What are the signs of a weak magnet?

Discussion in 'Just Pickups' started by Peckhammer, Jan 13, 2020.

  1. Peckhammer

    Peckhammer Tele-Holic Silver Supporter

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    I have a single coil pickup referenced in another thread that has very low volume output. I did some additional testing this morning, connecting the pickup directly to an amp and holding it over the strings of another electric guitar (that was not connected to an amp).

    I strummed the guitar and the pickup I was holding over the strings sounded weak. I then held the pickup about 2 inches from the guitar and it sounded full and loud. That was a head-scratcher, but I noticed that I was holding the pickup directly over the poles on the guitar's installed humbucker. If I moved the pickup over the strings between the two humbuckers, it sounded weak, When I held the pickup over the poles of either humbucker, the volume increased substantially.

    The magnet in this pickup has enough charge to hold a small pickguard screw on it . Could a weak magnet in the pickup be causing the low volume condition? Any suggested methods of testing for it?
     
    Last edited: Jan 13, 2020
  2. bgmacaw

    bgmacaw Friend of Leo's

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    A multimeter...

     
  3. sugarinthegourd

    sugarinthegourd Tele-Meister

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    Unless your multimeter tests gauss that won’t do it.

    low tech approach: how far away from that pickguard screw does the pickup need to be to lift it? How far for a similar pickup whose strength you like?

    Is your pickup Alnico or ceramic? If the latter, might check whether the bar magnet is held fast to the back.
     
  4. jvin248

    jvin248 Poster Extraordinaire

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    .

    Use a paperclip or screwdriver to stick to the weak pickup pole piece and then to a regular one and how does the pull feel? Same/different?

    You could support the pickup(s) stick a paperclip to it then hang various weights (like more paper clips) from it to see a actual relative measurement when the magnet lets go.

    Some of the alnico pickups will have one or two poles go out and the rest are ok.

    ... "if you had a ceramic magnet they never weaken nor drop their field" where alnico weakens, drops, and reverses. Which is higher quality ;)

    .
     
  5. chillybilly

    chillybilly TDPRI Member

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    When it skips leg day.
     
  6. Peckhammer

    Peckhammer Tele-Holic Silver Supporter

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    Thanks for the input. Since I work in the repair business, I took the pickup to a local winder. He could not explain why the pickup volume swelled when it was held over another magnetic field, but he quickly identified that the winder (TK Smith) had installed one of the magnets upside down. The pickup now sounds normal.
     
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  7. bgmacaw

    bgmacaw Friend of Leo's

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    It will test if the pickup output is correct. Damage to the windings is a lot more common than a weak magnet.
     
  8. TimTam

    TimTam Tele-Holic

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    I think your findings actual exemplify two competing theories of how pickups work. The usual theory says that the ferromagnetic string's movement creates a change in the magnetic field created by the pickup magnet, that is detected by the coil. Theory two says the string is actually magnetized by the pickup magnet, and the coil detects the movement of the magnetized string's field; that is, the string becomes the (moving) magnet.
    https://lawingmusicalproducts.com/dr-lawings-blog/how-does-a-pickup-really-work

    Your finding with the SC's output when over the HB is consistent with theory two - the HB magnet nearby has magnetized your string strongly and your SC pickup has detected the strong change in the string's field as it moves.

    I don't know enough of the science to know whether that is definitive, or whether there is more to it.
     
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  9. Antigua Tele

    Antigua Tele Friend of Leo's

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    Both are true, but #2 is the primary, #1 is secondary, and people misunderstand #1.

    #2, the magnetized string induces voltage, is very easy to prove, so it's not a theory. #1, the guitar string moves the magnetic field of the pole piece, that must be true so long as the pole piece has significant permeability, for the same reason that a screw driver becomes a magnet which it's close to a real magnet. Steel pole pieces are steel, just like a screw driver. AlNiCo is permeable also, but much less so than steel. AlNiCo 2/3/4 are more permeable than AlNiCo 5. #2 is not a theory either, it's well understood in transformer and inductor design as being the reluctance path of the core(s).

