Filter to remove AM radio from telephone lines

Grab a ferrite clip around filter.

Have you tried using a DSL filter? They already come with male and female phone connectors. They sell for about a dollar each at the electronics surplus stores in Silicon Valley, and usually less than $5 at regular retail stores (although I have seen them advertised for over $20). You may need one for each phone.

"Homer J Simpson" wrote in news:WqYYg.17305$P7.12287 @edtnps90:

Does anyone know whether a ferrite RFI filter like this (the kind that goes around a cable) also attenuates voltage spikes?

Seems I recall that a typical spike lasts about a microsecond, so the fequencies involved would be a megahertz and above, so it seems like it might work. (Just thinking about adding them to my surge protectors.)

I haven't tried one because this is the first I heard. I'll look for one. 90% of my phone calls are from my desk phone, so that ought to be enough.

Strangely, I rarely hear the radio when the conversation starts, but if I talk for 30 or 60 minutes, it starts and gradually gets louder and louder until I can recognize what is said (and know what station.)

Also it has stopped for a couple months for some reason. And then later for a few days. But when it restarted up again the second time, I thought I should look into curing this.

Remove NOPSAM to email me..

I may see these at hamfests. The last one of the season is this Sunday. I'll look.

Thanks to all.

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Contact the engineer at the station. They will help you. Note that it may be your phones, not the wire.

Geoff.

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IL Voice: (07)-7424-1667  Fax ONLY: 972-2-648-1443 U.S. Voice: 1-215-821-1838 
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Easiest is to find a DSL line filter. They're just the right specs for killing AM band signals. Plug it in series with the loudest phone.

If you can't get one of those, try a 0.01uF to 0.05uF capacitor, across the red and green wires. You may need one at EACH phone jack.

Way back when I used to live in the shadow of a 50KW AM station, it took a pair of capacitors across the phone line, with 2.5mH chokes in series, to eliminate the background chatter. But you can probably get by with less than that.

Unless that radio station is very close to you, the problem is likely caused by a corroded connection. You can filter it out, but if you can find the connection and clean it you'll solve it too. One problem with this is that the problem may exist internally in one of the phones. But you can track that down easily by unplugging phones.

This happens all the time with music equipment. Poor connections on cables start tuning local radio stations. All you really need to make an old fashion crystal radio is a diode. Corroded connections can do a fine job at this. Then the capacitance and inductance in the rest of the amplifier functions as the tuning stage.

In 1975 when I moved my bussiness to a building a quarter mile from an AM station we could hear it on the phone. The phone co. sent out people two times and put various filters on the line all to no avail. Finally on the third visit an old timer who looked about to retire came in, put a small mylar type cap across the ear end terminals in the handset. No more problem, not ever on other extensions when they were put in.

Chuck P.

This was a standard fix back then. I also probably saw it in one of the ARRL handbooks. That was the first thing I was going to say. Ferrite filters on the phone lines, handsets, power cords, should help. I would put an MOV across the phone line at the service entrance. Actually one on each line to ground. Good power surge suppressors have inductors and caps after the MOV's to filter RF and the spikes caused when MOV's are in action.

greg

Bound to. Even plain wire will do that and the filter will do it more.

I lived one street over from a 27.5kW (ERP) FM radio station in suburban Cleveland in the early 1970s; I could see the station's tower from my third-floor bedroom window at night. The signal didn't get into the telephones in the house where I was living at the time, but I was hearing the station darn nearly everyplace else--on my TV sets on channel 6, between local stations on an FM/stereo radio (cheap offshore brand), even in an Ampex Micro 88 cassette tape recorder my dad had. I didn't know much about this type of interference in those days, much less how to deal with it, but I lived in that suburb only three years, so I didn't have to tolerate the problem very long. I now live in a small town located about five miles from a 1kW AM station. The station's signal doesn't get into my telephones or anything else electronic in my apartment (except my AM radios, of course), but during the day I can hear the station at two points on the AM radio dial--1460 kHz (the station's fundamental frequency) and 560 kHz, 0.9 MHz (900 kHz) away. The problem is still there at night, but less noticeable as the station reduces power to 1kW after sundown.

