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Thread: Fixing vintage 7ender PU

  1. #1
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    Fixing vintage 7ender PU

    HI
    I have had here many good advices. So I'd like to share how I manged to fix (or at least make it live longer) my '65 P bass pickup. I'm sure that method
    won't always work, and rewinding must be done. But I think it is worth of trying.

    When I measured the PU closely, I noticed that the bass side coil has 140 kΩ DCR to the upper magnet. I checked more closely with a loupe, and there is rust on the magnet.

    I scraped the laquer away from the boppin and the magnet, and heated the magnet a little with soldering iron to break the joint and make a small fissure between the mag and the boppin. Then I put there several times a little contact spray. And same thorough the winding hole. And let the capillarity work.
    And hocuspocus: the DCR went up over 20 MΩ.
    PS. Just measured the PU. My Fluke 179 DCR range is 50 MΩ. Shows "OL" (over 50 M). Must check with EE 5000 the impedance.

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    Last edited by okabass; 05-21-2018 at 08:27 PM.

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    Senior Member ken's Avatar
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    What if any did your DCR change? It should be about the same as the treble side coil.

    ken

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    Sorry, don't quite understand.
    Both coils DCR is ok. Don't remember exact values, but PU's total DCR is ca. 10,7 kΩ, L is 5,8 H.
    The problem was (perhaps still is) the leak from the coil to the magnet. But it went from 140 kΩ to +50 MΩ. Its quite usual failure on PU of that era. Rust and bad insulation makes eventually a dead PU.

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    Quote Originally Posted by okabass View Post
    Sorry, don't quite understand.
    Both coils DCR is ok. Don't remember exact values, but PU's total DCR is ca. 10,7 kΩ, L is 5,8 H.
    The problem was (perhaps still is) the leak from the coil to the magnet. But it went from 140 kΩ to +50 MΩ. Its quite usual failure on PU of that era. Rust and bad insulation makes eventually a dead PU.
    Electrical contact between a magnet and one turn has no audible effect and will not show in the DCR or frequency response either. This changes when more turns are really shorted via the same magnet. Your symptoms indicate that the wire insulation at one or more contact points is damaged/degraded. While the contact cleaner and your manipulation of the magnet seem to have broken a high resistance contact bridge (conductive rust or whiskers), the cleaner may as well have increased the risk of a better magnet to winding contact, when magnet or coil get moved a little (e.g. by thermal expansion/contraction). The purpose and effect of contact cleaners is to reduce contact resistance and I would not use it in cases where you want no contact at all. Even though some cleaners (definitely not all) may help to prevent/reduce further oxidation of the magnet.

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    Yes. Thank you.
    I'm aware that it doesn't fix permanently the PU: rust never sleeps. And the effects of that 140 kΩ point wasn't audible. And I took a risk to warm the pole. But you may use it as a last means before rewinding.
    If it works you have a working original PU instead of a rewound one.

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    Quote Originally Posted by okabass View Post
    Yes. Thank you.
    I'm aware that it doesn't fix permanently the PU: rust never sleeps. And the effects of that 140 kΩ point wasn't audible. And I took a risk to warm the pole. But you may use it as a last means before rewinding.
    If it works you have a working original PU instead of a rewound one.
    Sorry, I don't get your point. The PU worked fine and most probably would have worked fine for a long time. No need for rewinding. You took a lot of risk and I am not convinced that your measures really reduced the risk of an eventual failure (i.e. partial short).

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    Ok. I understand, that the 7ender PU's (60's -70's) have a great tendency to break. I have both PUs of my '73 Jazz Bass broken. I believe that 140k point is an early symptom of that. Now I hope the method slows or even stops the degeneration. We'll see. If the PU eventually break, then I'll have to rewind it, no problemo.

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    Quote Originally Posted by Helmholtz View Post
    The purpose and effect of contact cleaners is to reduce contact resistance and I would not use it in cases where you want no contact at all. Even though some cleaners (definitely not all) may help to prevent/reduce further oxidation of the magnet.
    What about "non-conductive" cleaner/lubricants? Safer?
    https://www.digikey.com/en/product-h...rosol-cleaners
    3M Novec Contact Cleaner/Lubricantis a non-flammable, non-conductive aerosol cleaner and lubricant designed for critical electrical cleaning challenges. Designed to help make the workplace safer, it complies with green initiatives, and cuts electrical cleaning challenges down to size. It quickly penetrates and flushes away light and fluorinated oils and greases from sensitive electrical and electronic equipment including avionics, electro-mechanical devices, electrical contacts, connectors, and switches. It also leaves a thin protective silicone coating to help prevent excessive wear and abrasion. It supports U.S. DOD initiatives to procure environmentally preferable products and meets MIL-PRF 29608A Class L specifications for electrical contact cleaning and lubricating compounds.

    -rb

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    Quote Originally Posted by rjb View Post
    What about "non-conductive" cleaner/lubricants? Safer?
    https://www.digikey.com/en/product-h...rosol-cleaners
    3M Novec Contact Cleaner/Lubricantis a non-flammable, non-conductive aerosol cleaner and lubricant designed for critical electrical cleaning challenges. Designed to help make the workplace safer, it complies with green initiatives, and cuts electrical cleaning challenges down to size. It quickly penetrates and flushes away light and fluorinated oils and greases from sensitive electrical and electronic equipment including avionics, electro-mechanical devices, electrical contacts, connectors, and switches. It also leaves a thin protective silicone coating to help prevent excessive wear and abrasion. It supports U.S. DOD initiatives to procure environmentally preferable products and meets MIL-PRF 29608A Class L specifications for electrical contact cleaning and lubricating compounds.

    -rb
    I never encountered a conductive contact cleaner. Their purpose is to loosen and sweep away oxides and other debris but not to conduct.

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    Quote Originally Posted by okabass View Post
    Ok. I understand, that the 7ender PU's (60's -70's) have a great tendency to break. I have both PUs of my '73 Jazz Bass broken. I believe that 140k point is an early symptom of that. Now I hope the method slows or even stops the degeneration. We'll see. If the PU eventually break, then I'll have to rewind it, no problemo.
    Why should a leakage resistance indicate a future break? Electrolysis tends to eat up the ignoble (base, less noble) metal. And this is the magnet. The wire may eventually break from tension cycles.

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    I understand, that the 7ender PU's (60's -70's) have a great tendency to break. I have both PUs of my '73 Jazz Bass broken.
    I also had several Fender Strat PUs from the 60s and 70s with a wire break or shorted turns. The most probable and obvious reason was that some previous owner had tried to move (push- or hammer-in) the g-string magnet.

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    Rust and sweat on the magnet. Rust swells on the magnet and eats itself little by little to the coil and tensions the wire. If I remember me correct Formvar insulation is quite sensitive to moisture (air, sweat).
    https://static1.squarespace.com/stat...8/?format=500w

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    On my JB the PU wire (plain enamel) quite probably break. There was a tape around the magnets, no rust. The sound was very thin and quiet.

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