How do you measure volume pot resistance that are already wired up?

Discussion in 'Luthier's Corner: Guitar & Bass Technical Discussi' started by Fendert, Dec 26, 2011.

  1. Fendert

    Fendert Member

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    I'm not a total idiot, because I known how to measure a potentiometer, but for some reason my multi meter doesn't measure the volume pots on the guitar.

    They are vintage and dont say the value on them.

    Can anyone help? I would really like to no have to un solder them.
     
  2. kingdavidy2k

    kingdavidy2k Member

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    I'd like to know this too
     
  3. Mickey64

    Mickey64 Member

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  4. 8len8

    8len8 Member

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    For in-circuit resistors (or pots) you need to unsolder 1 terminal to measure its resistance. Otherwise you'll be measuring it along with everything in parallel with it (likely the pickup).
     
  5. VaughnC

    VaughnC Supporting Member

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    In order to measure the resistance of a pot in a guitar, the pot has to isolated from the pickups or, due to the shunt resistance of the pickup's coil, the reading will be inaccurate. The easiest way is to unsolder one of the pots outside terminals, then measure.

    However, depending on the guitar in question, you might be able get the pickup selector switch into a between position where its out of circuit (ex. Strat or Tele) which would isolate the pickup from the pot. If you're not the techie type and this doesn't make sense its probably best to just unsolder one end of the pot.
     
    Last edited: Dec 26, 2011
  6. walterw

    walterw Gold Supporting Member

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    good idea for a fender, but not on a gibson.

    gibson pickups are soldered directly to the pot; disconnecting the switch just disconnects the pot from the output jack.
     
  7. VaughnC

    VaughnC Supporting Member

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    Darn...thanks, I never knew that!
     
  8. mistercoffee1

    mistercoffee1 Member

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    Here's how you can get an idea of your pot's resistance without unsoldering, from the Les Paul Forum:

    "Select the pickup you want to test

    Measure the resistance at full volume, it will be about 6k to 15k, depending on the pickup
    Measure at zero volume - should read zero if you have modern or 50's wiring
    If those check out, adjust the volume to find the maximum reading. It will be 1/4 of the sum of volume pot plus pickup resistance.

    So for a perfect 500k pot, you would expect 125k to 130k. For a 300k pot, you'd get 75k to 80k. Actual pots may vary by 20%. These max readings will occur at about 5 with a linear pot and 7 with an audio pot. The tone pots makes no difference."
     
  9. Rockledge

    Rockledge Member

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    Makes me wonder if you could do the same thing using the formula 1 divided by (1/R1) plus (1/R2) , R1 being the pickup and R2 being the pot.
    Tried with the various suspected pot values.
    If I am right about this, the answer to the formula should be the reading you get at the outside pot lugs with the pot turned entirely down. .
     
  10. DT7

    DT7 Member

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    Just in case it isn't obvious to somebody, this will not give you the value of the pot...only the value of the pot plus the pickup. This lets you get the pot value only if you know the pickup value ahead of time...though if you know about where the pickup is at, you can obviously get very close for the volume pot.

    Unless you're one of those wisenheimers who doesn't use a cap on the tone-pot...in which case this whole trick won't work.
     

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