A Audio and hi-fi forum. AudioBanter

Go Back   Home » AudioBanter forum » rec.audio » Pro Audio
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Ground Loop from Cable TV Questions



 
 
Thread Tools Display Modes
  #1  
Old August 31st 19, 08:52 PM posted to rec.audio.pro
Bob Simon[_2_]
external usenet poster
 
Posts: 6
Default Ground Loop from Cable TV Questions

When I connect audio out from my cable box to my pre-amp, I get hum. I was able to eliminate it for a couple of years by inserting an isolator (ISO-MAX VRD-1FF) just ahead of the Contour cable box.

Two days ago, the cable company disconnected my cable TV service because they determined interference was coming from my house. Yesterday, a Cox technician looked at my connections and stated that the source of the interference was my isolator. He directed me to remove it, performed a measurement, and said that everything was ok now, so he reconnected my cable TV service.. No surprise - the hum is back.

1) Is the hum caused by cable ground being at a different potential than house ground?

2) There is a rod in the ground just below my electric meter. A clamp on this rod is connected to a thick copper wire which goes to a cold water pipe and then it goes into the back of the main electrical panel. Is this my house ground?

3) If the cable ground were connected to this grounding rod, would this solve my problem with hum? Should this be done or not?

4) Is the cable company responsible for doing this?

5) Is it a violation of code for the cable not to be bonded to house ground?

Ads
  #2  
Old September 1st 19, 04:31 AM posted to rec.audio.pro
Phil Allison[_4_]
external usenet poster
 
Posts: 397
Default Ground Loop from Cable TV Questions

Bob Simon wrote:
-------------------

> When I connect audio out from my cable box to my pre-amp, I get hum.
> I was able to eliminate it for a couple of years by inserting an isolator
>(ISO-MAX VRD-1FF) just ahead of the Contour cable box.
>
> Two days ago, the cable company disconnected my cable TV service because
> they determined interference was coming from my house. Yesterday, a
> Cox technician looked at my connections and stated that the source of
> the interference was my isolator.
>



** Very dubious....



> He directed me to remove it, performed a measurement, and said that
> everything was ok now, so he reconnected my cable TV service.
> No surprise - the hum is back.
>
>


** The isolator is probably just a simple 1:1 ratio ferrite balun that passes radio frequencies only. The hum you hear is 60Hz plus various harmonics.

The radiated RF interference is due to having no low impedance path to ground at the cable outlet - allowing items that follow to radiate easily.

The fix is ridiculously simple - re-fit the isolation balun and ADD a small capacitor across the input and output grounds. I may be possible to fit it internally but if not, is does not matter much.

Try a ceramic cap of about 100nF, voltage rating is not important either.



..... Phil






  #3  
Old September 2nd 19, 01:57 PM posted to rec.audio.pro
Scott Dorsey
external usenet poster
 
Posts: 16,614
Default Ground Loop from Cable TV Questions

Bob Simon > wrote:
>When I connect audio out from my cable box to my pre-amp, I get hum. I was=
> able to eliminate it for a couple of years by inserting an isolator (ISO-M=
>AX VRD-1FF) just ahead of the Contour cable box.


Because there's a lot of 60 Hz trash on the cable shield, possibly because
it's not properly bonded.

>Two days ago, the cable company disconnected my cable TV service because th=
>ey determined interference was coming from my house. Yesterday, a Cox tech=
>nician looked at my connections and stated that the source of the interfere=
>nce was my isolator. He directed me to remove it, performed a measurement,=
> and said that everything was ok now, so he reconnected my cable TV service=
>. No surprise - the hum is back.


Get a real technician out. Likely the "interference" they are referring to
is 60 Hz trash on the cable shield, and it's caused by the cable not being
properly bonded to ground at your house. When the ground isolator is removed,
the system is being grounded through your TV set (which also provides a
path to discharge lightning and other inadvertent transients), and so the
cable folks don't see the 60 Hz trash. This is an unsafe condition.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #4  
Old September 2nd 19, 04:54 PM posted to rec.audio.pro
Bob Simon[_2_]
external usenet poster
 
Posts: 6
Default Ground Loop from Cable TV Questions

On Monday, September 2, 2019 at 7:57:21 AM UTC-5, Scott Dorsey wrote:
> Because there's a lot of 60 Hz trash on the cable shield, possibly because
> it's not properly bonded.
>
> Get a real technician out. Likely the "interference" they are referring to
> is 60 Hz trash on the cable shield, and it's caused by the cable not being
> properly bonded to ground at your house. When the ground isolator is removed,
> the system is being grounded through your TV set (which also provides a
> path to discharge lightning and other inadvertent transients), and so the
> cable folks don't see the 60 Hz trash. This is an unsafe condition.
> --scott


Scott,
The cable is definitely not bonded to house ground. I found an article that discusses this: https://www.mikeholt.com/mojonewsarc...s~20030214.htm

"As the 820.33 FPN explains, one purpose of 820.33 is to limit the potential differences between CATV and other metallic systems (see Figure 820-14 un820-14 820-40D.cdr). So, connect the coax shield to the main grounding system."

and

"[820.40] When grounding the coax sheath:
Use an insulated grounding conductor listed for the purpose.
Use a grounding conductor made from a corrosion-resistant conductive material. If copper, it must not be smaller than 14 AWG."

Although 14 gauge copper wire is apparently large enough, do you have any information regarding whether increasing this to 10 or even 6 gauge would have an effect on the hum in my system?

The Cox technician who came out last Friday scheduled a return visit Thursday morning to ground the cable. Since he did not demonstrate understanding beyond his limited training, I expect that I will do a better job bonding the cable shield to house ground than he would. I certainly have more at stake in the outcome than he does.
Bob
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Ground Loop Bob Simon[_2_] Pro Audio 4 August 27th 16 05:11 AM
Ground Loop Questions Bob Simon Pro Audio 13 December 30th 06 03:03 AM
Ground Loop Bob Simon Tech 2 October 25th 05 11:13 AM
Ground Loop? (Ground Loop Isolators Have No Effect) [email protected] Car Audio 3 January 15th 05 06:18 PM
ground loop J. Sun High End Audio 5 May 2nd 04 07:10 PM


All times are GMT +1. The time now is 05:20 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright 2004-2019 AudioBanter.
The comments are property of their posters.