Strange Hum?

[farbs]

Hi Guys,
I recently got myself a new rig, and noticed when I rest my thumb on the back pickup I get a hum out of the amp/speakers. Wasn't using anything before. The bass is a Spector EuroLX. With the tone pump. Kind of sounds like a grounding or sheilding issue. It's a P/j config. The bass sounds great apart form that and only the the back Pickup not the front. Any ideas??

 

[mcnach]

You mean touching the polepieces?

If yes... I had that with a couple of basses, and it came from the polepieces not being grounded.
If the pickup is an alnico it'll not have a big chunky magnet uncer the polepieces, which will probably be exposed from teh inside. In that case all you have to do is stick some copper shielding to them on the underside, ground that, and enjoy the bass.
I had that with a ceramic pickup that sounded great otherwise, and couldn't access the polepieces from underneath. I tried to do it from teh top, after removing the cover, but it wasn't working. It was hard to get anything to stick to the polepieces properly without showing or blocking the holes in teh cover. I ended up cutting thin strips of adhesive copper foil used for shielding, and stick one across each polepiece. Then I put a strip joining all polepieces, and grounded that. Finally, I used some silver metallic nail varnish to coat the polepieces (and the copper). Applied 2-3 coats, and put it back together. It did the trick and you don't really notice anything odd with the polepieces unless you're really paying attention.

 

[mcnach]

Actually, I had some pictures still online...

Here's the pickup as it was, underneath. It actually had copper shielding to ground teh polepieces, but clearly it was not touching all of them (use a multimeter and you can verify that).

From the top: some copper shielding that was grounded, and now I just needed to get it to touch the polepieces. I tried to add a bit of solder between polepieces and copper (adding another strip of copper just tangent to the polepieces), but it didn't stick... so I went with plan B.

This is plan B: the copper strips across the polepieces.

I don't have pictures of the polepieces with the nail varnish, but it works
I painted them black at some point, then silver.

 

[farbs]

Thanks for the reply, they don't have exposed poles as they are the emg pickups, p/j config. So I think they are sealed.

 

[Stealth]

Very cool. Thanks for posting this, this explains a LOT about how shielding should be done.

* saves thread for future reading *

McNach, I checked your website - do you have any specific section for bass mods like this? I've checked the site far and wide, but no luck.