?where should i take pup ohms readings from?

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Groovey Records, Sep 24, 2008.

  1. Groovey Records

    Groovey Records Member

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    To get the most accurate reading where should I place the probes?

    Does the pup need to be out of the circuit?

    Can I take it at the base of the pup on the thinest wires or should it be done on the leads?

    Thanks
     
  2. Rosewood

    Rosewood Member

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    The most accurate reading will be to unsolder the pickup and read directly across the coil because the volume pot will lower your reading somewhat.
     
  3. walterw

    walterw Gold Supporting Member

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    and don't touch the leads with your fingers, because us humans average about 500k to 2meg from one hand to the other!

    you can just plug a cable into the guitar and read the other end of the cable to get a ballpark measurement; a 6.5k pickup might read 6.2k or something that way. be sure to turn the knobs all the way up and switch to just the pickup in question by itself.
     
  4. Groovey Records

    Groovey Records Member

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    Thanks fellas
    Walt thats a neat trick
     
  5. David Collins

    David Collins Member

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    Unless removing the pickup for some other reason, I read from the cable as Walter said. If you want a better approximation to the actual pickup value outside of the circuit, a bit of math can bring you closer.

    With the pickup in question switched on alone, and the volume turned at 10, you end up taking a reading of the pickup and potentiometer in parallel. So if you take the value of the pot (call it Rᴾ), multiplied by the total resistance read at the meter (call it Rᵀ), and divide that by (Rᴾ-Rᵀ), you will find the value of the pickup alone (call it Rᴾᵁ).

    (Rᴾ×Rᵀ) ÷ (Rᴾ-Rᵀ) = Rᴾᵁ.

    So if you have a 250kΩ pot, and your meter reads 6kΩ, then (250kΩ × 6kΩ) ÷ (250kΩ - 6kΩ) = 6.15kΩ (rounded) for your real pickup value.

    Of course pot values are not precise and are really an approximation when you put them in to this equation, but it's close enough. In the example above, even if the actual pot value were a full 20% above or below 250kΩ, you would end up with ±.03kΩ (half a percent of the pickup's total resistance) difference in your end answer. Since this is probably better than the accuracy of most meters, those approximate tolerances should be just fine.
     
  6. Jon C

    Jon C Silver Supporting Member

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    you can also plug the cord in and unscrew the sheath at the jack end and read from there ... can be easier than dealing w/ the cord snaking around &c.
     
  7. Groovey Records

    Groovey Records Member

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    David
    I always knew Guitar mantainence could be more challanging then Rocket Science

    thanks oh my head is spinning but I do get it !

    I didn't just drop out I left the country and went on the road!
     
  8. Bob V

    Bob V Member

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    I seem to collect multimeters, so I soldered an old set of leads onto a guitar cord plug so I can measure the pickups from the jack without fiddling with alligator clamps or probes - basically gets you a ballpark to know whether you've got hot pickups or not, or whether your coil-shunt switch is working to split a humbucker.
     
  9. Bob V

    Bob V Member

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    I've been called a tool before. But just for the record, I'm not enough of a geek to understand Collins's formula.:)
     
  10. leper messiah

    leper messiah Member

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    Collins is just employing the formula describing the total resistance of a circuit involving resistors wired up in parallel:

    1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4 + ... etc.

    The sum of the inverse of resistances is equal to the inverse of the total resistance. In this case there are only two resistors.

    R1 = pot resistance
    R2 = pickup resistance
    Rt= total resistance

    1/Rt = 1/R1 + 1/R2

    Just solve this for the resistance of the pickup w/ a little algebra:

    1/R2 = 1/Rt - 1/R1

    1/R2 = R1/(Rt*R1) - Rt/(Rt*R1) = (R1-Rt)/(Rt*R1)

    R2 = (Rt*R1)/(R1-Rt)

    This is the formula Collins referenced.

    So you just measure Rt at the jack and use the resistance value of your pot for R1 (250K, 500K) to calculate R2.

    For example, with my WCR Darkburst neck pickup engaged and the neck volume full out, I measure the total resistance at the jack to be 8.06K. I have 500K pots installed, and assuming a 20% tolerance the actual value is certainly between 400K and 600K. Plugging this into the above equations tells me that my pickup resistance is between 8.17K and 8.23K. So the pickup's resistance is about 8.2K.
     
  11. leper messiah

    leper messiah Member

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    Since the desired pickup's pot is wide open, aren't you actually taking a measurement of that pickup and the other pickup's pot wired in parallel? So isn't it important that the other pickup's pot is turned to zero (turned to its maximum rated resistance)?
     
  12. David Collins

    David Collins Member

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    There may be some rare isolated examples, but not usually. Conventional wiring will run individual pickup volumes before the switch. So if you have the guitar switched to a single pickup's position, any volume pots other than it's own will be removed from the circuit.

    Good call for you to bring that up though. Cases like having individual and master volume arrangement, you certainly would have to account for both pots.
     
  13. leper messiah

    leper messiah Member

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    Now I'm a bit confused. If the volume pot of the pickup you're measuring is turned up to 10, doesn't it have zero resistance? Where is the other resistor in the circuit?
     
  14. David Collins

    David Collins Member

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    The pot is connected at both sides - one to the pickup, the other to ground. So even when you are at 10 (with direct continuity from the wiper to one side of the pickup), the wiper is still connected through a resistor (the carbon ring of the pot) to ground on the other side.

    Les Paul example. Switch in neck position - this removes the bridge pickup and bridge volume control from the circuit. In this position, the tip of the jack is directly connected with the center wiper of the neck volume pot. The center wiper is connected to ground by two resistors in parallel (the carbon ring of the pot, and the coils of the pickup).

    We know the total resistance of the circuit (meter reading, or Rt). We know the value (±20%) of one resistor (the carbon ring, R1). We're solving for the second resistor (the pickup coils, R2).
     
  15. walterw

    walterw Gold Supporting Member

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    the pot's basically in parallel with the pickup, remember? zero resistance in parallel means a dead short, killing the signal, while infinite resistance (open) in parallel means full signal.that's why the higher the pot's resistance, the closer it comes to equaling the sound of the pickup with no pot on it at all.
     

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