Tracking down 900 Mhz Interference?

[Art Mulder]

There's a lot of bright technical people here so I thought I'd ask...

We have a pair of 900Mhz cordless speakers in our house. We've had them for ... I dunno, 2-3 years and they've given good service. We keep one in the kitchen up on a shelf, and one down in the basement (by the treadmill). It's a nice way to be able to listen to the main stereo without running wires.

Quite abruptly, about 4 days ago, both speakers started giving off a loud droning hummmmm whenever they are turned on. Curiously, the one in the basement seems to give the louder hum.

I have heard that wireless internet can interfere with certain cordless phones. However, we do not have a wireless router. I suppose it could be the speaker's broadcasting base station which has gone wonky? The stereo itself sounds find through the directly attached speakers, so I doubt that is the problem. I've tried turning off various appliances, freezer, fridge, my computer, just in case there is some feedback on the power line.

Anyone got any ideas? I suppose one of the neighbours could have bought a wireless router which is interfering. We do have cordless phones, but they are in the 2.4Gig range. I suppose it could be the line-out jacks on the stereo which are the problem.

puzzled,
...art

[David Epperson]

Just a wild shot in the dark here, but you might build an RF shield. Just a piece of aluminum foil with a ground wire attached. Move this shield around the speaker and/or base and see if it will reduce the hum when it gets between the source and the speaker or base station. I doubt that anything this Rube Goldberg would eliminate the hum, but it could tell you which direction the interference is coming from.

[Mike Henderson]

And, of course, it's possible that something failed in the speaker system and it's not interference at all. If the hum is around 60 Hz, it's likely something in the unit that failed.

If you didn't change anything that radiates in your house (like bring in a new wireless device or move an existing wireless device), I'd suspect the speakers themselves.

You didn't say how close the neighbors are, but 900MHz cordless phones aren't really sold any more (I suppose you could find one but it'd be tough) and the 900MHz signal doesn't travel that far.

Mike

[Art Mulder]

And, of course, it's possible that something failed in the speaker system and it's not interference at all. If the hum is around 60 Hz, it's likely something in the unit that failed.

In Both speakers (they're independent) at the same time?
Actually Both the speakers, and the base station, are DC powered via "wall wart" transformers (three in total). So I don't think they should be doing a 60 cycle hum.

If I had another stereo system (I don't) then I'd try swapping it to another unit. Maybe the hum is coming from the line-out on the stereo receiver.
Hmm, I should try it on my computer..... [short pause] Nope. I've eliminated the stereo from consideration. I plugged the base station into my computer speakers, and the hum is there again.

So that leaves, to my mind, either an external influence, or the base station itself.

[Leo Graywacz]

The wart transformer could have gone bad.

[Ken Fitzgerald]

Art...

Mike's suggestion of 60hz hum....very plausible. If the power supplies in the base station has failed or if a filter cap in the 900 Mhz amp/converters has failed you could get 60 hz hum or some multiple there of depending on the type of rectification used in the power supplies....generally 240 hz. Within amplifiers there are filter caps that prevent certain frequencies of feed back from happening between stages of amplification. If one of these fail it could happen. The fact that both speakers are experiencing the problem, I'd suspect problem in the base station or RF interference.

The suggestion about using an RF shield has some validity too!

Take one of the speakers. and place in very close proximity to the base system and make a tent out of aluminum foil, place the speaker inside the tent and ground the aluminum foil to the ground terminal on the stereo system. If the hum goes away it could be outside interference. If it doesn't go away, the power supply theory could be a very valid possibility.

JMHO.

[Jim Becker]

AYou didn't say how close the neighbors are, but 900MHz cordless phones aren't really sold any more (I suppose you could find one but it'd be tough) and the 900MHz signal doesn't travel that far.

900 MHz devices are indeed still sold...my Plantronics (Avaya) wireless headset is in the 900 MHz range...and I get about 200 feet of walk-around range with it with the base located in my current office which features 18" thick limestone walls and wire lathe under the plaster.
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Art, I'm also thinking that something is physically amiss with your wireless speakers although interference could certainly be causing what you are hearing.

[Mike Henderson]

So that leaves, to my mind, either an external influence, or the base station itself.
Sorry - I did mean to imply that the hum was coming from the base station - assuming that both sets of speakers were receiving from the same base station.

If two base stations, then it's unlikely to be power supply hum and more likely to be interference.

It might just be some type of failure in the base station that manifest itself as distortion in the speakers.

Turn off the base station (unplug it) and see if you get the hum or distortion in the speakers. If yes, then you probably have some outside interference. If not, I'd focus on the base station.

Ken's suggestion of putting the two units very close together is also good. That way, the desired signal should be very strong and it should be more difficult for interference to affect it.

Mike

[added note] Sorry, Jim. I meant to imply that 900 MHz cordless phones are no longer sold to any great degree. I'm not familiar with the frequency bands used by things like the headset. Cell phone headsets probably use Bluetooth which (if I remember correctly) operates in the 2.4GHz band.

[Jim Becker]

A little OT, but likely interesting to some...


Cell phone headsets probably use Bluetooth which (if I remember correctly) operates in the 2.4GHz band.

The headset I referred to isn't BlueTooth; it's a pro headset connected to my VPN enabled IP phone for work and is on 900 MHz...but BlueTooth is indeed in the 2.4Ghz band (http://www.mobileinfo.com/Bluetooth/air_&_band.htm). Given the range of BlueTooth is about 30 feet max (when you're lucky...) and very low power, interference issues are unlikely. (Although I've found it to be a lot farther outside as evidenced when I forget to shut off my hands-free device in the car and walk to the house)

Here's what my wireless headset looks like...its in the cradle to the right of the IP phone

74968

Completely unrelated, that Uniden phone on the right is DECT which has a dedicated frequency band and really great range. I've only had that system for a few weeks now, but it's the best I've ever owned.

[Art Mulder]

Here's what my wireless headset looks like...its in the cradle to the right of the IP phone

74968

Wow. Look at all those "leashes" that Jim has to wear... ;)

Thanks for the suggestions, folks. I'll try swapping the base station
wall wart transformer tomorrow, and also the idea of moving a speaker close to the transmitter. Hmm.

[Jim Becker]

Art, good buddy...everything is wireless at least!

[John Schreiber]

Art, good buddy...everything is wireless at least!
That's technology for you. Now our leashes are invisible, but always around our necks.

[Tim A. Mitchell]

You could rule out wireless routers. The opperate in the 2.4 range, and are more likely to interfere with the 2.4 phones than anything. Baby monitors are often on 900. Have there been any additions in the vacinity of your house?

[Art Mulder]

The wart transformer could have gone bad.

Hey folks,

Just to bring some closure to this topic. I managed to dig up a compatible transformer for the base station, and the hum was virtually eliminated.

So I'd say the conclusion is that the wall-wart AC adaptor for the base station went bad.

best,
...art