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going crazy wiring pbass

Discussion in 'Pickups & Electronics [BG]' started by traynorbassmate, Dec 18, 2012.

  1. traynorbassmate

    traynorbassmate

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    Hi all,

    I've been around the block a few times, I know my way around tube amp repair. I'm embarrassed to say that this simple wiring job is driving me bananas.

    A few months ago I decided to re-build my p-bass electronics - new pots, capacitor and output jack. The problem is that I can't seem to get the tone knob working properly - that is, the tone is always rolled off. I've re-done this wiring twice already with no luck. Tonight I came close - getting the tone knob to behave properly before being attached to the pick guard.

    The wiring is standard - http://cdn.seymourduncan.com/images/support/schematics/std_pbass.jpg

    However, at first, with the tone capacitor grounded to the tone pot chassis, the pot didn't work. I saw a picture on the mojotone website with the cap grounded to the volume pot, at last - the tone pot works, however, the bass played a very reduced volume (poor ground). When the pots are grounded together I get full volume again, and the tone pot ceases to work. I'm using a foil-lined pick guard.

    Any help is appreciated!!
  2. iiipopes

    iiipopes

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    Something in the tone circuit is grounding out too far upstream. Take the tone control jumper loose from the volume lug to make sure the volume control operates properly. Then solder the jumper from the volume pot to the tone pot and make sure nothing else is touching anything. Sometimes the "ramp" on the pot is too steep and rolls off too much too early. Often a linear pot is used for the tone control instead of the audio taper pot used for the volume control.

    I'd try again with the stock wiring, a 250 kohm audio taper volume pot, a 250 kohm linear tone pot, a .047 capacitor, and make sure everything is isolated. Since everything is grounded through the ground wire of the pickup to the case of the volume pot, one change you could try is instead of grounding the tone pot with the shielding on the back of the pickguard, ground the case of the pot with another jumper from the case of the volume pot to the case of the tone pot, so if there is any issue with the shielding it is obviated, and leave the 3rd lug of the tone pot open, as in the diagram, with the tone capacitor going from the center tone pot lug to ground.
  3. Stone Soup

    Stone Soup

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    Disclosures:
    GHS Strings, HipShot Products, Pick Guy custom picks
    Photograph your wiring job and post it.
  4. M.R. Ogle

    M.R. Ogle Supporting Member

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    Good one!

    I can't read a "real" schematic, so I use those SD "illustrated" wiring diagrams all the time. Seems like that setup's so straightforward, I don't how it could NOT work.

    If I were in your shoes (I'm guessing here) I'd get rid of that foil under the pickguard... it's either useless or redundant, and if that didn't work, I'd try a new tone pot. That one might be defective in some way.

    Past that, I'd have to call in the Geek Squad of electronics guys who REALLY technically understand this kind of thing.
  5. Stone Soup

    Stone Soup

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    the foil is not the problem
  6. traynorbassmate

    traynorbassmate

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    Please excuse the globs of old solder on the pots, these have been through a lot. The two black wires at the vol pot are p/u and bridge. As is, I believe both pots chassis need to be shorted together, but then I lose tone control.

    [​IMG]
  7. Meddle

    Meddle

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    I would run another wire from the volume pot back to the tone pot back. Don't worry about ground loops, that is stuff of urban myths. Wiggle each wire in turn.

    I was in the same boat last week doing a P/J wiring job. Bass worked fine until I screwed the guard down.... very annoying. Ended up swapping out an entire pot assembly. Also, it *could* be lacquer on the back of each of the pots. Make sure you file the top of each pot slightly before soldering to it. Also make sure your iron is hot enough to properly melt solder onto the pots without it crusting up.
  8. Stealth

    Stealth Supporting Member

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    For one, those solder blobs are hideous. I'd unsolder everything from each pot chassis and scour it with fine sandpaper. There might be a cold solder somewhere in there that you can't see.

    EDit: I re-read the thread, re-checked the symptoms and other than a blown potentiometer or cap, I got nothing. Everything appears wired as it should. Unless those pots are shorting one of their lugs against the pickguard foil. You might want to put a cardboard shim or similar in there.
  9. maurilio

    maurilio Musician/tech at Mo's Shop and Nordstrand Supporting Member

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    Disclosures:
    Endorsing artist: Nordstrand - DR strings - Sadowsky - Genz Benz
    it looks right (a little messy ;-) )
    You need a ground wire between the two pots (theoretically the ground foil on the PG should provide that, but I don't trust that).
    Make sure the tip's connector on the jack (hot) is not touching the inside cavity (especially if the cavity is covered with conductive paint or copper foil).
    Also make sure there are no cold joints on your connections.

    M

    ps: I'm too slow typing, some else already replied!
  10. traynorbassmate

    traynorbassmate

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    Thanks for the replies thus far. All the parts I'm using are (were) new, high quality pots (Alpha or Bourns). So does it matter where the capacitor grounds itself? and ALL the grounds need to be connected together (P/U, bridge, both pot chassis, capacitor and output neg), meaning, no grounds are ever isolated from one another?

    Here is a schematic, just to be sure, I hate diagrams.

    [​IMG]

    I'll clean & sand the pot backings, and connect the two pot chassis' together so that all grounds are connected. (for the record, this is what I did on the first attempt and it did not work).
  11. traynorbassmate

    traynorbassmate

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    Wow. I'm astounded at my mental block, unable to see what should be clearly visible.

    No dice.

    I cleaned off the old solder on the pots, scuffed the metal at fresh locations. I double checked both pots before commencing soldering, both are reading fine. Here's my grounding scheme now - the cap and bridge ground to the tone pot chassis, the low side of the vol pot and p/u are soldered to the volume pot chassis. The output uses a continuous wire to ground to both pot chassis'.

    Revised schematic and picture:

    [​IMG]

    [​IMG]


    Except for the tone, the bass plays fine - full volume, no drop-outs or intermittent noises. Both pots are 250K audio (vol reads in the 240's K and tone reads in the 230's K), both sweep fine. The taper isn't the issue, I don't hear any change between either full on or full off. This is the 3rd cap I've gone through, it tests fine as all the others did, plus it sounds like its doing its job anyway (tone is permanently rolled off), I just can't seem to get the brightness back. I can even test the pot with everything hooked up, it properly puts 2 ohms to 230K between the p/u signal and the cap depending on position. Surely the difference between 230K and 250K isn't significant, I should be hearing some change between the two extreme positions.

    Thanks again everyone.
  12. M.R. Ogle

    M.R. Ogle Supporting Member

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    Just TRY swapping out that tone pot. I know it's new, but if there's something going on with it you can't see....
  13. etoncrow

    etoncrow (aka Greg Harman, the curmudgeon with a conundrum) Gold Supporting Member

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    The Bill Lawrence (Wylde Pickups) schematic shows the capacitor wired from the middle (second) leg to the third leg (empty on your shot) of the tone pot and then a wire from that third leg to ground on the pot back. Maybe it will work for you.
  14. traynorbassmate

    traynorbassmate

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    New linear 250K pot. New capacitor.
    This is insanity.

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