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Grounding Problem vs. Shielding Problem

Discussion in 'Pickups & Electronics [BG]' started by jobu3, Dec 8, 2011.


  1. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    Few weeks ago I bumped the cable plugged into my (passive) bass at the jack and it immediately started to hum on the spot like there is a grounding issue.

    It hums with either pick up on or both pick ups on when the tone knob is turned on in either situation.

    It stops humming when I touch the strings, the knobs or the bridge.

    Not seeing anything visibly loose, I figured it was the jack so I replaced that with a new one and it's still doing it.

    I've unsoldered and resoldered all the grounds to where they were and it's still doing it.

    I pulled the pick ups out and didn't see anything amiss there in terms of the leads soldered to the housing.

    Oddly enough it gets worse (louder) as I move away from my amp.

    Is it grounding or shielding or both or...? The fact that it was dead silent until I bumped leads me to believe its grounding. the fact that it gets worse as i move away from the amp tells me its shielding. Could it be one exacerbating the other?

    Thoughts and recommendations?
     
  2. Bongolation

    Bongolation

    Nov 9, 2001
    California
    No Bogus Endorsements
    Did you change the cable?
     
  3. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Yes. The noise would normally get shunted to ground via the shielding, but if the ground is broken then the noise goes the only place it can--into the audio path. And if the connection from the shield to ground is broken, then the shield won't do its job properly.
     
  4. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    Yep, tried it trough two different amps in my house and three different cables.

    I tried the bass at a store too... Still did the same thing.

    I have another bass with a similar set up is dead quiet in terms of grounding noise. Just the usual 60hz single coil hum out of that one.
     
  5. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    So how do I make sure the path from shielding to ground is not broken, or reconnent the two?

    There is some copper shielding behind the pick guard (it's a PJ bass) over the cavity and is in contact with the jack where it secures in with the nut and washer.
     
  6. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Do you have a multimeter, or a simple continuity tester?
     
  7. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    My brother does. If not, I can get one.
     
  8. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    Just picked one up.

    Checked all the grounds under the pick guard and everything is coming back with good continuity from point to point including the bridge, backs of pots, the jack, the copper tape that's already there.

    The precision pup's pot does not have shielding under it. Could that be it? If so, not sure why it would start now after bumping the jack.

    The only thing not showing continuity to ground are the pole pieces of the p pup.
     
  9. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    Actually, the p pup doesn't have continuity to ground and the jazz pup in the bridge does (all but one pole piece).

    Checked on another pj bass and none of the pole pieces on either pick up show continuity to ground and there are no grounding issues with that bass.
     
  10. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Yeah to be honest I still haven't figured out why some pickup poles will have continuity with hot or ground while other poles do not. Must look into that.

    The lack of shielding under the P pup is not a problem, and definitely not related to the cable being bumped.

    Is it possible that when the cable got bumped, the area where the jack is attached got bent just enough so that the "hot" (signal) part of the jack is touching the shielding, when a plug is inserted?
     
    Dave Neal likes this.
  11. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    The lack of shielding is under the pot for the precision p-up. There is copper tape under the jazz/bridge pot and the tone pot but it stops before it gets to the first volume pot.

    There is nothing on the back of either pick up set in terms of shielding tape.

    The pickguard seemed ok. I'll double check to see if anything seems out of whack there.

    Thanks.
     
  12. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    Nothing wrong with the pick guard. Nothing seems to be making inappropriate contact. Good contact with the shielding tape there.

    I'm thinking that either I got a bad jack from the shop or it's the pick ups. Honestly, I don't know how it could be the pick ups.

    I've reheated solder on everything both grounds and hots and all the connections appear good.

    I've jumpered a piece of wire from everywhere to everything grounded and nothing is stopping the hum but my finger. The only thing that stopped the hum was when I touched it to the middle prong of the neck pick up which is not to ground but has a red wire from the pup soldered to it. By doing so it cut out the signal so that's not it either.

    I'm the immortal words of Charlie Brown, AARRRRRGGGHHH!
     
  13. Black Bart

    Black Bart

    Sep 11, 2010
    This is what I first suspected when I read the OP - cause and effect. I would pull the jack and take a look.
     
  14. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    I looked at it with a couple of different cables plugged in. Nothing looked like it was touching anything it shouldn't.
     
  15. bassbenj

    bassbenj

    Aug 11, 2009
    I agree. The trouble started with the jack and I'd take another look there. The place to check continuity is between the shields and grounds in the bass (bridge, back of pots, copper/paint shielding etc.) and plug a cable into the jack and then take the far end of that cable and test the OTHER end of continuity to the barrel of that plug. I suspect your shielding has somehow come ungrounded at the jack. All shielding and grounds need to be connected to the ground terminal at the jack. If you have conductive paint shielding, sometimes the screw that grounds the paint can lose contact and you have to screw it into a new place to make contact again.
     
  16. frianbisher

    frianbisher

    Dec 28, 2007
    Milwaukee WI
    Take the jack out and make sure no part of your cable tip is touching the ground wire of you'r OJ. If the ground wire of the output jack is hitting your cord you'll get buzz.
     
  17. Problemchild

    Problemchild

    Apr 9, 2011
    DFW
    I have had problems with the input jack contacting (or very
    close to) the electronic shielding- enough to remove the
    shielding in the remote area to get rid of the unwanted noise.

    +1 to the above - is worth checking !

    PC
     
  18. As other have said, it's either open ground, or short, ground to + signal. Are you SURE the barrel of the jack is in electrical contact with the tape on the back of the pickguard?
    Yes? Then next I would insert a cable in to the jack, and check for continuity between the tip of the cable, and ground(check from cable tip to shielding, for example). Obviously there should be no continuity there.
     
  19. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    Just checked and rechecked:

    The copper shield tape is in contact with grip washer holding the tone pot in place although there are some disruptions where it turned a bit. ie not perfectly flat or clean and at least 80% intact. I've tried with the grip washer and a flat washer (both good conductors) without result.

    Checked continuity from the ground shaft on my plugged in cable from the cable to the jack, the shielding tape, the backs of the pots, the bridge, and the entire series of the grounded wire. All good continuity.

    Checked all the hots vs. grounds and no continuity.

    Went back and checked the hot leads the same way as grounds (tip unplugged through the last hot) and vs. grounds and it all appears to be good.

    Still hums. Not sure what to do next but I'm really not wanting to get new pickups to rule those out. :(
     
  20. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    If it started humming when you bumped the jack, I'd replace the jack.
     

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