[aclug-L] Re: cable testers?

[Nate Bargmann <n0nb@xxxxxxxxxxxxxxxx>]

* David Carmichael <dec2955@xxxxxxxxxx> [2002 Aug 25 06:22 -0500]:
> 
> John -
> 
> Maybe I was shown the wrong way to test cables with a volt/ohm meter.. but
> the way I was show.. required that you have both ends of the cable in your
> hands... and the cable is already mounted in a wall.

Hi David.

Well, it was likely easier for the demonstrator to have both ends
handy.  Here's how I have gone about such things.

1.      Make sure the cable is open at both ends.  i.e. nothing attached
to it but the end connectors.

2.      Using the VOM (volt ohm meter) set on the lowest resistance or
continuity scale, verify the cable is open.  i.e. you want your meter to
do the oposite of what it does when you short the leads together with
the same setting.

3.      If the cable is open, use a alligator clip lead (or anything
else that's handy) to short the center conductor and the shield.

4.      Go to the other end of the cable and perform step 2.  Only this
time the meter should act the same way as touching the leads together.
If you're on the resistance scale you should have a very low reading,
likely less than 10 ohms.

5.      If the cable tests good, then move on down the line testing
other components.  Be aware, things like splitters and amplifiers will
be next to impossible to test with this method.  In this case
temporarily substituting a known good component for the unknown is a
good test.

6.      Be aware that end connectors can fail.  If your cable shows a
short in step 2, then you have little to lose by cutting off one of the
connectors and testing again.  If the short persists, cut the other end
off.  Cables just don't "go bad" unless they suffer sever physical
damage.  I have seen coax suffer quite a bit and still do its job of
transferring tens of watts of RF to an antenna.  You can get connectors
and a tool at Radio Shack in Derby (tell Mark or Bruce that Nate at
Marysville sent you).

7.      This troubleshooting technique divides your problem into small
parts so you can more easily identify which direction a problem lies.

8.      In a past life I had a site where the 4Mb/S Token Ring ran over
their Cat 3 telephone wiring.  Since I had no fancy meter, I used the
technique above to good effect.  Often I had to locate a blown lightning
protector in a pedestal somewhere.  Occasionally, we had a pair in a
cable go bad (some cables had been damaged and spliced before I worked
at the place and they were too cheap to dig them up and fix them right)
and had to route the circuit to another pair.

9.      Even though cable is labeled 75 Ohm or 50 Ohm, you won't measure
that value with a VOM.  This is the "characteristic impedance" and
matters at RF, not DC.

10.     Good Luck!  (I knew I could work 10 in there!)

- Nate >>

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