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Picking up radio, why and how do I stop it?

Discussion in 'Amps and Cabs [BG]' started by Tom Crofts, Feb 17, 2002.

  1. Tom Crofts

    Tom Crofts

    Mar 15, 2001
    I have a McGregor Bassman 200W 4 Ohm solid state head. I bought it 2nd hand today and it had really scratchy pots, they're all contact sprayed clean and they sound lovely. However I'm picking up French radio still. It increases with the volume and is more pronounced when a cable is plugged into the amp. How do I stop it? I'm willing to tinker a bit (then I'll fix the amp. Sorry.).
  2. dude that sucks my friends amp did that and picked up like phone calls or somin , but only at the drummers house
  3. try a different wall socket.
  4. Did you try another cable?
    Another Bass or even a guitar?

    I have a SWR WK15 and before I replaced the patch cord I could get portable phones and radio stations depending on what was going on in the aptartment building I was in.
  5. chucko58


    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Are you handy with electronics? Some strategically placed capacitors could solve the problem.

    If the input jack is not mounted on a circuit board, try a ferrite bead around the wire from the jack to the board.

    The radio signal could also be evidence of a general grounding (earthing) problem. I'd go through the amp and make sure every nut and bolt was tight.
  6. Tom Crofts

    Tom Crofts

    Mar 15, 2001
    Well my combo is fine. I'm using the head through the combo speaker plugged into the back of the combo in the effect return input (does this bypass the preamp/eq stage?). It sounds ok but I get radio, it may be the crappy cable I'm using from head to effects return. Would that do it?
  7. Stachio

    Stachio Supporting Member

    Jan 29, 2002
    It may be your bass' shielding
  8. yawnsie


    Apr 11, 2000
    This used to happen to me, back in the days when I used to borrow my uncle's Vox amp and Hohner bass - it used to pick up signals from taxis driving past. I remember that we found out later that the electronics inside the bass were shot, so maybe it's the same for you. Having said that, if it's just starting happening with this new amp, it's most likely that rather than the bass.
  9. First, make sure that you are using shielded instrument cable for the guitar connection, the head to combo connection, and any effects units. If you use speaker cables for any of these, you will pick up all kinds of noise. Use speaker cables for connecting speakers. All other connections should be shielded instument cable.
  10. Tom Crofts

    Tom Crofts

    Mar 15, 2001
    It's not the bass, I think it's the cable, it looks really crappy... Is this a common problem with older (1980) amps? What could it be if it isn't the cable?

    Thanks for your help so far.
  11. What is the cable going into the effects return plugged into? If you are trying to use the Bassman's power amp, you will have to connect directly from the speaker out to the speaker. Going into effects return will use the combo's power amp.

    Do you pick up radio when the bassman has no cables connected except from the speaker out to a speaker and if so, does the volume control change the volume of the radio signal?

    A loose ground connection to a metal part of the chassis may cause the input connections to not be shielded. You may just check for loose or disconnected wires. Don't do this if you are not familiar with electricity. Capacitors for the power supply can contain dangerous voltages even after the amp is unplugged.
  12. Tom Crofts

    Tom Crofts

    Mar 15, 2001
    Ok, this is what's going into what:

    Bassman LS1 (it also has LS2 and slave) output into the Carlsbro combo Effects Return input through what appears to be a crappy guitar lead (it came with it).

    I'm pretty much definite that it's the cable, I've tried it with my Planet Waves and it seems to be ok. That's when I just have the PW cable running from the LS1 to the Effects Return.
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    I don't know what the input connection is like on that amp, but first I'd make sure all the chassis-mount connectors are secure, and any ground (earth) connections are also.

    If the problem remains after that, a small disk capacitor, like maybe 0.01 µF or so, from the input's "hot" side to the chassis might help. If the input connector is connected by wire to the circuit board and not mounted directly to it, you can also unsolder the "hot" wire at one end and slip a ferrite bead over it, and then resolder the wire. These steps will usually kill most RF interference problems like the one you describe.

    Another cheap fix for the speaker cable, in case the RF is coming in through the amp output, is to buy one of those large ferrites (rectangular, about 2 inches long, in a plastic frame that opens on one end) that Radio Shack/Tandy sells and loop the cable through it a few times.

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