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Guitar Hummed When Bridge or Control Knobs Touched--I fixed it, but I don't get it!

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  • Guitar Hummed When Bridge or Control Knobs Touched--I fixed it, but I don't get it!

    My son, Dan, got new pickups (two humbuckers) for his Epiphone SG. As I had to rewire it for him, anyway, I thought if we used stacked, concentric pots, he could have individual volume and tone controls for each pickup in place of the single volume and tone control that it started out with. The guitar appears to have been constructed in a hurry and without much care--the inside front face of the control cavity to which the hardware mounts was not consistently flat nor of a consistent thickness--so when I went to mount the new hardware (we replaced the 3-way switch and mono jack with better quality parts, as well) it was not a pretty sight. Some shafts came through crooked and in the case of others, not enough of the threaded bushing came through for the nut to grab on and hold it in place. I responded to this by putting the guitar body flat on my drill press table and using Forstner bits of a diameter slightly greater than that of the individual pieces of hardware, I drilled away a bit of wood at the site of each device to leave a quite flat circular area consistently of nearly exactly 1/8" thickness at each place. Then the hardware mounted perfectly.

    I mention the following now because, as things turned out, I suspect it was part of the problem with the humming that this thread refers to. With the two humbuckers, there was no convincing need for pickup shielding, but my son had on his own taken some conductive paint I had on hand and painted the pickup routs and the control cavity. Plainly the guitar had worked "fine" when new without this shielding. Of course, when I drilled into the front face of the cavity as described above, the conductive paint was removed where I drilled. Inasmuch as I did not see need for the paint in the first place, I could not imagine that removing part of it in this fashion would have any adverse effect.

    I wired the guitar up initially based on a wiring diagram I found on line that was reported to be styled after Gibson's manner of wiring an SG with 2 humbuckers and a volume and tone control for each (my schematic attached). Basically the pickups were wired directly to the volume pots, the capacitors were wired between the volume pots and the tone pots, and the pickup selector switch went between the wiper on the volume pot and the tip of the jack. We found this worked out crazy. I could not find that I had wired anything up wrong, but the volume controls worked unreliably, the sound might turn on or off without obvious cause, and one of the tone controls could influence the tone of the other pickup, even when the pickup associated with the tone control was turned off!

    In response to that, I devised my own wiring, based more on typical Fender wiring practices (although I am not aware of any Strats with 2 humbuckers!; my schematic is attached) and rewired it. In this case, the selector switch was between the pickups and the volume controls, the capacitors were between the tone pot and ground, and the wiper of the volume pot went directly to the tip of the jack. We got it all wired up and, at least at first glance everything seemed to work fine. Both pickups worked reliably together and alone and the volume and tone controls influenced the sound of the pickup they were supposed to control.

    Night before last, I asked Dan how the guitar was working out, and he said it was great, "except if I touch the bridge or the knobs when I'm plugged in, I get this awful hum." I asked him to bring it down so I could take a look.

    Yesterday morning I took it apart and did a little head scratching. I was afraid I might have forgotten to solder the bridge to ground connection, but that was fine. Everything looked okay. Everything was soldered to ground as it should be, and I had grounded both pots to each other on the stacked pots.

    The only thing that looked odd (not in an electrical sense to me, but just in the sense that it was not a typical thing to see) was to have the cavity painted with conductive paint, except where it was drilled away to fit the hardware. Again, given that the guitar did not need conductive paint in the first place, I had a hard time believing that that would be part of the problem--those pieces of hardware had been attached directly to wood when the guitar was new, so I could not understand how having the hardware attached directly to wood now would cause a hum, just because there were some adjacent areas covered with conductive paint that none of the hardware came in contact with.

    Anyway, being unable to find anything else to try, in the end I covered the bare wood areas around the hardware with copper tape carefully overlapping the pieces, I soldered a bare ground wire from piece-to-piece to more definitely connect those pieces of tape together, and then I soldered the end of that wire to the back of one of the pots.

    When I first put it back together I freaked out, because the guitar was dead--no sound whatsoever. After a little more head scratching, I came to recognize that with the copper tape lying right under the volume pots with their hot leads, that the terminals were bent down and contacting the copper, shorting the signal to ground. I cautiously used a pair of needle nosed pliers to bend those terminals up away from the copper, and then the guitar worked fine--good sound, all controlled as it should be, and no more hum when I contacted the bridge or the metal control knobs. I banged away on the guitar for about an hour until Dan got out of bed and I could show him the improvement.