I didn't realize that line filters such as are used on telephone lines carrying DSL Internet service (to prevent the DSL signals from interfering with regular telephone usage) could also be used to filter out AM broadcast interference. You learn something new every day, I guess, which is one reason I read these forum posts all the time (they arrive each day in my inbox in digest format).

Every now and then I can hear another cordless phone when I am using my own SBC 900-MHz phone, and I even get interference at times, not very often, that sounds like crossed wires on my corded phone as well. I guess that's one drawback of living in an apartment building; the fact that all devices (radios, televisions, cordless telephones, etc.) certified to comply with Part 15 regulations must accept any and all interference they may receive is annoying at times as well, but that's life in the 21st century, like it or not.

73 (best of regards),

Jeff Strieble, WB8NHV (email addy not shown to deter spammers) Fairport Harbor, Ohio USA

Those clip-around filters are really good at high-frequencies, but at the AM band they're kinda negligible impedance. You need something in the millihenry range, not the low microhenries.

Buy an audio or power transformer, strip it down and re assemble around the power line?

Are there any unused wires in the phone cables? Try grounding them. For instance,when they ran the phone wiring here,they used CAT-5..We had two phone lines at the time (one mostly used for a dial-up connection),so there 4 unused wires in the CAT-5 cables..I went out to the phone box,and connected the unused wires to the earth ground connection. It killed the bit of cross-talk we had and some of the noise,and the dial-up modem would connect at a faster speed. I even went as far as to connect the extra grounded wires to the unused contacts in the phone jacks at a couple spots,to use the "extra" pair of wires (yellow and black) in the (long) phone cord runs to act as a quasi-shield also.But that trick only works with 4 conductor phone cords,not the cheaper 2 conductor ones.

Thanks to everyone who answered. Your answer turned out to be on the money.

I had a phone, but found one at a rummage sale that displays the number I just dialed. Since I'm never sure, I started using thzt phone, but I guess I had the downstairs phones unplugged then, and didn't notice the AM radio interference then, because it didn't start until I plugged them back in. And when I did that, I didn't put 2 and

2 together.

A year or more later, the new phone stopped working! Maybe I dropped it too many times, but I can't hear anything. So I went to a Southwestern Bell Sleekline phone (shaped like a Trimline), where the interference problem was much worse. I didn't think that associated with any Bell company could be that cheap.***

After a week or two I changed back to the previous phone, made by Master, a brand I have never heard of (except for padlocks) and it has NO radio interference**. It seems one can't go by brand too much.

**(although there is the slightest hum or whisper when the downstairs phones are plugged in**. I changed one of the downstairs phones from Stromberg to some cheap phone and that didn't make any difference, but that is only one extension out of 3 on that section of wire. Not enough to worry about. I changed phones largely because the Stromberg needed a thorough cleaning.)

***I got the SWBell Sleekline, still looking brand new with the "features" sticker on it, at a rummage sale or hamfest for free or a dollar, and found that the Redial didn't work right, and I think even the Flash didn't work right, plus the AM radio interference. That's why it was cheap. I"m not complaining. It still works as a phone.

If you are inclined to email me for some reason, remove NOPSAM :-)

This seemed like a really good and easy idea also, and I was going to do it, but as the other post says, I found the problem when I had to replace the phone that caused the problem, after it broke.

Thanks.

If you are inclined to email me for some reason, remove NOPSAM :-)

I was definitely going to do this. I talked to a phone company guy who said that rather than get one from him, and pay for a half hour phone company labor, I could get one at Radio Shack! :) But they wanted 15 dollars. That's still sounds like real money to me. Especially if I might end up needing more than one.

And after all, this is a repair newsgroup, which to me always means replacing or adding the smallest part which will do the job.

So this was my first choice. Thanks, and thanks especially for being the only one to address my specific question, which was, What size capacitor would I use.

Even though the problem is solved at this time in this place, I'm going to get one or two of these out of my stock of miscellaneous parts, or maybe just buy a few from Mouser.

Does it matter if they are polarized or not?

Still I appreciate the suggested value for later, more serious cases that may arise.

Thanks.

If you are inclined to email me for some reason, remove NOPSAM :-)

ALL Electronics sells DSL filters for around $3 apiece.

Jerry