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  1. #31
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    Feb 2010
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    Default Re: voltage in cable coaxial

    Could a problem in their lines (an intermittent short maybe) damage the modem and cause it to backfeed voltage? They have never checked the grounds and they have not re-done the ground which is attached to my main electrical ground below my disconnect on the back of the house.

  2. #32
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    Default Re: voltage in cable coaxial

    Ground loop --- go back and think about what causes a ground loop --- a potential difference between two interconnected components.

    In this case the cable company is grounded on their end ---- your home is connected to a different ground reference ---- which equals two different potentials. The cable resistance from the distribution to your home increases creating more voltage potential drop.

    This is why it's important for the cable to be properly bonded to your home ground in order to try and bring both points to the same potential.

    There also can be ground loops within your own home when different components are interconnected while being plugged into different receptacles.
    Differences in power supplies and electronic designs also affect ground loops with interconnected equipment that use ground reference in their designs.

    One thing you might try is bring the modem to the point of entry --- eliminating all splitters , interconnects , etc..
    This way the modem directly connects to the incoming cable and would eliminate any local ground loops.

    If all else fails you might try an isolation transformer on the cable connection.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #33
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    Aug 2007
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    Default Re: voltage in cable coaxial

    A demarc box is a small junction box where the cable company wire comes in and connects to YOUR cable wiring . Usually mounted on the side or the eve of a house , although some installations just connect together with no enclosure . At any rate , unscrew the connection that ties THEIR cable to YOURS . Now you are isolated .

  4. #34
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    Feb 2010
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    Default Re: voltage in cable coaxial

    Quote Originally Posted by djohns View Post
    A demarc box is a small junction box where the cable company wire comes in and connects to YOUR cable wiring . Usually mounted on the side or the eve of a house , although some installations just connect together with no enclosure . At any rate , unscrew the connection that ties THEIR cable to YOURS . Now you are isolated .
    Right - there is a splitter block that is tied to the telephone pole ground and it is up on the line. I can disconnect the feed up in the attic. That will isolate a major possibility for the ground loop - grounded at the pole and grounded at my electric service entry (neither of which have been checked). I brought the modem in to work but I suspect I will not see the voltage. It is starting to look more and more like a ground loop and maybe even one that damages the modems somehow. This will be fun to try to explain to Comcast.

  5. #35
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    Feb 2010
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    13

    Default Re: voltage in cable coaxial

    OK, it is 47 V AC (bleed through) from the signal wire - or the shield - to the receptacle ground at work. There is no voltage between the shield and the signal wire. I couldn't get an amp reading either.

    Does that mean the unit is damaged? Is the problem a ground loop (that damages the modem)? Are all these reported voltage problems the result of a ground loop?

    Can I tell them that the splitters cannot act as capacitors?

  6. #36
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    Default Re: voltage in cable coaxial

    I would say the modem is foobarred.
    Whether it's a TV or a modem there should be no voltage at the RG 59/6 cable input.

    The braided portion of coax is being used as ground which is connected to the F connector which connects to the chassis of the device.
    One way to confirm this --- take your multimeter and check for continuity between the neutral spade on the plug and the threaded male F connector on the device.

    One thing that might reduce the loop issue would be ensure the braid is only connected to ground on one end. This would lift the loop and still provide the RF shielding.It's really not a good idea to float the ground this way for saftey reasons.

    If there is a signifigant ground loop issue then an isolation transformer on the cable line is a definate resolve.


    The cable company should be able to resolve things --- afterall they run into situation more than once.
    Bottom line is they want the install to be as cheap for them as possible.

    In most cases the installers are hacks , especially when it comes to internet connections or digital cable terminal installs. Around here the installers for internet and digital cable are sub contractors --- usually some person that may have half knowledge to be able to turn on a PC , setup the account and that's it.

    Good grief ---- if they have to run cable ( in a professional manner ) or troubleshoot issues such as what you're experiencing.

    Ask them to prove the groundloop is your responcibility and that splitters store a charge ------- other than " I dunno , it's gotta be "

    My 2 cents worth.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  7. #37
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    Default Re: voltage in cable coaxial

    Quote Originally Posted by canuk View Post

    The cable company should be able to resolve things --- afterall they run into situation more than once.


    You're assuming way too much old buddy . I know I told this story once , but ..... I was having problems with my internet . First tech comes out . " You're going to have to rewire your house . This cable is old and no good anymore " .
    Mind you , he never took any test eqpt. out of his truck . Thanks , please go away . Second guy comes out and changes some splitters . No help . Third guy changes a bandpass filter in the pedestal . No help . FINALLY , on the fourth try , the tech brings out some test eqpt. and diagnoses a bad cable underground . Cable replaced , problem solved . These were all COX Cable techs .

  8. #38
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    Default Re: voltage in cable coaxial

    Yep --- lost count with having to battle with cable hacks and telco butt heads.
    Three quarters are too lazy or don't know what the heck to do especially when it might mean climbing a pole --- pretty bad when less than half know what they're doing or care to do anything at all.

    Just the other day , one of my customers had an issue with the fax line not working.
    I went to have a look at it for them --- the phone wiring into the office looked like a drunk monkey with no arms did the orginal work. The phone company couldn't send out a tech for 3 days. This was just crazy that a business had to wait 3 days --- geez --- so I fixed the problem .
    Last edited by canuk; 02-18-2010 at 11:43 PM.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  9. #39
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    Jun 2007
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    Shamokin, Pa.
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    Default Re: voltage in cable coaxial

    Quote Originally Posted by djohns View Post
    You're assuming way too much old buddy . I know I told this story once , but ..... I was having problems with my internet . First tech comes out . " You're going to have to rewire your house . This cable is old and no good anymore " .
    Mind you , he never took any test eqpt. out of his truck . Thanks , please go away . Second guy comes out and changes some splitters . No help . Third guy changes a bandpass filter in the pedestal . No help . FINALLY , on the fourth try , the tech brings out some test eqpt. and diagnoses a bad cable underground . Cable replaced , problem solved . These were all COX Cable techs .
    Entry level techs with no real world OTJ training. I too have seen way too many cases were they get it right the fourth time.

  10. #40
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    Aug 2007
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    The deep South
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    Default Re: voltage in cable coaxial

    Don't get me started on the phone company . It took me several weeks of constant hounding , to get them to come out and fix my elderly Mother's phone . A lot of folks here are complaining of poor customer service . I'm talking about THE phone company . Not one of the smaller Mom and Pop outfits .

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