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AM radio problem on telephone wiring

Discussion in 'Electronic Repair' started by [email protected], Feb 20, 2005.

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  1. Guest

    I'm going on a service call next week to address an issue of AM radio
    interference on a telephone line. The lady has her computer connected
    to this line and it seems to keep getting bumped off especially
    throughout the day. There is a 5KW AM transmitter operating on 610KHZ
    about a quarter of a mile from the customers home. At night they drop
    their power to 1.0KW and she tells me the interference decreases. She
    has tried filters purchased from Radio Shlock which were marked ".50MHZ
    to 3.0MHZ but apparently they did not help.
    The other day while discussing this with her on the phone, (she called
    me from her work so I have never heard the interference), she told me
    that someone had suggested that she plug her modem dierectly into the
    interface on the side of the house. The length of cord used was
    approximately 15 feet. When she did this she found that there was
    apparently no interference problem. I'm suspecting then that if the
    line is clean up to the interface, then the telephone company has
    perhaps already addressed the close proximity transmitter issue in her
    neighborhood using sheilded wire and perhaps filters on the street and
    that her problem is the straight untwisted wiring throughout her home
    which may be acting as an antenna.
    I've considered running a new sheilded wire from the interface,
    grounded to the telephone company's ground, directly to her computer.
    I would try that first with no other house wiring connected.
    In theory if that were the only line connected then that should work.
    However, I would suspect that as soon as I would connect her existing
    house wiring back up to the interface, (essentially reconnecting the
    "antenna") the problem I would suspect would return.
    Should I plan on completely rewiring the house with sheilded or does
    anyone have any another ideas they might suggest? Thanks very much for
    any assistance. Lenny Stein, Barlen Electronics.
  2. There's no reason why the modem needs to be near the computer. Put the
    modem physically very close to the telephone demarc. Run a long RS-232
    line and (if necessary) a long power cord.

  3. Graham

    Graham Guest

    Yes I think your game-plan is sound Lenny

    The demodulation of the AM carrier may not be happening within the modem, it
    might be happening within a telephone or other device connected to the line,
    even while that device is 'on hook', so start by disconnecting everything
    except the modem.

    Also try another modem.

    You might like to try a filter designed for use with DSL installations, I
    should imagine these would also be good for rejecting external AM


  4. Bill Jeffrey

    Bill Jeffrey Guest

    Lenny -

    The RF is being demodulated somewhere in the house, and I suspect it is
    the demodulated audio that is interfering with the modem. One thing you
    might do (though it's time-consuming if done on a service call) is run
    around the house and disconnect every device on the telephone circuit,
    one by one, and see when the interference quits. Or disconnect 'em all
    and then put 'em back one by one. You may be able to identify which
    specific device is causing the interference.

    BTW, if the audio level is high enough to interfere with a modem, it is
    probably high enough to hear on a telephone handset. Do you have one of
    those lineman's test sets so you can listen to the line even when you're
    on hook? That would enable you to determine instantly what effect you
    are having.

    If it turns out that the modem itself is reacting to the presence of RF,
    you might be able to construct a series-tuned LC filter and put it
    directly across the line as it enters her modem. It should be a much
    sharper (and deeper) filter than the Radio Shack jobbie.

    Where did she put the Radio Shack filter, by the way? As close to the
    modem as possible?

    Or, as Isaac suggested, just put the modem near the service entrance, if
    that really makes a difference (which would surprise me, by the way, but
    the lady thinks it does, so ...).

    Bill Jeffrey
  5. TimPerry

    TimPerry Guest

    Pick up a handful of inline filters at R shack. Add one at the service
    demarc for each branch and one at each device. Inspect the ground at the
    demarc (if any). Place a filter on each unused telephone jack.

    Pick up some RFI filters that plug into wall outlets. Place one on each
    telephone device that uses external power.
    Be sure to put on the modem. Get a ferrite RFI suppressor and clamp it to
    the power lead of the modem. Place it as close to the electronics as you can

    If the house telephone wiring is not twisted pair go ahead and replace it.
    (all of it, not just the run to the computer).

    If the transmitter is a directional array (2 or more towers) the stations
    engineer will have a field strength meter. The RF field can be measured at
    that location. You may be dealing with 2000 mV of RF or more.

    I once had a guy who run unshielded speaker wire throughout his house. A
    switch selected pairs of speakers in various rooms. the field was so strong
    that music came out his speakers even when the stereo was turned off and
    unplugged. a pair of low pass "L" filters at the amplifier took care of the
  6. All the other replies seem to be addressing a voice-grade modem being
    used here.

    It's _not_ xDSL - is it?
    If so, filtering the line to the computer (to the DSL modem) is _not_
    one of The Things you want to do.

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