Using signal generation to detect the location of a line break in my wall

[Tomi Holger Engdahl]

One possibilty is Time Domain Reflectometry (TDR)

Connect an oscilloscope and a pulse-generator ( with needle like pulses) to
the suspect wire and ensure it is terminated with the characteristic
impedance on both ends, something like 100 ohm for twisted pair.On the
oscilloscope you will see the pulse and a reflection at the short of open.
The speed of the signal in a cable is about 2/3 of the speed of light
(calibrate with a known piece of cable) so the fault will be at 0.5 (because
the signal has to go back and forth) * time / speed. Do not expect
millimeter accuracy, so you may still have to hack a long hole in the wall
:-(

Plans for building one such pulse generating circuit can be found at
http://www.epanorama.net/circuits/tdr.html

 

[John Fields]

If you don't mind a drop in audio quality.....

Perhaps you could get by with a common earth return? eg....

Three speakers need only 4 wires (not 6).

Two speakers need only 3 wires.

Prerhaps you have enough working wires to get something going?

 

[Frank Bemelman]

---
Unfortunately, using a common return will cause a voltage drop in the
return conductor which will vary with the sum of the instantaneous
currents in all three speakers. This will hopelessly muddle the high
registers and reduce the transparency of the soundstage.

--

Not only that. I would not blindly trust that every amplifier
has common minus terminals. But you can always try ;-) Silence
can be beautiful too.

 

[CWatters]

Good points.

What about investing in a digital amp for the three "remote" speakers and
running sigital over the working wires?

 

[Scott Newell]

I have three speaker wires running just behind sheetrock along a wall in my
house that have breaks in them somewhere (I believe from the contractor

[snippage]

I'd vote for TDR. Know anybody who works for the cable company or the
phone company?

Wouldn't TDR give you the length of the wire, not the location of
the break? If you know the section is 25' long, finding the break
behind the wall also means that you've got to know exactly how the
wire was run.


newell

 

[mike]

I have three speaker wires running just behind sheetrock along a wall in my
house that have breaks in them somewhere (I believe from the contractor

[snippage]


I'd vote for TDR. Know anybody who works for the cable company or the
phone company?

Wouldn't TDR give you the length of the wire, not the location of
the break? If you know the section is 25' long, finding the break
behind the wall also means that you've got to know exactly how the
wire was run.


newell

Post a better solution.
mike

--
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laptops and parts Test Equipment
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TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Michael]

I have three speaker wires running just behind sheetrock along a wall in my
house that have breaks in them somewhere (I believe from the contractor
firing nails through the wires when putting up the sheetrock). Two of the
wires are shorted from one end (wall terminal) and open from the other. The
third is open from both ends. I figure in all cases a nail-gunned nail went
through the wire in question, and in two cases the nail is probably shorting
one half of the wire.

I would like to detect where the breaks are in the wall.

I've tried generating emf in the shorted lines in the wall using a strong
magnet, and it works to a degree but not enough - I need a stronger magnet
or a more sensitive multimeter.

What I'm thinking now is that if I can use the wire(s) in the wall as an
antenna, generating some kind of signal, I could use a receiver held up to
the wall to detect that signal, and thus detect the break. Would this work?
How would I best do this?

Also, I've used a small electric motor as a generator to create an AC
current in the wires. This also works to a small degree even with the cut
(i.e. open) wires in the walls due to the capacitance of the wire, though
there's no closed circuit. Can I use something to detect the AC Current in
the wires behind the wall? Could this work even with the broken (not
shorted) wires, relying on nothing but the wire capacitance to allow a
trickle of AC current to get through?

Are there other ways to do what I'm trying to do? It seems like it should be
possible.

Thanks,
Eric

If you end up knocking a hole and finding the break right behind it,
please let us know.

Murphy's Law Guarantees that the break will not be where you punch the
hole,

 

[Jim Thompson]

I have three speaker wires running just behind sheetrock along a wall in my
house that have breaks in them somewhere (I believe from the contractor
firing nails through the wires when putting up the sheetrock). Two of the
wires are shorted from one end (wall terminal) and open from the other. The
third is open from both ends. I figure in all cases a nail-gunned nail went
through the wire in question, and in two cases the nail is probably shorting
one half of the wire.

I would like to detect where the breaks are in the wall.

[snip]

I've had good success using a Progressive Electronics model 701K Tone
and Probe Kit... allowed me to find speaker wires that were put in the
walls of my new house while I was out of town and promptly covered
with dry wall, so I didn't get my usual set of pictures to work from.

