Humm - Ground Issue?

Discussion in 'Pickups & Electronics [BG]' started by ErikP.Bass, May 21, 2005.

  1. ErikP.Bass

    ErikP.Bass Supporting Member

    Nov 23, 2004
    I decided to put some DiMarzio Model J's in my Jazz bass, to shield the control cavity and also rewire the controls to Volume-Blend-Tone. I have attached the wiring diagram I worked from.

    Shielding the cavity was no problem. Then I installed the pickups into the body of the bass, no soldering yet. Lastly, I proceeded to wire up the new pots according to the attached diagram....and then finally I attached the pickup leads.

    Initially I chained all the grounds in the diagram (and the bridge and pickup plate grounds) and had them run to a single ground, the standard Fender one screwed to the body, from the back of the volume pot. Then tried it out...everything functioned as it should, but there was humm.

    Next I tried installing more grounds, separate ones for each pot and from the jack (as well as the bridge and pickup grounds), this time running them directly to the shielding foil. 3 points in total two wires to each solder point, and I left the standard Fender screw ground in too. Plugged in again and still humm.

    I hope someone can follow what I am saying...I know it is hard without actually seeing it.

    Other considerations : the two other wires from each pickup, black and white, are connected - for humbucking series. The bridge ground is in tact.

    I think I am going to let it sit for a few days and then revisit it...but I figured I would post to see if I missed something completely obvious. :eyebrow:
  2. What I like to do is as follows:

    Ground from Jack to Volume Pot case
    From there I run one ground to the balance blend, a separate ground from volume to the tone control, a separate ground from volume to bridge, a separate ground from volume to cavity shielding and finally a separate one from volume to bridge. Note: if you have shielding on the pickguard then do not run grounds to other controls as they will be grounded through the pickguard from volume pot. You do not want more than one ground path to a given item or it can cause ground loop hum.
  3. ErikP.Bass

    ErikP.Bass Supporting Member

    Nov 23, 2004
    I just thought I would update my thread for those who are/were curious.

    It turns out that though I initially thought that "sheilding the cavity was no problem", it may have been my problem. After corresponding with Lyle Caldwell and re-reading one of his threads.....I realized that I never checked the impedance of the shielding I had put in (or soldered the points where the sheilding overlapped) before assembling everything else.

    So I re-sheilded the cavity, this time using only four pieces of copper shielding in total, and then soldering all the joints where it overlapped. I then ensured, with a multimeter, that the impedance throughout was 0 ohm.

    I plan on fininshing up the wiring tonight...hopefully it works.....more to come. :rolleyes:
  4. ErikP.Bass

    ErikP.Bass Supporting Member

    Nov 23, 2004
    I completed the wiring tonight and fixing the shielding worked wonders for the humm I was getting. The bass is certainly much quieter.

    I do have another question though, as this is the first bass I have owned where the pickups have exposed pole pieces. Is it normal that when your finger comes in contact with these that a hum is produced? Just wondering if this may be another weak link in my wiring somehow. :eyebrow: Seems like I may be completing a ground loop with my thumb...?

    I did do a search and have noted that some people suggest putting nail polish or tape on the poles to help prevent this problem. NOTE: I play with the floating thumb technique so I don't anchor my thumb on top of the pickup.

    Any advice is greatly appreciated.
  5. ErikP.Bass

    ErikP.Bass Supporting Member

    Nov 23, 2004
    Well I am happy, and sad at the same time, to say my Jazz bass modification is complete. Documented above are some of the steps I have gone through, and hopefully they will serve to benefit someone.

    I ended up calling a guitar repair shop near me in Boston to see what the fee would be to get my wiring job assessed and/or fixed....turns out after talking to the guy on the phone and explaining my situation about the pole pieces and what I had done thus far....he told me how to fix it - noble in my book considering he could have turned a quick buck on the deal.

    Now I will pass his words of wisdom on to you :

    He told me that the poles did not have a connection to ground. And when you think about it, the metal plate in the pickup cavities doesn't seem to have much of a purpose since it does not come in direct contact with the bottom of the pickup and poles because of the foam.

    So to make a long story short he told me to put a piece of copper tape on top of the foam ensuring that it was pressed tightly against the poles and bottom of the pickup. Then solder a wire to it (the tape, not the pickup), run the wire through the hole to the control cavity and then lastly solder it to the common ground. NOTE : before connecting the tape and wire to the ground...ensure that the connection/impedance is 0 ohms when, with a multimeter, touching the tape and the end of the wire which will be soldered to the ground.

    Needless to say this worked wonders, and the soldering iron and I can both sleep well tonight. I know these are long posts, but hopefully someone will read them and learn from my mistakes/misfortunes.