Is this hum a shielding or a grounding issue?

Discussion in 'Pickups & Electronics [BG]' started by boristhespider7, Nov 28, 2012.

  1. boristhespider7


    Jan 27, 2008
    OK guys...really appreciate any thoughts....

    I have an Ep Viola bass (which sounds great BTW) apart from the hum:crying: I have hum when facing the amp or with my back to it. Hum disappears if i have my bass end on to my amp (either end doesn't matter). Also dissapears if bass is side on to the amp. Wouldn't be so much of an issue if i could have a decent distance from my amp but most gigs i do this isn't practical.

    Now for some details:
    The PU's are mini humbuckers with a VVT arrangement. There's a working ground attached to all pots, the metal PU covers (enclosing the PU's), the bridge and strings. Hum is not effected by whether i touch any part of the bass or not. There is no shielding inside the bass (would be kind of hard to get it in the hollow body but not impossible...just v fiddly). Hum does not increase/decrease if i have both PU's on max or just 1 PU on. Amp doesn't show any hum with any other bass i have (including a 50's single coil P bass)

    Is the hum a shielding issue? Or could it be something else eg. rubbish PU's or electronics?
  2. bassbenj


    Aug 11, 2009
    Rule is that if the hum goes away you you touch the strings (or any ground like jack body) it's a shielding problem, but if the hum depends on the orientation of the bass but does NOT go away when you touch ground it's a single coil hum problem.

    Since your pickups are supposed to be humbuckers this can only mean your pickups are either wired wrong (you'll have to reverse one of the coils) or they are crap and the two coils are not cancelling hum.

    You should solo each pickup to see if both have the same problem. Sometimes one coil of a humbucker can short out (or short to the grounded metal can) leading to a miswiring. If each pickup has only two wires then there is little you can do save new pickups, if there are 3 or four wires a wiring check is in order. Also as last resort taking pickups apart to look for shorts and testing both coils (neither should show low ohms) may be an option.

    Hope this helps.
  3. bumperbass


    Jun 19, 2012
    I think you have this backwards. There could be a shielding problem with orientation. I agree about the possible pickup problem....turning into a single coil due to defect, etc.
  4. boristhespider7


    Jan 27, 2008
    Thanks for the useful advice! I have solo'd each PU, exactly the same with each one. Each PU only has 2 wires:(
    I tested the ohms of each PU with an Ohmeter just testing the ends of the cable where they are soldered on (i think that will give me the reading across the PU?) and got about 3.5 ohm. If thats right isn't that a bit low for a humbucker? I'm not an expert on this but aren't they usually at least double this? Could it be that they aren't reverse wired and if i reverse the polarity of one of the PU's they'd hum less?