    I am sorry for the length of this. I hope it does not inhibit anybody from reading it enough to respond! It was just a complex enough unfolding that I did not know how else to put it. Does it make sense to anybody, given that the guitar as initially built did not need to have grounding where the hardware was attached to the bare wooden face of the cavity, that it should need grounding there now just because some adjacent areas are covered with conductive shielding paint?

    Thanks for consideration! Rob R
    Attached Files

  • #2
    If the conductive paint wasn't grounded, it could make the hum worse than before. Applying copper tape to it, and grounding the tape, would also ground the paint.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    • #3
      Originally posted by Steve Conner View Post
      If the conductive paint wasn't grounded, it could make the hum worse than before. Applying copper tape to it, and grounding the tape, would also ground the paint.
      That's interesting and plainly answers my question. I had it backward in my mind. I was wondering why the hardware that had not needed to be grounded to the wood before the conductive paint now needed to be grounded. In fact, then, it was not the hardware at all, but the conductive paint that needed to be grounded! Thanks. If I had simply applied a single small piece of copper tape to the paint, not necessarily even in the neighborhood of the pots, switch, or jack, and run it to ground or screwed a ground wire into an area painted with the conductive paint and run it to ground, would it have had the same effect, then?

      I have a simpler question (at least simpler to express! ) along the same lines. It is almost too obvious to bother asking, but if I am wrong in my presumption I'll have to unsolder and re-solder a bunch of ground connections, so I'd rather make sure before I jump in.

      On my own current guitar project, I have quite a number of connections to ground to make, but I have only one (stacked) pot with limited area to solder to, and having ground wires running all over the place would be crazy. Of course, primarily for shielding, I have lined the cavity with copper tape. I want to presume though, that once having grounded the copper tape to the pot, I can run any other ground connections needed to the copper tape. That would allow shorter wires running to ground, keep them nearer to where they originate, not have me trying to solder too many connections to the back of the one pot, and limit the number of wires criss-crossing through the cavity. Is that a legitimate way to proceed?

      Thanks again Rob R

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      • #4
        I've done that before with no problems.

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        • #5
          Thank you very much. That's all I needed to know. Now I'm rarin' to go.

          Rob R

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          • #6
            Thinking about it, I would make sure that all the ground connections go to the same piece of copper tape. I wouldn't care to rely on the conductive glue as part of the ground system.
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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            • #7
              For sure.

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              • #8
                Thanks for the input. I didn't trust the conductive glue, either, so I soldered all the joints. I figured that would assure the electrical continuity of all the tape and maybe give some rigidity to the tape to make it less likely over time to start pulling away from the wood (especially in the bridge pickup cavity, where the convoluted shape of the rout required use of multiple oddly shaped little pieces of tape that did not want to stick so readily). I've attached an image of the cavity and the pickup routs with the tape and soldered joints. The image does not show the full control cavity, but all tape joints were treated the same. I think with the joints all soldered I should be safe to go to ground anywhere in the cavity. Do you think that is correct?

                I have 3 questions unrelated to that issue for you, though they are related to each other. As I recall, you are big time into making pickups, and I am interested in getting into that. The first question--when I look on the pickup page here, there is mention of a book that people have used, but I have been unable to find it or any other on the subject on line (I don't recall the author's name at this moment, but I think it began with "L" and I believe you might have been among those that recommended it several years ago). I don't know if it might be out of print. Are you aware of any currently available books on the topic? The second question--there is certainly a lot of info on the Pickup page here, but it is hard to tie it all together and know that you are not missing something. If there are no available books in print on the topic, what would you list as the major considerations that I should investigate on the Pickup page here to get started without fearing that I am missing something important? The third question: I have seen mention of a number of motorized rotating devices as a primary mechanism for winding the copper wire on the bobbins. Among them I have seen mention of the mini-lathe. I do not have a mini-lathe, but I do have a full -sized lathe with a continuously variable speed of rotation. I have attached an image of my lathe with the dimensions and operating characteristics. Is there any reason that with some ingenuity that would not be as good an idea to use as the mini-lathe for this purpose?

                Thanks, again Rob R
                Attached Files

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