Re: How to use a meter to determine if a cable is good



w989531@xxxxxxxxxxx wrote:

On Tue, 13 Mar 2007 22:33:18 -0400, "Patrick Keenan" <test@xxxxxxxx>
wrote:

<w989531@xxxxxxxxxxx> wrote in message
news:2dpev2p4sbaa9tqr7nbk4m40mvoea1ujsu@xxxxxxxxxx

Could someone please inform me how to use a digital/analog multimeter
to determine if there is a short or a broken wire in a guitar/PA cable
that is equipped with 1/4" jacks at either end.

What I would like to do is to be able to connect the probes to either
end of the cable and have the resistance displayed indicate whether or
not there is a short. Also, could you please let me know what the Ohm
readings should be for a good cable as well as for a broken cable.

I have checked onine and scoured the Yamaha Sound Reinforcement
manual, but I cannot find any information that I can use.

Thanks very much...

Doug

A working cable should show very little to no resistance between connected
conductors, and infinite resistance between unconnected connectors.

That is, the tips of the 1/4" connector should give a reading of 0 ohms and
so should the sleeves (this is where the continuity function on a meter
comes in handy - it's audible and you don't have to watch the meter). If
you get higher readings, and if they are higher they will tend to be range
of infinity, there is a break in the conductor (or solder connection).

If you have the meter set to a low range, you may get a reading of a few
ohms if the cable is long. That would depend on the cable itself.

On the other hand, you should get no continuity (and an infinite impedance)
between the tip and sleeve. There should be no connection between those two
at all. If there is, then there is a short.

HTH
-pk


Thanks very much...I really do appreciate your time and effort. This
is one of those problems that I just could not find an answer to on
the 'Net. I will try out my newly found information immediately.

Thanks again...

Doug


If your meter is an autoranger, make sure your fingers are not touching the probe
tips when taking readings from tip to sleeve.

Your skin resistance will be displayed if they are. Otherwise,
set your (non autoranging) meter for the lowest ohms scale
when reading tip to tip and sleeve to sleeve. When reading
tip to sleeve, set it on the *highest* ohms range; this is because
some (of the better-quality) cable used to make guitar patchcords
with has a triboelectric shield on the inner conductor. This is
made from conductive plastic or carbon-impregnated cloth,
and if any of it is touching the tip contact inside either of
the plugs, there will be a high-resistance path from tip
to sleeve. This will not keep the cable from passing a signal
like a dead short will, but it will cause the signal to be
attenuated. I've seen many cables assembled by do-it-yourselfers
which displayed this condition. Here's an old NG post I put
up on AGA a few years ago which deals with this:

_________________________________________________

Monte Barnett wrote:

Speaking of cable, I ran across something last week
which was new to me: conductive "insulation". In the
process of minimizing the cable situation on my
pedalboard, I had cut a pre-made 3' Peavey cable in
half and made it into two 1.5' cables using some
solderless straight .25" connectors. I checked the
connections using the "audible short" function on my
meter, and everything was fine. Two hours later at
practice, I had almost no signal from the pedalboard
to the amp, so I started bypassing effects and swapping
cables until I had a good signal with at least my
Fulldrive II & DejaVibe. In troubleshooting the problem
later, I found that both of the smaller cables I'd made
had high-resistance shorts between the shield and center
conductor. Turns out that the black sleeve between the
braid & center conductor insulation was conductive,
and was barely making contact with a *single* wire from
the center conductors on each cable. I'll admit to not
having done much cable-building in the past couple of
years, but this is the first time I've run across this.
Is it common with good cable nowadays, or mostly with
cheaper stuff?

Lord Valve Speaketh:

Actually, it's the *good* stuff that has that. The
"conductive insulation" you ran across isn't insulation,
it's electrostatic shielding. This can be plastic or
cloth based; Canare cable uses plastic and Belden uses
cloth. It's there to reduce mechanically generated
noises; high-impedance cable can be quite microphonic,
and can also make scritchy-scratchy noises when it's
slithered around on the floor. This is due to random
"stray" charges generated within the cable itself (like
"static" electricity) and also to changes in cable
capacitance as the wire flexes, slightly changing the
distance between the outer shielding and the inner
conductor. If you're making your own cables and you
encounter what appears to be an extra layer of
"insulation" between the center conductor and the
shield, take care to remove it from the exposed portion
of the center conductor before you solder everything in
place. If *any* of it touches the copper portion of the
center conductor, you will have a high-resistance short
from tip to sleeve; as you found out, using the low-ohms
range on your meter to check from tip to sleeve will *not*
detect this condition. What appeared to be a perfectly
good cable when you were finished making it became a
source of signal attenuation in the field. (BTW, this
is *another* reason not to use "solderless" connectors;
you cannot *see* the actual connections inside the plugs,
and all kinds of bogus crud could be in there.) I learned
early on that whenever I sell someone a piece of good-
quality cable, I'll either need to spend five minutes (now)
telling the dude (or dude-ette) how to prepare it for
soldering or spend ten minutes (later) listening to him/her
bitch about the "shitty" cable I sold. For guys what rolls
their own, here's the best combo I've found: Canare GS-6
cable and Switchcraft 280 plugs. I know the temptation
is to use the snazzy-looking Neutrik plugs; they are hard
to solder to without a *really* powerful iron, and the strain
relief system is bogus. They cost twice as much, too. Stick
with the Switchcraft; they were good enough for Grampaw and
they're *still* the best. (I sell both kinds, in case anyone
is wondering.) If you want to make a cable that is damn near
indestructible, use a piece of 3/8" HST over the inner part
of the plug; fill the inner portion with hot-glue and slide
the shrink over the glue while it's still hot; it'll begin
to shrink immediately. Finish shrinking it with your heat-
gun (or a 1000W Par-64 can, or a propane torch set on low,
or a cigarette lighter, or a hot-air popcorn popper [all of
which I have used successfully in the field]) and wait for
it to cool before removing any glue that squished out the
edges while you were shrinking it. This is the *best*
termination system that I've been able to devise; I have
guitar cables in the field that are still going strong after
more than a decade of use. So Sayeth the Lord.

____________________________________________

Lord Valve
Expert

VISIT MY WEBSITE: http://www.nebsnow.com/LordValve
I specialize in top quality HAND SELECTED NOS and
current-production vacuum tubes for guitar and
bass amps. Good prices, fast service.
Authorized dealer for QSC amps, Sovtek/Electro-Harmonix,
Behringer, Hammond-Suzuki, Leslie, Rolls, Weber VST, etc.


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