Low voltage paging systems, AM radio Interference

[jet122]

We have installed a 24 volt paging system from Valcom in a construction trailer complex. There a numerous trailers hooked together creating a single large room comprising 63 speakers. Each speaker has a 2 pair of cat 6 4 pair cabling in which 1 pair is the audio tip/ring and the other pair is supplying the 24 volts. We have a large antenna tower 4000 yards away. The issue is that we have a AM radio station bleeding over onto the paging system. It is still there when we remove the tip and ring but leave the power lead on. We have tried the following.
- using shielded 24 gauge. Leaving the shield alone, grouding 1 side, grounding 2 sides
- grounding the power supply in the closet
- grounding the individual speakers
- placing RF filters in line
- using a power conditioner between the 110 outlet and the power supply,& control unit.
We have contacted the speaker manufacturer, Valcom, BICSI, APC and Powerware UPS without any success. We are contiplating trying to use a 70 volt paging system. That will be a expensive solution. Do you have any insight as to what a solution would be?

Thank you,
Jeff Toia, RCDD
Texel Corp.
703 709-2955
jtoia@texelcorp.com

[ July 14, 2003, 11:43 AM: Message edited by: jet122 ]

 

[j_braddock]

Re: Low voltage paging systems, AM radio Interference

jeff,

i am assuming that the antenna 4,000 yds away is the am radio antenna.

several things come to mind.
1. the bandwidth of the radio station matches the bandwidth of your system. if so, get another system outside the bandwidth of the radio station. you could also send the unit to an electronics repair shop and see if they can tighten the bandwidth filters.

2. the radio station is emmitting a signal over the grounding system of the utility company. some of my family have told me that they did this on a small scale in order to get around the license issue in the early radio days. this shouldn't happen if this is a commercial station.

3. the radio station fundamental signal could be off / the harmonic frequencies are interfering with your system.

hope this information helps.

 

[ron]

Re: Low voltage paging systems, AM radio Interference

Are the circuits installed in raceway? What type?

 

[dereckbc]

Re: Low voltage paging systems, AM radio Interference

First thing is to determine which line it is entering on, either the 24 VDC or signal line. Sounds like you may have done that already by disconnecting the T/R which indicates power. What I cannot tell is where you disconnected the T/R line.

If it is entering the power line the RF is being demodulated in the rectifiers and riding on the DC (oscilloscope will verify). Solution would be to install capacitor filters across the DC line periodically to short out the RF.

Grounding the cables, power supply is only likely to increse the problem.

On the other hand it could be brute force RFI entering all the cables. If that is the case then as Ron was eluding to using metalic conduit may help.

At any rate you have a common mode noise problem.

 

[leshutter]

Re: Low voltage paging systems, AM radio Interference

Assuming that each of these amplified speaker sets is getting 24volt power from a single source and assuming that with the t/r lines disconnected at the remote speaker u still get AM interference,
FIRST...Take all four conductors right before they enter the amp(over-all Jacket off of course) and wrap them around a big old nail(Iron) as many times as you can. Radio shack also sells a split ferrite core for wrapping wiring around(one or two turns).
SECOND....Place a .1uF/100V disc ceramic capacitor accross the input terminals of the amp from V+ to V-.
THIRD....IF you still get RF put cap accross the terminals from Tip to V+/Tip to V-/Ring to V+/Ring to V-
If it works do it to all of them.
If it don't the problem is the Amplifier design for the speakers...Cheap engineering

 

[hbiss]

Re: Low voltage paging systems, AM radio Interference

Maybe a stupid answer but did you ever consider asking Valcom about your problem?? That's the first thing I would do. It's their equipment and they should know it better than any of us. I understand their tech support is top notch.

 

[hbiss]

Re: Low voltage paging systems, AM radio Interference

Scratch that. I reread your question and see that in the list of people you contacted, Valcom actually was one of them.

In my experience Valcom is a good company, but I never did like the idea or see the necessity for powered paging speakers. Valcom is geared towards the telephone industry hence their products are designed to use wiring and methods telephone techs are familiar with. Nothing wrong with that except reinventing the wheel so some people won't have to learn something new or buy from another distributor has its drawbacks as you have found.

The tried and true (and simplest)way to do a paging system like this is a distributed 70 volt system. One amplifier, 67 speakers and some 18ga wire. I can't see how this would cost more than that Valcom stuff and it's much more immune to RFI.

Everybody here gave you some good suggestions but if you are still banging your head against the wall the only way to solve this is to replace it.

 

[hurk27]

Re: Low voltage paging systems, AM radio Interference

We have a very simular problem with phones in one subdivision that is very close to a tower (1460 am) the problem is the radio station is getting into some of the amplifiers in the phone equipment and the only thing I found that works is phone filters for RFI across the mic input inside of the phone. sounds like this paging system uses the same princables as a phone so these filters might work but they have to be as close to the main amp as possable. also if you can determine which line it is comming in on then install a filter/trap on it too.

 

[hbiss]

Re: Low voltage paging systems, AM radio Interference

Hurk, I don't know what filter you used but I am totally against installing anything inside of a phone. Only the manufacturer has any business being in there. For one it voids the warranty, it will void any UL and FCC approvals, could cause a shock or fire hazard and besides, who knows it's in there. The customer replaces the phone with something else in a few years and now they suddenly have an RFI problem.

If you have any kind of RFI problem with a phone or phone system the first place to look for help is Mike Sandman. (sandman.com) He offers a large number of not so common items for the telecom industry. Among them is a "system" of RFI elimination filters that he claims will work in 99% of all cases. If you diagnose the problem properly these will fix it. He has a complete diagnostic flow chart and a tech bulletin in his catalog as well as I beleve on his website.

These filters go outside the equipment although he does talk about an NEC service bulletin that authorizes installing capacitors inside the phone across the dynamic microphone element.

 

[ronaldrc]

Re: Low voltage paging systems, AM radio Interference

Could cause a fire or shock hazard? Hum

That use to be common practice around here.

 

[hbiss]

Re: Low voltage paging systems, AM radio Interference

Yes, all communications equipment that connects to the outside network is tested by UL for certification and also to meet FCC specs. Once certified, the manufacturer cannot change the design in any way without having to have the product recertified.

Basically, it must be designed to withstand a certain level of surge voltage from power crosses and lightning without causing a hazard or a danger to the user. It doesn't have to survive, but it can't burst into flames, burn or shock someone using it.

In order to meet these specs the designers use flameproof components and circuit boards, fuses, MOVs, spark gaps, isolation and other measures at critical locations. Also, lead dress and component location can be critical to prevent arcing and a fault to something like metal case parts or the handset during a surge or overvoltage episode.

Obviously then, "unauthorized" components, modifications and unauthorized service can cause serious safety problems and would certainly void any UL certification.

That said, we are primarily a telecommunications company. (I am also an electrician from many years before.) Often our customers will spill soda or coffee on their phones or otherwise beat the hell out of them. We regularly disassemble, clean and reassemble phones that can be salvaged. These are usually "system" phones, those that operate behind a KSU or PBX and are not subject to the same safety requirements as a phone that connects to the outside network. Even so, we are limited to taking them apart, maybe swapping boards, displays and mechanical or case parts with another phone to make one that works or looks good and putting them back together. We would never replace or add components or wiring.