TV aerial socket resistance ?

Discussion in 'General Electronics Chat' started by Man_in_UK, Mar 14, 2010.

  1. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0
    My TV aerial connects to a 4 way amp outlet that is situated in the loft / attic. This amp feeds 3 TV sets around the house.

    I have been having signal troubles recently and decided to upgrade the amp to a better quality model as this is easier than getting to the aerial. This new amp has a short circuit fault protection indicator on it that warns if there is any wiring problems (I thought this was a silly feature, not so).

    Plugging in 2 of the TV sets shows a fault on the amp indicator. I took resistance readings on the cables but there is no short on any of them. I then found that there is 0 ohm reading on both TV RF input sockets. I guess this is not supposed to be. The last TV reads open circuit on the RF socket.

    I also suspect that my HD freeview recorder OR cable box has also gone the same way but I have yet to disconnect both to measure them seperatly. Both chained together in series reads 0 ohm. My poor signal problem appears to be caused by this short on the equipment.

    Is it possible that a fault with my old TV amp can have caused all RF inputs to go short ?
    Is this easily repairable ? (both TV's are recently new Samsung LCD's)
     
  2. Bychon

    Member

    Mar 12, 2010
    469
    41
    When I worked on T.V.s it was normal to show zero ohms DC resistance at the antenna terminals because the input was a tiny transformer. This means that having a "short" at DC frequency doesn't necessarily mean the TV is bad. However, if the amplifier doesn't like the DC short, you are looking for a device that blocks DC in an antenna cable. I know they exist but I can't remember what they are called. A way to test this idea is to connect a small capacitor (1000 pf) from the center conductor of your cable to the center conductor in the connector in the TV. The TV will probably work in this condition, at least enough to see it tune in some channels. It's a crude method, but if the amplifier suddenly quits complaining, you have found the problem and only need to find the DC blocking modules.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    I'd try one item at a time, testing with a new piece of cable direct to the amp if possible.

    It's possible the amp is designed to work with infra red repeater units to allow control of a central satellite receiver or dvd etc. from each distributed point, so the ouput cables may have power feeds to run the IR receivers. (eg. some SLx brand units).

    As far as the weak signal, check the condition of the coaxial cables.
    It's unfortunately common for aerial installers to leave the cable end unprotected so water can penetrate.
    If the copper of either the screen foil/braid or the inner conductor is anything but bright, clean copper the cable will be useless. If it's gone black for even a couple of feet, you will lose most of the signal.

    Note that DC tests with a meter are useless for checking coaxial cable quality, as you are then testing the bulk copper. RF signals rely on skin effect, only the surface matters for this.
     
  4. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0
    I managed to get hold of an ex-TV engineer who told me that I should not get a short on the input. He thought it could be damaged caused by a lightning strike. I'm not sure about this as 2 of the TV sets are measuring fine, it just happens that both goods set are old CRT types.

    The amp is a reasonable quality with (woooo) gold terminals and is described as an SLX with digital bypass.
    I don't use it with any remote switching. Have I gone and purchased the wrong thing for the job ?
     
  5. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0
    dataman19,

    Do you know what sort of cap I might find in a DC blocker ?

    Maplin do not show anything other than a lightning protector adaptor and I don't think that is what I'm after .... or is it EXACTLY what I am after ?
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    I spit out my pepsi.. I don't know why I found that so funny.

    Awww...jeeez...

    But yes, a lightning supressor (75ohm type) should block DC to around 1500Mhz. That should do ya.
     
  7. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0

    That is a very kind offer.
    I will see how much of a mess I make trying to fit a cap inside an RF plug, it will be fun. If it all goes wrong I will shout you back.

    Cheers
     
  8. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0

    At least it brightened up someones day.
    Usually I find that "surge protected" and "spike protected" items cost 3 times the price of normal items, and only have a 10p MOV soldered across the lines.

    Now I do know that will not stop DC volts.
     
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