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Intercom Troubleshooting

Discussion in 'Question of the Day' started by Andy_Leviss, Mar 5, 2008.

  1. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    I ended up going out on a service call to a client's theatre this afternoon to troubleshoot some "humming" on their intercom system. It was an interesting enough solution, with both some fairly common and unusual aspects to it, so I thought I'd post it here for everybody's benefit.

    So, here's the setup. They've got a simple single channel intercom system, with a power supply station (in this case a Production Intercom PS-2, although this could equally as well have been a ClearCom system, since the systems are interchangeably compatible). Three single-channel beltpacks are chained on the first channel of the power supply, and the other two channels are unused.

    The client was reporting "buzz" in the system, any time they tried to use it unless a mic was keyed. If the stage manager turned her mic on, the buzz went away, but only at a very specific (and too low) volume setting on her beltpack, turning it up loud enough so she could hear anything resulted in what she described as a "squeal".

    When I listened, the "buzz" sounded like a fairly low pitched hum, but higher in pitch than ground hum. The "squeal" was still a hum, but noticeably higher in pitch (maybe around 200 Hz).

    The power supply metered out both proper voltage (24V between pins 1 and 2) and termination (4kOhm between 1 and 3).

    There was one other symptom that I noticed in testing, but that the client did not notice, and would not in their usage of the power supply station, but that did help in narrowing down the problem. If nobody comes up with it in the next day or so, I'll post it, but in the meantime, see if you can either solve the problem without it, or at least start down the road on troubleshooting it and find this symptom on your own. Feel free to ask things like what happens when you turn X on, push button Y, jiggle cables, etc and I'll post back.

    --A
     
  2. TimMiller

    TimMiller Well-Known Member

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    I have had plenty of buzzes come up when lights are dimmed and things like that. I have also ran into an occasional weird problem when people try to talk on the base station. If the belt packs are metal, i have run into other problems of buzzes when you place them on a spotlight or the audio console.
     
  3. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    In this case, it was not dimmer buzz coming in on a ground, although that is a common problem. ClearCom systems are especially notorious for being susceptible to ground hum, particularly since they have a blazing "pin 1 problem" that's a headache w/r/t buzz in any pack I've ever used (maybe finally fixed in the new generation of packs, I'm not sure, but definitely still bad in the RS-501 generation). And, as Tim pointed out, it's far worse with the old style metal packs. In this case, the packs are Production Intercom's BP-1, which is a painted (or otherwise coated?) metal case with rubber feet on the bottom.

    This time, it's definitely not lighting/ground related, as it's constant no matter what state the lights are in, and is not at 60 Hz/a harmonic thereof.

    I will say that an additional problem I found in this system, although not the primary cause of the problem I described above, is something that is one of the biggest culprits in ClearCom/compatible ground hum issues. Bonus points for anybody who knows/can figure out what that is.
     
  4. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    Here are a couple posts that got lost in the database crash, but that I found copies of in notification e-mails in my inbox:
    Getting very warm. There were a number of cables in the system that had pin 1 jumped to the shell, but this was not the problem. As for termination, there was no switch, and the power supply metered proper termination coming out of it.
    Yes.
     
  5. David Ashton

    David Ashton Active Member

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    OK what was it, it has exactly the symptoms of a termination failure but it's not, so the answer is?
     
  6. harry1989

    harry1989 Member

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    That has happened to myself as well with an intercom with two beltpacks. I had used a similar system in the city with no problems which meant that it was something to do with our intercom system. Was like this for a year until I became Lighting Designer for last years musical. (Lighting Designer was usually seen as the lead of the crew at that point (due to the amount of work they had to put in). Anywhos... I was sick of programming cues so I decided to set up the intercom system for the spots (as it isnt kept up all year)... and decided I would actually have a look at the base station.

    Lo and Behold... A Gain button hidden at the back. Pressed in. Did a few tests with two headsets and beltpacks and figured that is what caused the squealing. (feedback from the headset mic to the headset "speakers").

    To cut a long story short...see if there is a Gain button/option which has been enabled...causing the squeal. (Yeah I know I coulda just said that...I just wanted to tell a story lol)
     
  7. David Ashton

    David Ashton Active Member

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  8. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    Here's a bit more to go on, before I give you the solution. As I went through testing, and dug the PSU off the shelf that it was buried on, I noticed that the station, while not having an audio i/o, does have a call light, and that the call light was not lighting up when the beltpacks were signalled, although all of the beltpacks did light up properly.

    That should give you the key to figuring the problem out, but if not, I'll post back before the weekend's over with the rest of the solution :)

    To summarize the symptoms so far:
    -System is showing indications of an unterminated system, but the PSU is properly terminated
    -Call signal is not working properly on the PSU, although it does throughout the rest of the system.

    --A
     
  9. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    So, here's the solution. As those of you better versed in the inner-workings of ClearCom know, the call signal travels on Pin 3, the audio conductor. Since the PSU wasn't showing call signals, this was a big indicator towards finding the problem. Disconnecting the system, and checking with a single beltpack connected by a new cable directly to the PSU confirmed proper call light operation, so the problem was somewhere else in the system.

    It could be further narrowed down by the fact that (save the oscillations) audio and call signal did work properly everywhere else in the system, so it was likely a problem in that link from the PSU to the first beltpack. Indeed, it turned out that the lead to pin 3 had broken inside one of the connectors. Since the termination is on the audio line, that meant that the rest of the system was unterminated, resulting in the oscillations noted above.

    Had it been an active base station, rather than a PSU only, it would have been easier to find, since the lack of audio at the base station would have been a bigger red flag, but since the PSU doesn't output audio, that symptom didn't show up. Anywhere else in the system, pin 3 lifting would have caused interruption of audio and call signal as well as the oscillation, so it would have been easier to narrow down, but since it was at the front end of the system, it was a bit trickier!

    --A
     
  10. tech2000

    tech2000 Active Member

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    I have a problem with the clearcom intercom which seems to be in the same area as the problems previously stated in this thread.
    When the main station is on, sometimes there is this really loud buzzing/grinding sound that will not go away. When either the talk or call buttons are pressed and held, the sound stops but starts again when the buttons are let go. The only way to stop it is to turn off the main station, however it takes a few tries to get it to stop.
    This caused the theater I work at to just go out and buy 2 way radios to replace the system. Is there any way to fix this problem, or what could the cause be?
     
  11. napoleond

    napoleond Member

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    This is my first post. Love the forum. That said:
    One thing that I've done in my space is construct a small, centrally located patch bay (8 circuits, 4 A channel, 4 B channel.) Each circuit to a different location - Stage, Spots, Orchestra pit, Dimmer room. If (read when) I encounter a problem I can pull circuits one at a time to find the general area of the problem (the stage circuit has 5, 2 channel boxes attached.) It saves a lot of headaches.

    I decided to construct it after coms went down 5 minutes before a show and 4 trips to the spot booth. 144 steps at a dead bolt. Again, it saves aggravation and shortness of breath :)
     

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