ok for shorter (tens of metres) cables. The rule of thumb is to treat the trace for the first metre or so with some skepticism, but after that it's fine. For a fault near the end of cable, TDRing from the other end is often useful. You can usually see all the joins, kinks and imperfections - in fact "bending the cable over" and looking for a blip on the trace is common. Link to comment Share on other sites More sharing options...
mervaka Posted November 28, 2007 Author Share Posted November 28, 2007 cool, I'll look out for a TDR then. time to go bugging the BT engineers! Link to comment Share on other sites More sharing options...
Rob_Beech Posted November 28, 2007 Share Posted November 28, 2007 Is it tails at one end and a box at the other? Be sure to check both ends, I had a couple of these a few years ago and the cable was perfectly useable the connectors at both ends were really not up to the job. My intention was to replace the connectors, but I put them down to move my van 10 feet an came back and they'd gone. Never mind eh, saved me a job. Link to comment Share on other sites More sharing options...
mervaka Posted November 28, 2007 Author Share Posted November 28, 2007 it is indeed a tails/box job, but like I said I've disconnected the cable at both ends and checked the connectors and cable separately, its definitely the cable :D anyway, bedtime for me now I think, I'm turning nocturnal! another thankfully smaller gig tomorrow, so shouldnt need this secondary box :( Link to comment Share on other sites More sharing options...
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