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Path: utzoo!watmath!clyde!cbosgd!ihnp4!houxm!vax135!petsd!peora!pesnta!hplabs!hp-pcd!raan
From: raan@hp-pcd.UUCP (raan)
Newsgroups: net.video
Subject: Re: AC on Cable?
Message-ID: <20600016@hp-pcd.UUCP>
Date: Sun, 17-Feb-85 20:20:00 EST
Article-I.D.: hp-pcd.20600016
Posted: Sun Feb 17 20:20:00 1985
Date-Received: Mon, 4-Mar-85 20:13:47 EST
References: <35@bbnccv.UUCP>
Organization: Hewlett-Packard - Corvallis, OR
Lines: 31
Nf-ID: #R:bbnccv:-3500:hpcvry:20600016:000:1613
Nf-From: hpcvry!raan    Feb 28 17:20:00 1985



I encountered the same phenomenon on my system.  I believe that what is
happening is that the tv cable shielding is picking up an induced voltage
(relative to earth).  The source of this induced voltage is the power lines
that the cable shares the telephone poles with.  (You get the same affect
if you ground one of the incoming wires on your telephone.  You'll be 
rewarded with a LOUD buzz in the telephone.  The telephone works because
it is a balanced line and the induced voltage is common to both lines.
The cable is not balanced, since the shield is grounded.)  

The first thing to check is to make sure that the cable shield is grounded
to a good earth ground (like a cold water pipe or a long metal stake in the
ground).  This should be at the point of entry to your house.  

I still had problems with attaching the cable to the rest of my system
because of ground loops, so I constructed an isolation transformer by
putting a 75 to 300 ohm adapter back-to-back with a 300 to 75 ohm adapter.
(The 75 to 300 does not block the ground path, but the 300 to 75 does.
This means that you need this combination even if the cable is being 
fed to a 300 ohm input.)

The voltage on the cable should be very low current, in other words it
should not have sufficent power to do anything useful (ie light a light
bulb, I assume that your test light was a neon type, which uses low
current.)  If it does light up a regular bulb then you have a fault in
the system (a potentially lethal and certainly damaging one) and should
contact the power company and or cable company immediately!

Raan Young
(hp-pcd!raan)