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finding a ground problem

Discussion in 'Luthier's Corner' started by lizard_king_21, Jul 8, 2005.

  1. i have a profile p-bass copy with a ground problem.
    I need help!
    I have a wiring diagram for my bass and everything looks ok, except that the last owner ran all the ground wires to the body of the pots instead of to the actual ground terminal, and one of the pots is a 250k and the other is a 500k, and the ground wire doesn't actually attach to the back of the bridge, but then it doesn't look like it ever did, it just kinda sits there and you bolt the bridge back on over it.

    does any of this sound like a problem?

    I know electronics and wiring but not where it applies to a bass. where would i start to look to trace down a ground problem?
    there is only one shop in town that does repairs and he's an a$$. i dont like him and he does everything half a$$ed.

    please, please help? :help:
  2. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    From what I've seen it is typical for the bridge ground wire to not actually be attached to the bridge, but to only press against it when the bridge is bolted down. Running all the wires to the back of one of the pots is also typical. If there is no connection to the actual ground on the jack, then there's a problem. What are your pickups ground wires hooked to?
  3. the ground wire on the pups goes toe the body of the volume pot, then to the body of the tone pot, then to the jack.
    the ground wire from the bridge connects to the tone pot also.
  4. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    This all sounds to be what has been my experience with some cheaper basses... what is the ground issue you are having?
  5. if i am not touching the metal on the bass it hums or buzzes.
    it's driving me nuts.
    I've tried different outlets in my house. my patch cord is brand new. i've even moved into the bedroom where the only other thing running is my alarm clock and it wont go away by any simple means. i've even taken to playing with a pick so i can rest my hand on the bridge....urgh!
    it plays alright but it means that i cant turn the volume up on my amp and play LOUD
  6. nateo

    nateo Schubie Fan #1

    Mar 2, 2003
    Ottawa, Ontario
    It sounds like you're picking up the 60 Hz hum from the household wiring (or 50 Hz, if you're not in North America). The easiest way to get rid of it is to shield your control and pickup cavities. Just get yourself a roll of conducting tape (copper or aluminium work fine) and cover the walls of the cavities (including the underside of the covers). Then connect the shielding to your ground (either through physical contact with a pot body or by soldering a connection wire from the ground terminal to the shielding).

    There are a few other tricks you can try, like rewiring your grounds to take any "ground loops" out of the circuit. Loops act as antennae that pick up electromagnetic interference. Ideally all of your grounds should run to the same point (called "star" grounding by some) instead of wiring from one ground to the next ("daisychain" style).

    There are plenty of good resources on the topic of grounding and shielding out there on the interweb. Unfortunately I can't recall any just now. A Google search on "cavity shielding" should turn some up. Hope it helps.

  7. theres a pictorial in the pickups forum for shielding thats very good.
    just do a search in that forum.
  8. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    You can try looking here. The grounding and shielding done here is very good.

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