    Therefore the sum output is a mixture of cause #1 and cause #2. With AlNiCo 5 Strat pickups, most of the output is caused by cause #2, but with all manner of steel pole/screw pickups, cause #1 becomes a more prominent factor, because the permeability is so much greater. It's hard to say what the exact ratio is without some complicated math, but a lot of the relevant math has been worked out http://www.physics.princeton.edu/~mcdonald/examples/guitar.pdf
     
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  10. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

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    Not able to leap over a building in a single bound?? :p

    r
     
  11. Antigua Tele

    Antigua Tele Friend of Leo's

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    Did he re-magnetize it or flip the magnet over?
     
  12. Peckhammer

    Peckhammer Tele-Holic Silver Supporter

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    I think he re-magnetized it with a machine that is dangerous for people with pacemakers.

    The TK Smith CC blade pickup is constructed as a single coil with wire wound a bobbin and a blade in the middle. The magnets are placed on either side of the bobbin (lengthwise). Imagine a P90, but take the magnets from below the bobbin and place them on each side of the bobbin/coil. Wolf noticed that one magnet was South up and the other was North up.

    I think the magnets were epoxied in but I am not 100% sure. Wolf did place the pickup on a machine used to charge magnets or change their polarity. Both magnets are now North up, and the pickup works as expected.

    Mistakes happen. I assume one magnet was installed upside down during assembly. The technical problem was solvable. TK Smith’s customer service, maybe not so much. “I’ve never had any problems with my pickups... maybe my pickups just aren’t a good match for you.” He would not fix or replace this pickup; he would only offer me a refund if I returned it.
     
    Last edited: Jan 14, 2020
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  13. Peckhammer

    Peckhammer Tele-Holic Silver Supporter

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    Interesting information. What I'd also like to gain from this experience is something that can be used for future troubleshooting. The pickup exhibited symptoms consistent with a shorted coil or a magnet problem: weak output, but fairly normal DC resistance reading. I was originally convinced that this was a coil short because the magnets seemed to be charged. In my serendipitous test of holding the pickup over another charged magnetic field and getting strong output, I was able to rule out the shorted coil theory.

    The results of my test were repeatable on a strat, too. Holding the CC pickup over any of the strat pickups resulted in increased volume from the CC pickup. Given what we now know about the CC pickup, I still don't fully understand why placing the pickup into the magnetic field of another pickup raised it's volume level. I suspect that increasing the magnetic field allowed the coil to do its job.
     
  14. Antigua Tele

    Antigua Tele Friend of Leo's

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    Seems like an obvious statement, right?

    When the pole pieces are arranged "N N N N N N", the magnetic fields all point up and out. If you have like "N N N S N N", the magnetic fields of "N S N" are all complimentary, because the polarity on the sides of the N oriented magnets (the return paths) has a polarity of S, and vice versa. So if you have, in a given point in space, N point one direction and S pointing the other, the net flux N minus S, less magnetism.

    This is why pickups like the Fender Super 55, which are "N N N S S S" are notorious for having a dead spot in the middle of the pickup. What people don't realize about those pickups is that not only is there a dead spot in the middle, but the D and G pole pieces are weaker than the otherwise would be. Here's an ascii art representation:


    https://www.strat-talk.com/threads/fender-super55-split-coil-analysis-and-review.441252/
    Fender Super55 Neck/Middle

    pole____offset__raw_____adjusted
    ________0.5_____-48_____1__*
    E_______1_______-43_____6__******
    ________1.5_____-43_____6__******
    A_______2_______-42_____7__*******
    ________2.5_____-44_____5__*****
    D_______3_D_____-45_____4__****
    ________3.5_____-49_____0__
    G_______4_G_____-45_____4__****
    ________4.5_____-43_____6__******
    B_______5_______-41_____8__********
    ________5.5_____-41_____8__********
    E_______6_______-41_____8__********
    ________6.5_____-48_____1__*
     
    Last edited: Jan 14, 2020
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