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Electronics experts - help!

Discussion in 'Pickups & Electronics [BG]' started by lump, Oct 20, 2000.


  1. lump

    lump

    Jan 17, 2000
    St. Neots, UK
    Sorry for the new thread, but I figured it would increase the odds of a response. I'll move it/delete it if I get an answer, as appropriate.

    Here be the deal. I installed new AERO Instrument pickups in my Carvin LB70 (I'll post the good stuff in the AERO/Lindy Fralin thread), and have run into a snag I'm hoping one of you electronics gurus can help me with.

    I'm getting a ton of ground hum when I lift my fingers off of the strings now, and I'm trying to find the cause.

    The old J99s had THREE wires from the pickups - pos, neg and a third ground wire, all going into the volume/pan pot circuit board. The the circuit board routes the two ground wires (one for each pup) to a ground wire connected to the output jack. The there is another ground wire coming from the output jack soldered to the copper shielding, and the bridge ground is also soldered to the copper shielding. Is there a way to compensated for the now non-existent pup ground wires to eliminate the hum?

    I brought the problem to the attention of Larry at AERO (who has been too cool for words), and he suggested that the problem might be the bridge ground. And although I'm confident the problem is related to the installation of the new pups (or more correctly, the removal of the old ones), I checked it out just in case, and the connection is good (I removed the bridge and reseated the wire with some copper foil underneath, just to make sure).

    Any ideas? I emailed Carvin, but unless you're buying something, I think they gaff their email. And I don't want to bother Larry anymore - he's been too helpful already, especially for a guy that makes pups for $6,000 Foderas. So, I'm bugging you guys. It could just be the normal amount of hum for standard single-coil pups, and I'm just not used to it. And it's really only a problem when the volume pot is full on or close to it. But if I can fix it, I'd like to. I'd also like to know if I'm gonna go up in a puff of smoke if grab a mic stand...

    And while we're here, I have a question about pups with exposed pole pieces. I get a buzz if I touch any of the outermost pole pieces, particularly on the neck pup outside of the E string. This sucks, cuz it's where my thumb travels, and I'm now having to adjust to avoid running it across the pole piece. Is this normal? Argh.

    Any advice would be most sincerely appreciated.


    [Edited by lump on 10-20-2000 at 10:12 PM]
     
  2. Christopher

    Christopher

    Apr 28, 2000
    New York, NY
    Assuming that you're still getting noise despite everything having been properly shielded and star-ground, I can't recommend any easy fixes. The hard fix would involve installing ground wires for the pickups (like those on the J99s or Lane Poors), but this would mean dismantling the pickups; not an easy operation. Another option would be to purchase an electronic noise gate. One final consideration: are the pickups matched, ie. is one reverse-wound with respect to another? If not, this could be contributing to the noise problem. The bottom line is that any single coils of vintage design are going to produce some sort of extraneous noise.
     
  3. Phat Ham

    Phat Ham

    Feb 13, 2000
    DC
    I was thinking the same thing as Christopher. J-basses have the pickups wound in opposite directions, so the hums from the two pickups cancel each other out. The pickups you have might have been wound in the same direction, or you may have installed one of them backwards.

    You might try switching the positive and negative on one of the pickups. Keep in mind though that I don't really know what I'm talking about so don't blame me if something goes terribly wrong
     
  4. Shield the pickup cavity. you can just put a brass plate on the floor of the pup cavity with a piece of foam rubber between the pup and the brass, a wire goes from this to ground. Or eaven better shield the pup cavitys with copper foil sides and bottom and ground this. It might be one of your ground wires have a cold solder joint so reheat the new ones. The single coil hum will increase as you get closer to a amp or floresent light. Having both pickups at equal volumes will minimize or eliminate the hum.
     
  5. lump

    lump

    Jan 17, 2000
    St. Neots, UK
    Hey, thanks guys. Further analysis reveals that my biggest problem is the crappy gov't-contracted Korean housing I live in, located in EMI central (with both 110v and 220v outlets/fixtures scattered about in the most inconvenient possible configuration). Once I got out of my apartment, the buzz wasn't noticeable at all. I should note that there was zero buzz with the old pickup configuration, but I think, like Chris said, that it's just the nature of the beast. I prefer the "retro" sound, but the price is the retro buzz. I do like the idea of shielding the pup cavities though, and will probably do that, especially since it's a cheap solution. I might also replace my output jack, because I'm not totally convinced that it's grounding properly (and that's a cheap fix too).

    Thanks again.

    [Edited by lump on 10-25-2000 at 03:17 AM]
     
  6. lump

    lump

    Jan 17, 2000
    St. Neots, UK
    An update.

    I ordered some copper foil and a new jack plug from Carvin, and today I performed a little mad science on my bass. Per Bassdude's advice, I shielded and grounded my pickup cavities, and replaced the jack plug for my own piece of mind. One or the other of those procedures reduced the hum considerably (despite the fact I SUCK at soldering). I tend to think the shielding was the big help though, because now I can hold the bass right up to my computer monitor and get no more hum than when I had the J99s installed. And I finally figured that the worst source of EMI is a 220v, 5-bulb "chandelier" that hangs in my living room, and apparently doubles as an active jamming platform. If the North Koreans attack, I'm flipping that baby on.

    Thanks again for the help!