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How to find the location of a broken wire in a wrapped cable

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#1 gzsfrk OFFLINE  

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Posted Fri Sep 28, 2012 2:48 PM

So here's a situation I found myself in last week. I was enjoying a nice game of Moon Patrol on my newly-acquired Atari 5200 when the 1, 2, 3, and Start buttons all decided to stop responding. At first it was intemittent, depending on the angle at which I had the controller held, but then it got to the point where it wouldn't work at all.

So I do some quick research, find that the 1, 2, 3, and Start buttons on the 5200 controller are all tied to Pin 7. I open up the controller, find the red/white wire for pin 7 and do an end-to-end continuity check. Nothing. All the other wires were fine, though. So I start incrementally checking the red/white wire as far as it's exposed leading up to the cable's collar, and it's whole. Unwillingly, I carefully trimmed off the collar and continued checking the wire, hoping to find the break very close to that end. Still nothing--I've got continuity to over 2 inches past the collar. Crap. So then I start checking continuity at the plug end. By that point, it's getting messy; I've already made several surgical incisions to expose the red/white wire for testing, and I STILL hadn't found the break. FINALLY, I go back to the controller end of the cable, cut back the wrap another 3 inches and THEN I find the break in the wire--some 5 inches down from the collar. What rotten luck, even by 5200 controller standards. So I'm able to patch it back up and get it working again, although by the time I had to tape all the cuts back closed, it truly looks like a ghetto controller.

But the whole situation got me thinking--was there an easier way to find the break without having to make all the exploratory incisions in the cable? I know that there are wire break detectors commonly used by owners of buried invisible fence lines which use something like an AM radio signal using the suspect wire as an antenna and a receiver to figure out the approximate location of a the break. But is there anything that works at a much smaller (and therefore more precise) scale? What I'm thinking is a device that you hook to one end of the wire and emits a simple square wave over a radio frequency, and then have a receiver which picks up the tone as you move it along the cable (presumably, the tone would change once you pass the break). But I have no idea if that would work on the scale of locating a break within an inch or so.

Has anyone run across a similar problem for which they've found a solution? It's pretty much a moot point for me now since the damage is already done. But I'm intrigued by the challenge of coming up with a more elegant way of addressing the problem should I run across it again in the future.

Edited by gzsfrk, Fri Sep 28, 2012 2:50 PM.


#2 edintv OFFLINE  

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Posted Fri Sep 28, 2012 3:02 PM

There are network analizers that can detect breaking in network cables, but normally are for very long cables that cost a lot to replace (Since that Tester cost about 2 grand) for a couple of feet cable there is not a more affordable slution that just replace the cable.

I think there are more non-working 5200 Controllers than the working ones, so getting a replacement or just do what you did is cheaper.

#3 gzsfrk OFFLINE  

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Posted Fri Sep 28, 2012 3:04 PM

Right, but at this point it's not so much about the cost-effectiveness of coming up with a tool to reduce this problem in the future so much as it is the fun of trying to come up with a viable and efficient solution to a technical issue. :)

#4 Ransom OFFLINE  

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Posted Fri Sep 28, 2012 3:12 PM

Cut the cable in half, and test each half. Whichever one has a continuity problem, cut it in half again. Repeat until you know exactly where the original break was.

No incisions needed with this method!

:grin:

#5 Dave Neuman OFFLINE  

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Posted Fri Sep 28, 2012 3:25 PM

Use an ohmeter with a safety pin attached to one of the probes. Place a probe at one end of the wire and pierce the insulation at various points with the "safety pin enhanced" probe until you find the break. I just reread your post - this method only works with single wires.

Edited by Dave Neuman, Fri Sep 28, 2012 3:31 PM.


#6 Keatah OFFLINE  

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Posted Sat Sep 29, 2012 12:39 AM

X-ray or TDR. Or sometimes careful flexing and can narrow it down to a few cm. I've gone done and built a small RF generator and coil to test where it sounds different as you move up and down the wire. You connect one end of the suspect wire to the generator, and the other end to GND. You'll see (or hear) a difference as you move your detect-O-coil up and down the wire.

You can apply the same principle if you do the complement phase to each end. And use two coils through connected in a summing op-amp config, and then look at the o-scope. This is as close as you'll get without x-ray or cutting into the cable.

There are other ways of making the pulse, and other ways of detecting it. But those worked for me.

#7 gzsfrk OFFLINE  

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Posted Sat Sep 29, 2012 4:17 PM

Cut the cable in half, and test each half. Whichever one has a continuity problem, cut it in half again. Repeat until you know exactly where the original break was.

No incisions needed with this method!

:grin:


Ah yes, the "Solomon's Baby" method. No, I think I'd prefer to keep the baby whole if at all possible. ;)

#8 gzsfrk OFFLINE  

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Posted Sat Sep 29, 2012 4:18 PM

X-ray or TDR. Or sometimes careful flexing and can narrow it down to a few cm. I've gone done and built a small RF generator and coil to test where it sounds different as you move up and down the wire. You connect one end of the suspect wire to the generator, and the other end to GND. You'll see (or hear) a difference as you move your detect-O-coil up and down the wire.

You can apply the same principle if you do the complement phase to each end. And use two coils through connected in a summing op-amp config, and then look at the o-scope. This is as close as you'll get without x-ray or cutting into the cable.

There are other ways of making the pulse, and other ways of detecting it. But those worked for me.


This was more an answer that I was looking for. Don't have access to any kind of X-ray (and given how the wires are twisted around in the outer sheath, I'm not sure it would be very effective anyway), so I'm thinking the RF signal would be the route to take. Might have to do some tinkering and see what I can come up with.

#9 Keatah OFFLINE  

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Posted Sun Sep 30, 2012 11:48 AM

Doesn't matter if they are twisted or spiraled or straight. You'd still see a density change.

If you do the RF thing, the signal goes in one end, and the other end is shorted to ground. One time I just used a telephone pickup coil from RS and stereo amplifier. And the generator was little more than a simple two transistor oscillator. This typically gets you a 1" resolution.

#10 Keatah OFFLINE  

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Posted Sun Sep 30, 2012 11:49 AM

It is also possible to use the output of transistor radio tuned to some kind of motor noise or interference as a source. Or just use your soundcard to make a tone.





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