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Precision Pickup Problem

Discussion in 'Pickups & Electronics [BG]' started by slagbass, Oct 31, 2013.

  1. slagbass

    slagbass Supporting Member

    Apr 5, 2005
    When I plugged in my '77P today it sounded weak and tinny. A quick inspection didn't reveal any problems. Then I broke out the multi-meter. I measured 305kOhms resistance across the pickups. No, I'm not missing a decimal place. I checked my '66P just to make sure I wasn't hallucinating and, sure enough, it measures 10.4kOhms.

    Any idea what would cause this? Any idea how to fix it (short of replacing the PUs)? I'm also curious as to how it could have deteriorated so quickly. I never measured it in the past, but I assume that since it sounded fine the resistance was ok.
  2. GlennW


    Sep 6, 2006
    Where did you have the prongs of the meter when you took your reading?

    If on the end of the leads at the pot, try again at the pickup itself.

    If you're getting the high reading at the pickups, I'd try re-flowing the solder joints where the leads are soldered to the eyelets on the pickups. Oxidation may have weakened the connection.
  3. slagbass

    slagbass Supporting Member

    Apr 5, 2005
    Thanks GlennW. The reading was taken across the pickup. I'll try your suggestion in the next couple of days.
  4. GlennW


    Sep 6, 2006
    Also check each coil individually.

    One coil might read 5.5K +/-, and the other much higher. If that's the case, leave the good one alone.
  5. slagbass

    slagbass Supporting Member

    Apr 5, 2005
    The fun continues.... Across both coils individually, I get an open circuit. Across both coils in series, 308KOhms. :confused:
  6. GlennW


    Sep 6, 2006
    The open circuit readings might be because your probes aren't really making contact...maybe potting lacquer is preventing it. Sometimes I'd have to carefully "scribble" with the probe on the solder blob to get a reading. The scratching around cuts through any invisible funk. If you try that, make sure you don't cut the coil wire.

    Also try your probes with one probe on the inside coil wire (at the eyelet), and the other to each magnet (one coil at a time, and one coil at a time). Again, scratch/scribble around on the magnet. You're looking for a short betweem the coil and the magnets. If you get a reading doing that, that coil needs a rewind.

    And be sure your meter is set on the right scale.
  7. slagbass

    slagbass Supporting Member

    Apr 5, 2005
    I'm definitely getting contact on the connection at the back of the pickups. Across both coils I get 308Kohms. Across the remaining two connections (i.e. straight wire between pickups) I get 000Kohms (as you would expect). Therefore, I can get contact at all 4 connections. But I still get an OC across both coils individually. Changing the scale on the multimeter doesn't help.

    When you say "inside coil wire at the eyelet" do you mean taking off the PU cover and getting a reading right next to the winding? I'm no expert but I don't see the difference between the eyelet on the outside of the coil vs. inside the coil as long as I can get continuity on the dab of solder on the outside eyelet (which I can).
  8. GlennW


    Sep 6, 2006
    Your meeter should be set to the 20kOhm scale (assuming it has one).

    You're taking your readings from the right places. The eyelets are similar to those in shoes/boots where the shoestring runs through them. They have holes with the coil wire wrapped around it a few times, the lead inserted, and all joined/connected with solder.

    I specified the eyelet for two reasons: 1) the coil wire itself is easy to break, 2) the coil wire is also insulated.

    The straight wire between the coils is often called the jumper wire.

    It sounds like both coils are bad, but that doesn't explain the 308kOhm reading across both together, unless it's self-capacitance (which means it's dead but doesn't know it).

    Did this just start all of a sudden?

    I'd try re-flowing the eyelets at this point.
  9. slagbass

    slagbass Supporting Member

    Apr 5, 2005
    Yes, started very recently. Prior to this, I did remove the pickup and bolster the foam backing (the pickup was sitting a bit low) but didn't take the PUs out of their covers or mess with the electronics.
    The only unusual thing I see is a black tar-like substance under the PUs. It's like the black foam backing melted at some point. When I lift the PU covers, it doesn't seem to have infiltrated the coils, so I'm not sure what, if anything, this has to do with the problem.
    I'll try reflowing the joints as you suggest. Failing that, I think its off to the repairman for a PU replacement.
    Thanks again for your help! Much appreciated.
  10. You could get the pickups rewound instead of replaced. Lindy Fralin does that work, as does Curtis Novak I believe. That way you keep the original pickup hardware, just redone so that it works correctly again.
  11. GlennW


    Sep 6, 2006
    Since the covers weren't removed, I'll guess that taking the pickups out might have shifted the leads in the eyelets enough to compromise one or more connections. Re-flowing might fix it for you.

    That black stuff under the pickups...that's the mojo...leave that there.

    Good luck.
  12. slagbass

    slagbass Supporting Member

    Apr 5, 2005
    Tried reflowing the eyelets. That seemed to make a few changes. Now:
    308 kOhms across both coils
    529 kOhms across one coil
    264 kOhms across the other
    (note that I'm on the 2000k scale to get these readings).

    I also tried each magnet from one each eyelet. All were OC (or not registering) except ONE of the eight that gave me 7.3 kOhms.

    Anything else I could try or is it time to spend some cash?? :)
  13. GlennW


    Sep 6, 2006
    You need to have your meter set to the 20kOhm scale, if that's an option.

    Your readings don't make sense.
    When you say 308k across both coils, you should have each probe on an eyelet that has a lead that goes to the pot, so you're measuring both coils in series. Is that correct?
    That should give you 529k + 264k = 793k (which is an out-to-lunch reading).

    But you have made progress.

    The 7.3k reading from one eyelet to one magnet is a bad sign because it indicates the coil wire is shorted to that magnet, but the reading also makes no sense. It should be less than a full reading for one good coil (which is normally between 5-6k on a regular P bass). If you have a short to the magnet, it needs a rewind.

    If we disregard the numerical values of the readings, the bottom line is one coil is shorted to one magnet, which means it's bad.

    It's probably a lost cause, but try reflowing the eyelets on the shorted coil, and take ALL of the readings again ON THE 20K scale.

    Or send it out for a rewind, or buy another pickup.

    It just seems weird that it was ok, then you removed the pickups (but not the covers), and now it's screwed up. Something happened.
  14. Ric5

    Ric5 Supporting Member Commercial User

    Jan 29, 2008
    I grow organic carrots and they are not for sale
    Perhaps you have a short or a wiring problem.
  15. megafiddle


    May 25, 2011
    If the pickup is still in the circuit, and the volume pot is 500K, you could get those kinds of readings.

    It would indicate that each coil is hundreds of Kohms, and a total pickup resistance up around 1 Megohm.
    In that case, when you try to read one coil, you are also reading the combination of the other coil and pot
    in parallel with it.

    Since both coils apparently opened, one possibility is that the coil wires were never soldered properly, and the
    magnet wire insulation coating was never removed in the process. One tiny spot may have made just enough
    contact to make it work (like at the very end of the wire).

    Wicking or vacuuming off the solder might reveal wire that's still insulated. It could then be resoldered properly.
    If you are going to can them anyway, you have nothing to lose. Just don't damage the coil wire.

    As GlenW said, one coil is shorted to a magnet. It's possible that it's still usable if the opens can be corrected.
    For example if the short is very close to the grounded pickup lead (small number of turns between short and
    ground). The short may have been there all along, even when they were working.


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