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Signal conflict?

Discussion in 'Electronic Repair' started by mm, May 7, 2010.

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  1. mm

    mm Guest

    What should I do about signal conflict (overload?) between two
    sources, both on co-axial cable? Is there an easy way to weaken a tv
    signal in a co-axial cable?

    I have two inputs to my maybe 10-year old analog tv, selected with an
    A-B switch, with co-ax input and output. One input A is from the
    2-year-old DVDR with digiital output and an RF modulator in the other
    room, and the other input B is a 1-year-old set-top box atop the set.

    In the last week, when I am watching in the A setting and slide the
    switch to the B setting to use the set-top box, there is an image for
    a split second, and then the screen turns all blue. But when I
    disconnect the A signal from the A side of the switch, the B side
    works fine.

    (This doesn't happen in reverse. The A signal will display fine when
    the switch is on A, even if there is a B signal connected to the

    I've concluded that the A signal is combining with the B signal to
    cancel both signals, or at least to change the signal such that the
    static suppressor in the tv stops trying to display a picture and just
    displays the blue screen.

    When I disconnect the B and the A signals from the switch, it still
    shows the A signal pretty well, I guess because all three co-ax cables
    run side by side and the A signal must be stronger. Do you think the
    signal is going right through the braid of the co-ax, or is it
    sneaking out through the open end of the unconected F-connector?

    Another clue is that I first noticed this a week ago, very soon after
    the old RF modulator failed and I installed a Belkin brand RF mod. The
    first RF mod lasted only two years. Maybe Belkin is a better brand
    with a stronger output? Except now it is too strong for this setup.

    Is there an easy way to reduce the signal strength of the A signal, or
    can I get an A-B switch with greater separation? I"m using an RCA
    brand (though identical switches are sold with other or no brand)
    pretty compact A-B switch with a flat top and bottom that I like,
    because it is stackable with more A-B switches. (At one point
    somewhere I needed two of them.) I could change to another brand, or
    change to a remote controlled Radio Shack A-B switch. Would that be
    likely to help?

    My original plan was to output one signal on channel 3 and the other
    on channel 4 and use the tv remote to change channels, but iirc there
    was interference between ch 3 and 4, and one or both didn't come in

    Then I tried just using a splitter to TV channel 3 and making sure
    only one signal was on at a time, but one of the signals was too weak
    iirc to work this way.

    Then I got really cheap a device that, when a video game is on, it
    takes the video game for input, and when the game is off, it takes the
    other input. Unfortunately, this switching seemed to depend on some
    sort of DC voltatage present in video game output, that isn't present
    in either the set-top box or the DVDR output. (Does that sound right?)

    So the A-B switch seemed straight-forward, but now this. :)

    What would you guys do? Thanks.
  2. N_Cook

    N_Cook Guest

    Have you tried changing one or more of the modulator frequencies? usually a
    recessed screw head near the connections, but put a sleeve over the
    screwdriver before inserting .
  3. UCLAN

    UCLAN Guest


    Two choices:

    1) [Best Choice] Get a programable modulator that lets you choose any channel.

    Set it to a much higher channel. Tune directly with with your TV tuner. Use
    signal combiner for A and B signals. Trash A/B switch.

    2) Get a better A/B switch.

    Or do it all with:

  4. mm

    mm Guest

    Wrong sig! Sorry.

    I just looked on the settop box and didn't find any screw. Not on
    the RF mod either, but it has a slide switch for 3 or 4.

    The on-screen Settings for the set-top box only permits 3 or 4. At
    least afaicr. Before I buy what UCLAN suggested, I'd better check
    again. Yes, only 3 or 4. This was a fairly expensive one, Dish TV,
    70 dollars (coupon plus 30 cash. It has features the others don't.)
  5. mm

    mm Guest

    Wow. At first I didn't like this because it meant spending more money
    than a switch would cost, buying a whole new device, but A) it's only
    aobut 50 dollars, B) one device would solve the problem at EVERY tv
    with a set-top box. Right now there is only one, but I have two more
    boxes to be connected. (I always imagine that something I've
    never bought costs a lot of money. I was amazed when I saw how cheap
    plexiglas and lexan are.)
    This is only about 5 dollars more. I have to read the manual more
    closely and also think about if I'm ever going to have a second
    source. Or just assume I might someday and buy it.
    Hey, this is even better. This one has 2, 3, or 4 inputs, depending
    on which model one buys, and costs 30 dollars more (78 to 100) for the
    one with 2 inputs, and 102 to 120 for the one with 3 outputs**, 125 to
    145 for the one with 4 inputs.

    **(Geekbro has it for 184 dollars. You really have to be careful you
    don't overpay. I came across a digital camera last month whose price
    ranged from 80 to 280 dollars, same model, none of them

    And yet they look like the first one you gave. The sketch in the
    manual reminds me that maybe I will have satellite some day, plus the
    dvdr (which has an antenna), and with a VCR too (although I would have
    plugged that into the DVDR, it's good to be able to plug it into this)
    and have those 2, or 3 or 4, inputs on separate channels, and still
    just use channel 3 for the settop box whereever there is one.

    I'm never going to have a camera at the front door, but one of these
    is still the best for me.

    Thanks a lot.

    Even though now it's almost obvious, it didnt' occur to me that they
    would make an RF mod with a different channel output, much less an
    almost any channel output, and even multiple inputs, and even at an
    affordable price. Wow. I don't think I came across these when I
    was googling for RF mods, but if I did, their names or blurbs didn't
    explain why I would spend the extra money.
  6. Jamie

    Jamie Guest

    put a attenuator in the line ?
  7. mm

    mm Guest

    Thanks. I didn't know for sure they had something like this.

    I have saved your post. I may need one later.

    But I also didnt' know about RF modulators that would output to almost
    any channel.

    That will solve my problem both at this tv and at any other one in the
    house if I connect a set-top box. And I'll be able to change inputs
    by using the tv remote, which I already have, instead of getting up to
    switching with an A-B switch, or buying one that works remotely.
  8. mm

    mm Guest

    Thank you for replying. I had to read this a couple times to
    understand and that's part of the reaosn from my delay in writing.

    Now, which is better? Should I just say thank you or say that I had
    done this before and it didn't work. I think the signal was too weak
    from one or both sources after having gone through the mixer/splitter
    Yes, a second box, but only for another tv, so still only two signals
    at most at any one tv.
    Okay. That is helpful. Thanks again.
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