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Shielding that simple?

Discussion in 'Pickups & Electronics [BG]' started by eganbass, Feb 27, 2013.


  1. eganbass

    eganbass

    Sep 28, 2004
    Last night I read through the many posts about hum, shielding, grounding, some of them pretty heated!....

    Ive had some slight hum problems, and out of curiosity I unscrewed the plastic cover on the back of my circa '84/'87 Japanese Jazz Special (no shielding at all) and put a single piece of aluminum foil on the back of the cover -- that's about the extent of modifications that I feel comfortable doing.

    Wow, the result was the elimination of any hum at all, regardless of whether I was touching the strings.

    Sound too good to be true? Maybe something else is going on? Let me know your thoughts -- would two pieces of aluminum foil provide more shielding?

    Thanks!

    Bruce
     
  2. I put flats and ashtrays on SX jazz. Bada-Bing...no more hum. :D I was quite pleased with that.
     
  3. uOpt

    uOpt

    Jul 21, 2008
    Boston, MA, USA
    Shielding is very simple. You need a conductor that is grounded around "hot" wires. Doesn't matter how thick.

    However, this only works against the random buzz from RF interference. It does nothing against the magnetically induced 50/60 Hz cycle hum that single coil pickups pick up.

    If you killed all noise by shielding the cavity you probably have noiseless pickups.
     
  4. bassbenj

    bassbenj

    Aug 11, 2009
    Shielding electric hum (changes when you touch the strings) is pretty simple, The ideal is to enclose all wiring and parts inside a water-tight conductive (metal foil) box. That box MUST be grounded to the outside of the jack or something connected to it. An unconnected piece of foil will not shield anything. But it's not possible to really enclose everything as tops of pickups must stick out etc. but every bit helps.

    So one big area of shielding is obviously the cover plate on the back. Covering it with aluminum foil is good start, but somehow the screws need to press tht foil down into something connected to ground to make the ground connection. If the cavity has been painted with conductive paint (usually black or silver) that usually makes the connection. I suspect your bass may have had conductive paint but nothing on the cover. Hence once you coated the cover with foil you effectively enclosed all the circuits and the hum went away.
     
  5. I'm in the process right now of becoming a 'whiz' with StewMac's 'Conductive Shielding Paint'. I'll complete two guitars before I get near my Fender 'P-Lyte' bass with it. One of the wonderful things I discovered during this very steep learning curve is the use of 'decorating wire', or identified by the older name: "pipe cleaners". Short of running a grounding wire or a piece of "wicking braid" through those narrow wire channels, there is no way I get get enough paint inside them to form conductive continuity from cavity to cavity. Those fuzzy, colorful 'pipe cleaners' can pull enough paint through those narrow channels (several coats however) to complete a conducting circuit. They are flexible, cheap, plentiful. And best of disposable. Why is that a big deal? Wait 'till you see what awaits you when you clean your brush.. it ain't pretty. Wear gloves, have a wet rag handy for accidental drips or over-brushing to the finish. And most of all.. move SLOWLY.. Make sure you get some very low 'tack' "painter's tape" for masking off the perimeter of the cavities.. You'll be so glad if you take that extra step. Next, if you don't already have a foil-backed pickguard, you can certainly "make it so". But glue is such a messy thing. I picked up a can of "3M Spray Adhesive" to avoid brushes and rollers. Lay out a very flat sheet of foil on a very flat, hard surface. Do a outline of the pickguard..maybe even 'score' it if you can (without making any tears of course) careful spray on the adhesive.. Wait for it to "tack up", and then lay the pickguard on it. Now, this is your ONLY chance to roll out any imperfections on the foil, so carefully do that. Now for the hard part.. Getting rid of the 'waste' so none of the foil will show once it has been installed. This part takes much patience and cleverness. No, you won't have to attach a wire to the now foil-backed pickguard to complete your shielded "Faraday Cage". Just install a foil strip (or copper tape) up the side of the shielded cavity and up over on the finish (slightly). I did that and also painted over the foil strip with additional conductive paint. Remember: everything thing you do has to "pass muster" with your multimeter set on "continuity" or it's all just in vain.. just pretty work to look at, but ineffective as a shield.
     

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