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unusual grounding and shielding bug

Discussion in 'Pickups & Electronics [BG]' started by mellowgerman, Jan 25, 2013.

  1. mellowgerman


    Jan 23, 2008
    Orlando, FL
    At least I'm pretty sure it's unusual. I've got a pretty decent understanding of grounding and shielding. However, I've been having an issue with the simplest of Precision bass circuitry that it's sort of making me question what I know. Bass is wired pickup to straight to the jack. Bridge ground and cavity shielding also to the ground prong on the jack. No controls.
    Anyway, I understand that:
    (1) if when you touch metal on your bass and the buzz gets louder, it's a grounding issue
    (2) if when you touch metal on your bass and the buzz goes away, it's a shielding issue

    To start off, there is a VERY VERY slight buzz when the bass is plugged in and I'm not touching anything; hands up in the air. It's almost inaudible unless the amp is turned up really loud, then you can hear it, but again very quietly. This doesn't bother me and seems relatively normal when playing a bass in an old house with a bunch of other apartments that have computers, light bulbs, TVs, cellphones, etc.
    My confusion comes from the following:
    -When I touch the bridge or the washer on the input jack and nothing else, the VERY VERY slight buzz gets goes away. Okay, normally I'd say that's a shielding-related thing. BUUUT...
    -When I touch the mounting screws or pole pieces of my bass's standard split-coil precision pickup and nothing else, there is a loud grounding-related buzz. (This was also going on when I first put the bass together, with the original SX pickup, before I shielded the cavity and before performing any pickup swaps.)

    I've never had both of these things happen before at the same time... The wiring on this bass is as simple as it gets. Also, i've tried three separate P bass pickups (stock SX and two different American Standard P bass pickups) and tried each of them through both of my amps and in different houses. The same problems were still occurring.

    So I'm stumped! What could possibly be causing this? Is my bass possessed? Any help would be greatly appreciated!
  2. Cadfael


    Jan 4, 2013
    Germany, EU
    No, it isn't! :D
    We have written about this phenomenon one, two or three times the last 14 days ... ;)

    Dave (or others) might eyplain it much better than me ...

    The magnets (=pole pieces on P PUs) are NOT grounded.
    The coils are reversed wired and the magnets are revers.

    You will find out that touching the magnets on one coil hum, on the other they don't ...

    Solving the prolem:
    1.) Easy but not really solving: Change coils or wires (black white).
    The other coil you normally never touch will hum.
    2.) Put scotch tape or nail polish onto the magnets.
    You can't diretly touch the magnets anymore - but the problem remains under the protection.
    3.) Take two pieces of adhensive copper foil and solder wires to them. One "short" wire between the two foils and a long one to the cavity ...
    Stick the foils under the pickups (onto the magnet - between magnets and foam). Solder the wire to GND.
    Now, it doesn't matter anymore if the PU is reverse or not. The "pole pieces" / magnets are grounded and won't hum anymore. It has no influence on sound or working of the PUs!

    As you shielded everything, method 3 would be the best way ...

    P.S.: Same with Jazz Bass! One pickup hums, the other PU doesn't!
    Normally bass players play close to bridge or neck and only touch one of the JB pickups. Put copper foil under both PUs or change HOT and GND of BOTH PUs. Then, the "normally non-touched PU will hum ...
  3. mellowgerman


    Jan 23, 2008
    Orlando, FL
    I've spent a majority of my time playing all sorts of basses other than just normal j's and p's... Now I know!
    Certainly a humbling revelation :D
    Thank you sir and I'm sorry for bringing up a commonly discussed topic! I hate it when people do that and usually try to make sure I don't
  4. Cadfael


    Jan 4, 2013
    Germany, EU
    NO PROBLEM AT ALL!!! :smug:

    Even though we discussed it - it's not "common topic"! :)
    It's noble not trying to bore people by "common topics". But in the end we are only human - bound to make mistakes. ;)

    Let me/us know how you solved the problem and if it wored as described! Feedback always helps others - and id good for the helpers.

    And don't call me "sir"! I'm just a freaky bloke ... :D
  5. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    The pickup pole pieces are not grounded. So touching them induces noise into the pickup. Same with the mounting screws.

    With a Jazz pickup type pickup, generally the start of the coil, which is close to, or touching the magnets, is connected to ground. That stops you from hearing the buzz when you touch the poles.

    You can try and reverse the polarity on both pickups and see if that helps. To do that, swap the ground and hot wire on each one.

    Don't do that if the pickups use shielded coax cable.
  6. mellowgerman


    Jan 23, 2008
    Orlando, FL
    Thanks for the help gentlemen! Did the copper tape on bottom of pickups method and it did the trick!