[mythtv-users] Electrical power in my cable line? Is that bad?
beww at beww.org
Tue Sep 11 01:26:31 UTC 2007
Keith Pyle wrote:
> Thanks to Brian Wood, David Brodbeck, and others for their help in
> diagnosing my hum bar problem. For the benefit of others on the list, I
> did two things that seems to have solved the problem (based on a limited
> number of recordings). First, I checked and improved the cable TV
> grounding. The cable installer had clamped a ground wire from the
> incoming splitter to the conduit between my electric meter and the
> distribution panel. However, he neglected to notice that the conduit
> was painted. There was a 145 ohm resistance to ground due to the
> paint. Fixing this substantially reduced the hum bar, but it still
> seemed slightly present. I'm not convinced this is the best grounding
> option and need to check a bit more.
The (probably steel) conduit is not the best conductor in the world
(silver is). There might be (should be in fact) a ground rod (usually an
8-footer) near the AC power service entrance. Grounding the CATV ground
block to that point might improve things a bit.
The second change was to create a
> temporary, crude isolation transformer from a couple of 75-300 ohm
> baluns (one Radio Shack 15-1253 and one 15-1140). I haven't seen a hum
> bar with this in place.
Ahhh. One of my favorite tricks, and pretty much proves the problem is
in the external grounding system and not a problem with your computer's
Using 1" or bigger copper strap or braided copper as a grounding
conductor can help, as well as keeping the runs as short as possible.
> I'm now looking for a better isolation option. I've seen some mention
> of the MAGIC (Mondial Antenna Ground Isolation Circuit) splitter as
> being a good one. If anyone has an opinion on this or other isolation
> options, let me know.
I'll ask around. HumBuckers don't go high enough in frequency. There
might be some sort of optical isolator though.
Glad you're making progress.
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