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DuKane School Intercom

Discussion in 'Electronic Repair' started by RootBoy, Oct 25, 2003.

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  1. RootBoy

    RootBoy Guest

    I have a school intercom, a Dukane MACS II system that has a problem.
    Two class rooms where added and in one of the class rooms only the
    office can call in to the room but the teacher cannot be heard. All
    other classrooms work fine. I have test all the wires and replaced all
    the parts and swaps channel at the PA, and no change.
    Can someone explain why the VOX doesn't work with this one classroom.
    Also what activates the switching of the VOX system at the ACS
    (console unit).
  2. I am going to "guess" because I am not familiar with the particular
    unit. But if there may be a form of "anti-vox" circuit so that while the
    office is talking, the classroom VOX won't activate. So if there is any
    form of noise on the wiring, the anti-vox is activated. You did use
    balanced, twisted pairs for the wiring? Maybe one lead is shorted to
    ground. Is there any noticeable hum or buzz? Dis you swap the units in
    the classrooms? Could also be a level or sensitiviity problem.
  3. Mark

    Mark Guest

    Exactly how were the two new rooms added? They must be home
    runs, and must use 70V (or 25V) speakers. More details on
    what you have, and have done, would be helpfull.
    IIRC, the master uses VOX only in an off-hook state, in
    which case it's simple audio sense/switch. If the master is
    being used hands-free, it's PTT.

    The truth as I perceive it to be.
    Your perception may be different.

    Triple Z is spam control.
  4. RootBoy

    RootBoy Guest

    Thanks for the responses.
    The classrooms are in a mobile unit install over a year ago.
    The two cables where install from the main office to the
    Mobile unit with no splices and are aprox. 450 feet long.
    The cable is four conductor with shield. The white and shield
    Are connect to the normally open call button and the black and red
    are connected to the speaker, 70V, and lowest wattage tap.
    I have used a meter to check for any shorts or grounds there are none.
    Both cables test the same. In one room the system works, the other
    room does not.
    I have swapped the speaker and used different combinations of wires
    including the green that is a spare. The results are the same.
    Iswapped connects at the head end to a known good input and the same
    results. The office gets the call from the classroom
    and answers. The teacher in the classroom can hear the secretary but
    the secretary cannot hear the classroom, no background noise, nothing.
    All calls are made and answered with the receiver off the hook.
    Thanks again for the help.
  5. Mark

    Mark Guest

    Well, that is a tough one. You've done all the right things
    to troubleshoot, and it sounds as if the rooms have been
    added correctly.

    My hunch is that one of the audio pair to that trailer
    (red/green) is shorted to Earth, NOT necessarily to shield.
    It has probably occurred where the cable enters the skin of
    the trailer, but could be in the main building as well.

    Since the trailers are probably in close proximity, get
    ahold of any two-conductor (even an extension cord would
    work temporarily) and with some jumpers, connect the "bad"
    speaker to the audio pair of the "good" speaker in the other
    trailer. If you get two-way audio, you'll know you need to
    pull new 357 to the "bad" trailer.

    Another alternative, since this is configured as a three-
    wire system, is to substitute the green wire at BOTH ends
    for the red, then the black, and see if that resolves the
    issue. If it works, you may be able to leave it that way if
    the noise level isn't too high, as the green is unshielded.


    Just had another thought. You said you checked for shorts,
    but did you check for opens? Disconnect both red/green at
    both the speaker and the head end. Read across them from
    either end, you should get infinite Ohms (if not, they're
    shorted somewhere). Now twist one end together and read
    across them from the other end, you should get something
    less than 10 Ohms. If not, obviously an open conductor.
    The most likely place will be at the splice in the trailer,
    which should be either at the switch or at the speaker.

    And yes, it is still possible to get some one-way audio on a
    system such as this with one conductor open.


    The truth as I perceive it to be.
    Your perception may be different.

    Triple Z is spam control.
  6. BOB URZ

    BOB URZ Guest

    1# To test 70 volts lines, your need an IMPEDANCE meter and NOT
    a DVM. AN i impedance meter puts an AC signal down the line to test
    its AC load. Some (like a TOA ZM104, use a 1000 HZ tone than can be
    used as a trouble shooting too by listening to the speaker.

    There are basically two kinds of intercom. Annunciator and non Annunicator.
    On a annunciator unit, pressing the classroom button lights a light on the
    main console on which room it is that's calling. On a non annunciator unit,
    any class
    room can call in, but there is no ID at the main console. They have to say
    "its room 205" or such. Then room 205 has to be manually selected at the
    main console. If its NOT manually selected, the office will NOT be heard in
    the class room.

    What i would do is remove the speaker and take it into the office close to
    the main unit and temporarily wire it in. trouble shoot it from that point.
    Does it work now? if it does, you have a wiring problem. IF not, your
    either not hooking up the correct terminals on the main unit, or the main
    has a problem or is NOT internally configured for that many stations.
    If you had a Toa ZM104 you could put a 1000 HZ tone down the line from the
    office, listen to the tone in the classroom to verify the wiring, and do a 70
    volt line load check at the same time. If you do this kind of work, its
    highly recommended.

  7. RootBoy

    RootBoy Guest

    Thanks for the help.
    There was water in the underground
    conduit that connects to the mobile
    classroom to the main school building.
    Water migrated in to the cabling creating a
    high impedance that placed the classroom
    In "privacy mode". Replacing the last portion
    of cable in the conduit and under the mobile
    classroom fixed the problem.
    Thanks again,
  8. Jason D.

    Jason D. Guest

    How do you determine that there was high impedance? Replaced the
    conduit? Hope you do want no water there or you'll be there again
    next year.


  9. RootBoy

    RootBoy Guest

    Checked the cable with a DVM but a a higher range (resistance), the
    DVM is not auto ranging. After I replaced the cable I physical
    inspected the old cable and found water in the cable and the shield
    was blacken.
    The new cable is a temp fix, new direct burial cable is on the way,
    this should fix the problem.
    Looking into getting a impedance meter.
  10. gene


    Jan 6, 2011
    have you checked the station card for a bad port or checked to make sure that you did not use a arch number already assigned.
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