[aclug-L] Re: cable testers?

[David Carmichael <dec2955@xxxxxxxxxx>]

Nate -

THANKS!  I will give your outline a try!

--David


----- Original Message -----
From: "Nate Bargmann" <n0nb@xxxxxxxxxxxxxxxx>
To: <discussion@xxxxxxxxx>
Sent: Sunday, August 25, 2002 6:51 AM
Subject: [aclug-L] Re: cable testers?


>
> * 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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