[mythtv-users] Electrical power in my cable line? Is that bad?

Brian Wood 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
power supply.

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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