Cut six holes for in-wall speakers and didn't miss any wires ;-)

...Jim Thompson

 

[Michael A. Terrell]

If you end up knocking a hole and finding the break right behind it,
please let us know.

Murphy's Law Guarantees that the break will not be where you punch the
hole,

I don't know about that. I had a drywall company fill all my in wall
boxes for a fire alarm, burglar alarm, and background music system with
plaster, then they textured the finished wall. I found over two dozen
boxes without breaking anything except the plug of plaster in each box.

 

[[email protected]]

I have three speaker wires running just behind sheetrock along a wall in my
house that have breaks in them somewhere (I believe from the contractor
firing nails through the wires when putting up the sheetrock). Two of the
wires are shorted from one end (wall terminal) and open from the other. The
third is open from both ends. I figure in all cases a nail-gunned nail went
through the wire in question, and in two cases the nail is probably shorting
one half of the wire.

You very well might be better off biting the bullet and
abandoning the wires, and rerunning them. If you have
access to the wall from above/below it could be worth
the effort.

Are these wires on the surface of the studs such that
when the rock was put up it pressed the wires between
the rock and the studs, or are they run through holes
bored in the studs - or are they vertical and stapled
to the sides of the studs?

I have a couple of "fears" about the practicality
of the repair, even assuming you can find the exact
location of the break/short.
1) You may not have enough slack in the wire at the
point where you make a hole in the wall to be able to
repair it, necessitating a bigger hole - or a second
hole.
2) You may have additional damage that hasn't shown
up yet that may necessitate future repairs.

Installing brand new runs eliminates both. When you
plan it, you'll know precisely where, and how big,
to make your holes. Obviously, I have no clue how
practical/impractal installing new runs would be
in your situation.

For a cheap, easy signal source you could try
connecting a 120 to 6 or 12 volt transformer
secondary to each pair of wires, one at a time
through a 1 K resistor. Then try to use a
transistor radio to detect the 60 cycle hum.
If that won't work, and audio amplifier with
a telephone pickup coil probably will. I don't
know how well that method will pinpoint the
source of the trouble, but it may be work trying.

An even better signal source could be made by
adding a triac circuit to the secondary and
setting it to produce say half the output
voltage.

 

[Rich Grise]

Um, if he could get to the far end to properly terminate it, he
should just fix it then and there. ;-)

I've used a TDR before; it's driven with a step, not a spike. And it
reflects a Z/s (impedance vs. distance) graph on the scope. A step to
infinity is an open, and a step to zero is a short. And they're
surprisingly accurate, if you have a fast enough scope.

Cheers!
Rich

 

[Tomi Holger Engdahl]

Um, if he could get to the far end to properly terminate it, he
should just fix it then and there. ;-)

I've used a TDR before; it's driven with a step, not a spike. And it
reflects a Z/s (impedance vs. distance) graph on the scope. A step to
infinity is an open, and a step to zero is a short. And they're
surprisingly accurate, if you have a fast enough scope.

Cheers!
Rich

TDR can be performed with step or impulse drive.
Both technologies are in use.

 

[N. Thornton]

Hi Eric.

I posted a solution before, but it doesnt seem to be here. Take a CMOS
input opamp, put a metal plate on the end of a plastic box, on the
outside, and connect plate to opamp input (plus a few protection
components). Compete the circuit of course. Now, put your amp on and
run this plate over the wall, and where it crosses the nail connected
to the cable you'll get a volume max coming out the opamp/piezo. Works
quite well.


Regards, NT

 

[N. Thornton]

In fact if youre lucky you may just need to pull the offending nail
out, the shorted cable may then work OK.


Regards, NT

 

[Cliff Wright]

Um, if he could get to the far end to properly terminate it, he
should just fix it then and there. ;-)

I've used a TDR before; it's driven with a step, not a spike. And it
reflects a Z/s (impedance vs. distance) graph on the scope. A step to
infinity is an open, and a step to zero is a short. And they're
surprisingly accurate, if you have a fast enough scope.

Cheers!
Rich

Hi
The opamp solution may just work. BUT it must be mentioned that the shorted cable should be
driven through a 4 to 10 Ohm WW resistor of several watts rating at least.
I would hate to see your power amplifier destroyed by it driving a short circuit!
Regards Cliff wright.

 

[CWatters]

The problem is finding the nail. I assume his metal detector is fooled by
the wire so he can't find it that way.

 

[N. Thornton]

The problem is finding the nail. I assume his metal detector is fooled by
the wire so he can't find it that way.

Yes, hence we're suggesting how to find it. I assumed a metal detector
wouldnt distinguish one nail from another, but I dont know a lot about
them. Maybe I'm missing something.

Regards, NT