help troubleshooting an amp picking up radio signal

[kjw215]

Hi everyone, first post. I recently started playing bass and am having a problem with picking up a local radio station on my equipment. I've searched through the forums for similar problems (http://www.talkbass.com/forum/showthread.php?t=529882&highlight=radio+signal) and have tried to be systematic about troubleshooting, but I haven't had much success. Help?

Background info:

1. I am picking up FM radio stations, sometimes two different ones at the same time, as far as I can tell. I can tune somewhat by moving my instrument cable around. But, generally I pick up 104.1, which is no surprise, as I live about 1 mile from the top of the building (the Prudential in Boston), where the 104.1 broadcast antenna sits. In fact, I have a clear line of sight from my window.

2. I tested the outlets and they are properly grounded.

3. I've moved my equipment around the room, different orientations, and it can help somewhat, but the radio is still pretty loud and annoying.

4. my gear is all used - stock Fender MIM Jazz Bass and Ampeg BA112. I've used a generic 15' instrument cable from Monoprice, and tonight I tried out a 10' Mogami Gold. The cables don't make any difference.

5. As far as isolating components, the amp plugged in to the wall outlet by itself is quiet. The amp with just a cable (either one), starts to pick up the radio loudly. Amp+cable+bass sounds the same. So it must be that the cable is acting like an antenna. The radio signal is not being picked up by any other electronics in the apartment.

The cables are shielded, so at this point I suspect either:

A. the used ampeg ba112 might have an issue with a bad input jack/ground connection. I bought the amp used and it was shipped, so there is a possibility of damage.

if this is the case, can anyone point me to any directions on how to open up the ba112 so I can inspect the input jack? (I took an analog electronics lab back in college, so hopefully I can figure it out...)

B. I'm SOL living too close to a powerful transmitter.

if THIS is the case, then I'll look forward to when my lease is up in 6 months.

Thoughts?

 

[johnk_10]

as far as B goes, try to borrow or have a friend bring over his guitar or bass amp to see if it does the same thing.

if it does, either learn to like what station you're receiving, or move ASAP.

 

[kb9wyz]

Before you open up the amp, open up the axe. Check the shielding. If it isn't shielded, shield it. You might also try getting a choke magnet to slap on the instrument cable. You can get them at Radio Shack, or some other electronics supply place. Also, the shortest cable possible would be best for dealing with RF interference. Be aware that major radio stations regularly broadcast at 50,000 watts. Even "weak" stations can be as much as 1000-5000 watts. At a mile from ground zero, you are very close regardless of the power of the station.

 

[kjw215]

Before you open up the amp, open up the axe. Check the shielding. If it isn't shielded, shield it. You might also try getting a choke magnet to slap on the instrument cable. You can get them at Radio Shack, or some other electronics supply place. Also, the shortest cable possible would be best for dealing with RF interference. Be aware that major radio stations regularly broadcast at 50,000 watts. Even "weak" stations can be as much as 1000-5000 watts. At a mile from ground zero, you are very close regardless of the power of the station.

Thanks for your suggestions.

1. the setup picks up radio even without my guitar plugged in; just the cable and amp are enough. so I was hoping not to have to open up my guitar. but i'll check this out.

2. I tried a choke magnet, no help.

3. WBMX transmits at 104.1 MHz with an effective radiated power of 21,000 watts from the Prudential Tower upper master antenna. This antenna is shared with WZLX, WMJX, and WXKS-FM... which would explain the other stations I get.

 

[beans-on-toast]

I would get in touch with a broadcast engineer at the radio station and explain your problem. They take these sort of things seriously and might be able to help you out.

I live within a mile of a set of TV and FM broadcast towers and don't have any of the problems that you described with any of my amps.

Usually the problem is AM stations. Many amps have low pass filters built into them that filter out RF transmissions at the input. Things can be done. One solution is to isolate your input jacks and install a small capacitor between the chassis and the jack ground.

 

[throbbinnut]

Have you tried a new cable? Last time this happened to a guitarist I knew, he switched instrument cables and it was fixed.

Chris

 

[FunkMetalBass]

It's far more likely that it's your pickups on your bass or guitar than in your amp.

 

[COOL AND DEADLY]

ground/ lift switch?

 

[Bassmec]

It might be getting in via the mains supply especially if its via overhead cables so buy an extension lead with strong
RF and spike filtering and check your houses earth by using a DMM set to ohms between your amp chassis and a real ground, like a wire attached to a wire coat hanger driven into a patch of damp earth.
Should read 0 Ohms mate!

 

[B-string]

They maybe a little hard to locate but find some ferrite beads, these will be slipped over the center conductor of your instument cable. They are non conductive so don't worry about shorting. They will block the RF from the radio transmitters (maybe the station engineers can hook you up?).

 

[kjw215]

It's far more likely that it's your pickups on your bass or guitar than in your amp.

yes, but if the amp picks up radio with just an instrument cable and no bass guitar, then I can rule this out in this case, no?

Have you tried a new cable? Last time this happened to a guitarist I knew, he switched instrument cables and it was fixed.

yep, tried switching up the cable; no difference. (monoprice 15' generic vs mogami gold 10')

ground/ lift switch?

only have one piece of equipment plugged in so theoretically shouldn't be getting any ground loops, and the noise isn't a 50-60hz buzz

They maybe a little hard to locate but find some ferrite beads, these will be slipped over the center conductor of your instument cable. ...

tried that, unfortunately no help.

