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I've looked everywhere... please help with shielding?

Discussion in 'Pickups & Electronics [BG]' started by theleteri, Jul 10, 2012.


  1. theleteri

    theleteri

    Aug 9, 2008
    I have a tregan shaman bass. A while back i put dimarzio pickups in it and wired it in parallel with one vol knob (it is a pj setup). It has been sounding a bit week since my band got an additional guitarist so i figured i would wire it back to normal so i could take a bit of the j pickup out of the mix to fatten it up. i did not like the tone so i decided to try series wiring. i also figured that i would not be adjusting the volumes between the pickups so i wired the 2 pickups (both are duel coil) and have 4 leads) in series. it sounds great and powerful and i will get to try it in the mix tonight at practice.

    the trouble is there is a wicked hum/buz now. if i turn the highs down on the amp the hums volume lessens but it is still there. my guess is this is a shielding issue. i have checked all the connections more than once. the noise stops when i touch the strings or the bridge or if i touch the black paint that is on the inside of the control cavity.

    any ideas? these pickups used to be silent.
     
  2. bassbenj

    bassbenj

    Aug 11, 2009
    Since the hum goes away when you touch ground it's likely you need some shielding. That includes foil on the cavity cover, foil in the pickup cavities and if possible all the poles should be grounded too. The conductive black paint seems to be working since when you touch it hum goes away, but often I'll do foil over the paint too just to make sure. Also wires to pickups should be shielded.

    Note that when you wire in parallel one end of the coil is grounded which makes it act sort of like a shield. But in series one pickup is grounded but the other is "floating" which allows it to easily pick up hum. Hence the need for covering all things electrical with a conductive metal "box" as much as possible. The conductive box must be grounded to jack ground.
     
  3. The point Bassbenj is well made and quite right - with one end grounded and the other unshielded, you have what is effectively a radio antenna picking up 60 Hz hum. Shielding is your only solution if that's the case.

    A couple of other things you can try & maybe narrow down the problem.

    1) connect each pickup by itself (i.e. independently). You can also go further and connect each coil independently. (alligator clip leads make this a quick job). If you've got hum on one, go further down. If no hum, step 2)

    2) Stupid as it sounds, try swapping the connections between the coils within each of the two pickups. You've got 4 leads from each pickup, and all are supposed to be in series. If you start with one wire on each pickup, there are 3 other possible wires on each pickup. Hook up your first pickup, adding one lead at a time. Hum? You just found your problem - bad connection or swapped lead. No hum, carry on. If Pickup #1 shows no hum with all four wires correctly connected in series, do the exact same check with pickup #2. (leaving #1 out of it). Check hooking up each lead in turn. If no hum, go to step 3

    3) You've now got two pickups with coils hooked up in series and you've established that each one does not hum by itself. Each seriesed-up pickup has two leads. Again, start with one pair. Then try a second pair. Any hum yet? Work your way through the connections. It helps if you write each check of each pair down, if your memory is like mine.

    If you get a hum at this point, then you know that there's a problem in either pickup #1 or #2 that only shows up when you series up the two - possibly a swapped lead on one coil in one pickup, or something touching metal or some other connection it shouldn't.

    (A DVM with an ohmmeter is really useful here for checking connections and to confirm that you have things actually in series).

    If a check of the leads produces no culprit, then look at shielding.

    The foil tape (actual metal tape sold for sealing air ducts, not "duct tape") sold in hardware stores is good useful stuff as you can cut and shape it to line cavities.

    Remember that your goal is to create an electric shield "can" with your pickups inside. (If you can get it, copper tape or copper foil are better, because you can solder them. But use what's available - I've made shield parts out of aluminum cut from soft drink cans, and out of the heavy gauge aluminum pans they sell for oven liners, or one-use turkey roasters.

    Ideally, you want to connect all parts of the shield to each other. With copper, you can solder it. With foil,, you can crimp it together.

    You say that you got your hum/buzz when you disconnected the pickups from the volume controls and connected them directly in series.

    There's an old adage among radio techs that 95% of the time a problem can be traced to a connector. I strongly suspect that's the case here.

    I don't know how you connected your pickups, or if there's any chance that one coil is wired backwards, or that one connection is not quite properly insulated and is making contact with some other metal surface.

    If I had to look, that's where I'd start looking. The key is not to disconnect everything at once, then reconnect everything at once.

    The trick is to start with one piece, and just keep adding coil sections one by one until they're all connected. If you get them together and there's no hum, then it was a connection problem. I
    f you get them all together with no hum, then suddenly get a hum when some other element is introduced, chances are it's either a grounding or a shielding problem.

    You may have a ground where you don't want one, or be missing a ground where you need one.

    Likewise, bypassing the pots may also mean that you have wires that are now unshielded, so shielding them becomes the next step. If your pickup wires to the pots were shielded, was the braid soldered to the pot cases? There's the possibility that the pot cases had direct connections or bypass capacitors to ground, specifically to filter out hum. With no pots, that's no longer working.

    Sorry to meander on so long. Troubleshooting is a PITA, and doing it step by step is slow, but unless you have a blue-sky bolt of luck, the only way to find a problem like you describe.
     

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