Sounds like there are more aspects to this than I had guessed, particuarly
the cable TV element. And the cable TV part sounds like it has no easy cure,
if I want to run its sound into my stereo.
Actually, it probably does.
You can buy, or built, an isolation-transformer for the cable-TV feed.
Installing this between your cable TV line and your TV (or A/V
receiver) will break the ground loop.
For a commercially-built device, check out part #180-075 at
www.partsexpress.com (type "isolation" in the keyword search box, and
scroll down to the "CATV Misc" section).
Attached below is one possible approach for a homemade isolator,
courtesy of Jamie Hanrahan and my USENET-snippings archive. I don't
know if the Radio Shack baluns he specifies are still available - they
may have changed designs in the ten years since this article was
written.
Another thing which you could do, is examine your cable-TV
installation. In most areas, cable-TV installers are required by the
local electrical code to "bond" the cable to the house's grounding
system at the point where it enters the building - the cable-TV
coaxial shield is connected to the building ground. If this was not
done when your cable feed was installed, it could exacerbate the hum
problem, because the cable's idea of "ground" would be whatever ground
voltage is present back at the pole, or at your neighbor's house, or
some combination thereof. If you find that your cable-TV line wasn't
properly grounded on entry to your building, you could call your cable
TV company and complain about the defective installation, and request
that they bring it "up to code".
###############################
From:
Newsgroups: sci.electronics,rec.audio,rec.video
Subject: Revisited: CATV baluns for eliminating AC hum
Date: 1 Nov 93 20:15:24 PST
Some time ago, someone in one of these groups had a question about eliminating
AC hum from their stereo system. The problem started when they connected
a VCR into the system, the VCR being connected to cable TV.
I suggested (as I and others have done in the past) using a pair of 75/300
baluns back-to-back in series in the CATV lead, in order to isolate the
stereo system from the cable's connection to earth ground. The problem is
especially bad if there's another earth ground connection in the system (like a
unit with a three-prong plug).
Recently, someone objected to this, stating that a balun has a common
connection between its two "sides", and hence offers no isolation.
And of course, he's right. Most of the diagrams of baluns that I've seen
show the "bottoms" of the two transformer windings connected together.
But, I've used this trick of two back-to-back baluns several times to
eliminate these CATV-induced ground loops, and it's always worked.
Well, today I had to do it again. I went to Rat Shack and bought two baluns --
an "inline" one with a female F-connector and pigtails for the 300-ohm lead,
and a compact one with a push-on male F connector and screw terminals for the
300-ohm side. (This type is often supplied with TV sets.) Before hooking
things up, I got out my trusty Fluke and measured the baluns. (Gasp, what a
concept!)
It turns out that the in-line one is a true transformer -- there's no common
connection between the 300 ohm and 75 ohm sides. The compact one does have a
common connection, but of course any single break in the "ground" chain is
sufficient.
So there you have it. I don't know *why* RS makes their in-line baluns this
way, but I'm glad that they do...
--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
Internet: (JH645) Uucp: uunet!cmkrnl!jeh CIS: 74140,2055
###############################
--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!