I would get in touch with a broadcast engineer at the radio station and explain your problem. They take these sort of things seriously and might be able to help you out.
... One solution is to isolate your input jacks and install a small capacitor between the chassis and the jack ground.

I will try this... Thanks.

 

[David Jayne]

You can test ground all the way through the amp easily, without taking it apart, with a meter.
1)Plug instrument cable into amp.
2)Insert power cable into amp, but do not plug into the wall.
3)Test continuity between instrument cable sleeve and power plug ground lug(3rd lug).

 

[scottbass]

Sorry, it's B, perhaps accentuated by overhead power lines into your building. All I can suggest is that you contact the radio station(s) engineer(s) and ask for their recommendations.

I would suggest trying a battery-powered amp to determine how much of it is in the air and how much is in your power lines. I'm speculating most of it is in your power lines, in which case you will get no relief unless you can get your building (which you don't own) to isolate their power supply.

 

[BassBrass]

Does this happen when the amp/bass etc are used in another location or just at your apartment? ALLOT of old wiring around there. Berklee Coll of Music doesn't seem to have this problem and they are right under that antenna. I used to get some radio at the old Sound Museum rehearsal space on Albany Street...(you could see the Pru from our window) but nowhere else.

btw, that used to be WBCN's frequency...sadly reminding me that when they closed this year, an era was over. I first heard Live At Leeds on that station in the 70s. I just changed my car radio over to 88.1 (M.I.T.) from 104.1 last week.

 

[David Jayne]

I'm speculating most of it is in your power lines, ...

Not likely, because:

5. As far as isolating components, the amp plugged in to the wall outlet by itself is quiet.

I suggest you try my test.

 

[scottbass]

Not likely, because:

Quote:
Originally Posted by kjw215

5. As far as isolating components, the amp plugged in to the wall outlet by itself is quiet.

Oops - I missed that part!

So, Amp alone = OK
Amp with instrument cable but no instrument = radio receiver

That means it's through the air and being picked up by your instrument cables - yes, both of the ones you tried. Forget what I said about your building's electrical system.

 

[kjw215]

Does this happen when the amp/bass etc are used in another location or just at your apartment? ALLOT of old wiring around there. Berklee Coll of Music doesn't seem to have this problem and they are right under that antenna. I used to get some radio at the old Sound Museum rehearsal space on Albany Street...(you could see the Pru from our window) but nowhere else.

btw, that used to be WBCN's frequency...sadly reminding me that when they closed this year, an era was over. I first heard Live At Leeds on that station in the 70s. I just changed my car radio over to 88.1 (M.I.T.) from 104.1 last week.

Yeah, sad that WBCN is gone. Now I resort to 92.9 as a poor alternative. Will have to try out 88.1.

I've only had a chance to use the bass once in NH so far, and it seemed to be ok, so I didn't suspect my amp. But more bypass capacitors might be the solution regardless.

The kicker here might be that I live on the 36th floor in my building, and somehow might be in a high power spot due to the radial pattern of the antenna.

 

[coreyfyfe]

So, Amp alone = OK
Amp with instrument cable but no instrument = radio receiver

We have the same problem at our practice space in allston. We're close to something, but I'm not sure who the station is because the programing varies. Used to be sports, now I'm not sure. From time to time it's in spanish. I don't have any problem at home with the same amp, so it's just the location. My recommendation, if you really can't deal with it, is the cap between the jack group and chassis, but unless you're recording I wouldn't worry about it. There's nothing wrong with the amp, just a little annoying noise when not playing. I thought about doing that at one point too, I was recording in woonsocket and we were picking up a lot of radio, but I never got around to it and haven't really cared since. You must be fairly close to the Pru though if you're getting their signals? I've recorded in the berklee studios and I used to live on boylston and the same amp that picks up radio in allston didn't get any noise in those locations.

 

[Steve_64068]

A Far-Field receiver's response will generally be accentuated by the following:

1) A conductor having an impedance of approximately 300-ohms. This matches the impedance of "free-space" in broadcasting and essentially forms an antenna with a gain of at least -10 dB. Which is sufficient to cause your pre-amp to respond... if coupled. Note: the gain can be much higher...

2) Any conductor having a wavelength longer than 1/20th. In other words... longer (troublesome) wavelengths will be ratios of 1:1, 1:2, 1:4. Eventually... at 1:20 the "tuning" of the antenna offers sufficient attenuation of the signal that your AMPs' front-end may reject the broadcast signal.

At your stated frequency (104.1MHz.) the wavelength will be approximately 2.88 Meters or 9.4488 Feet. Your goal is to ensure that "electrically" your AMP doesn't see the fullwave or fractions thereof at its' pre-amp input.

I would recommend a series of quick tests:

1) Power-up and monitor your AMP "without" an input connected (no cord, no Bass). If you are NOT receiving the FM station... gradually rotate your AMP through a full 360-deg. Taking about 1/8th turn every minute or so, until complete. If the AMP remains quiet, you have eliminated the AMP as the receiver. Naturally... ramp the gain to at least 1:4 during the test to raise the AMP above the noise floor.

2) If you're looking at #2 that's a good thing... because that means you're looking outside the AMP. And... this should be cut & dry.

Please let me know what you've found...

 

[WingKL]

Try another instrument cable, something with even better shielding. Perhaps something like the [Invalid or Expired Link Removed]. There was another thread where a guy had noise issues with his GK mb 500 and he tried 2 different cables and it turned out later both cables had broken shielding.