sub hum from cable TV coax redux

Discussion in 'Archived Threads 2001-2004' started by Rick Radford, Jan 25, 2002.

  1. Rick Radford

    Rick Radford Supporting Actor

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    I have a distinct subwoofer hum when I crank my Samson 700 to max. This is caused by the cable TV coax.. verified by pulling the cable to the TV from the CATV surge protector and the hum goes away.

    I did a search here and found info that referenced using a couple of Radio Shack 75 to 300 ohm antenna transformers. Further on in this reference was a msg by a Continental Cable tech who suggested the use of an inline attenuator 6 dB or less.

    At RadioShack I found a 75 ohm TV/VCR signal overload attenuator (6 dB attenuation) (part no. 15-1257a) and tried it.. but it didn't work. Is this the same thing as the tech referenced?

    I guess I'll return the attenuator to RS and try the antenna transformers unless someone has a better idea.

    Any thoughts?
     
  2. RicP

    RicP Screenwriter

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    Get a Model VR-1FF Ground Isolator from Jensen Transformers. It's more expensive than the Radio Shack solution but you will have very little signal attenuation.
    See it here...
    http://www.jensentransformers.com/vr1ff.html
     
  3. Jim A. Banville

    Jim A. Banville Supporting Actor

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    What's the difference between that $50 unit and this $5 one from Rat Shack... http://www.radioshack.com/searchsku.asp?find=15-579 ? And what's the difference bewteen these units and the $99 Mondial MAGIC box? I think I would try coupling a pair of $2 "300-to-75 ohm transformers" to see if it works without cuting the signal too much. I DON'T think you need to attenuate the signal.
     
  4. Rick Radford

    Rick Radford Supporting Actor

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    RicP: thanks for the link. I'd sure like to find a cheaper solution, if possible. ($51.95 for any lurkers)
    Jim A: in this thread, it appears that the RS part you referenced worked for Matt Stryker.
    I believe I'll try the RF interference filter ($5) or the $6 solution (the 2 antenna xformers). Hopefully, this inexpensive fix will solve my hum.
    I'll post back when I know more.
    Btw, here's the link that put me on to the xformer/attenuator solution for cable TV induced hum. (from this thread)
     
  5. Marc H

    Marc H Second Unit

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    Hiya Rick,

    Try and find a ground loop interupter (aka ground breaker). Shouldn't be more than about ten bucks. Looks similar to a matching tranformer; black cylinder about an inch and a half long with coax ends on it. Radio Shack used to have them, most dish installers carry them with them or any custom install shop would have them too.
     
  6. Rick Radford

    Rick Radford Supporting Actor

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    Hi Marc,

    I made a note of the GLI and will inquire at the Shack when I stop by later today.

    Thanks.
     
  7. RicP

    RicP Screenwriter

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    I think you misunderstand. By using the 300ohm to 75ohm breaker, you are attenuating the signal...by a pretty large margin. In fact without an inline amplifier you may find that your cable signal becomes unwatchable after installing a 300ohm breaker.
    The Jensen transformer has the lowest attenuation possible while still blocking the DC that causes the ground loop hum. I use one in my system and it works perfectly.
     
  8. Rick Radford

    Rick Radford Supporting Actor

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    RicP, you are right on the money. The interference filter (15-579a) from RS did NOT work.
    On to plan C.
    Hmm. Further searching has turned up this link.
    Any thoughts?
     
  9. RicP

    RicP Screenwriter

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    Again Rick, the Xantech model 63400 they list there may in fact work well enough to block the ground loop hum, but by how much will it attenuate the signal?
    At $6.95 it may be worth a try, but the Jensen unit -- while more expensive -- will guarantee you virtually no signal loss. Look at it this way, it's a once in a life purchase, you'll never have to buy one again. [​IMG]
     
  10. Rick Radford

    Rick Radford Supporting Actor

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    RicP,
    If my MO works as usual, I'll spend at least $50 worth of gas and untold hours trying to find a cheaper solution before finally deciding the Jensen unit is exactly what I need.. and it'll only have cost me ~$100. [​IMG]
    Fwiw, although I wasn't looking critically at the TV, when I tried the 6 dB attenuator, I noticed no loss of PQ. Since this is a 32" DV set, it may not be as critical as cable on a big screen.
     
  11. RicP

    RicP Screenwriter

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  12. Ron Reda

    Ron Reda Cinematographer

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    Rick Radford,

    Do a search on "Ground Loop Isolator" and look at the second post which is from John Morris. When I thought I had a ground loop problem, I contacted Electrotex and ordered one of their Holland "Ground Loop Isolators" (as seen in John's picture). I think it was $6. For my application (I thought the hum was coming from my sat receiver), it doesn't work, but it sounds like it may for you. You're more than welcome to the one I have as it's just sitting unused in my "HT parts and pieces" drawer! If you want to send me a few bucks for shipping, it's all yours!

    Regards,

    Ron
     
  13. Robert Cranwell

    Robert Cranwell Stunt Coordinator

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    I think alot of people are just using a bandaid on this subject? CATV has to to be bonded not grounded to the electic in the home. If you have a poor electric neutral connection by way of panel,meter pen or electric company connection,the electric will seek another ground source ie: Cable TV ; which is grounded to the electric at the pole or pedestal, which ever you may have.

    Rob
     
  14. Rick Radford

    Rick Radford Supporting Actor

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    Rob,
     
  15. Ron Reda

    Ron Reda Cinematographer

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    Rick Radford,
    Now, I'm not making any promises, but you really can't go wrong for less than $10! [​IMG]
     
  16. Rick Radford

    Rick Radford Supporting Actor

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    Since I'm on a roll, I think I'll also try this RS product for $1.99 (referenced by Marc H).
    Stand by for more reports!
     
  17. RicP

    RicP Screenwriter

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    Rick, don't bother. It won't work in your case. Incidentally, the part you linked to is not what MarcH was talking about. He's talking about the exact same thing that I am...the Jensen Transformer or equivalent.
    Trust me, I've been down the exact same road you're traveling now. [​IMG] I was right about the RF filter because i tried it, I tried the DC Blocker too, and it didn't work either. It's not meant to be a Ground Loop breaker. It's meant to block dc that's used for remote controlling.
     
  18. Marc H

    Marc H Second Unit

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    G'day gentlemen,
    This is the ultimate solution:
    [​IMG]
     
  19. RicP

    RicP Screenwriter

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    Yep that'll do it. BTW what's the insertion loss on that particular one Marc?
     
  20. Marc H

    Marc H Second Unit

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    I'm sorry but I really don't know the loss. There's no specs or packaging included.

    We buy them in bulk from Positive Marketing here in Canada for our Custom install department.

    You should be able to find them through any custom install place, dish installer or cablevision company.
     

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