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How do I check the Cap and pots?

Discussion in 'Pickups & Electronics [BG]' started by TJBass, Jan 4, 2012.


  1. I borrowed a multimeter so I can make sure I didnt fry the cap and pots when I soldered them, Just dont know what the readings should be and what I'm looking for.

    Anyone know how to tell if a cap or pot is bad besides the obvious no worky?
     
  2. ACalbass

    ACalbass

    Dec 16, 2011
    A pot is a resistor,the most common problem they have is the resistor wears out over time,or the surface is cut/broken.
    You generally do not produce this damages by soldering them.

    Now the cap is another story,they are made of 2 metal surfaces separated by a non conductive material,like a sandwich,and when too much heat is applied this material can melt and let the metals touch (a short circuit),or can desolder the connection between the pins and the metals ( open circuit).
    It also happen they go off specs,but this cannot be caused by soldering.

    To measure caps,the best way is to do it when is not connected to anything,because the meter will read whatever is connected to,giving you a false reading.
    But if you have to,first thing,make sure is not shorted,ohmmeter should never be 0.
    Any reading on the Kohms would say is not shorted.
    You have not much option for open circuit caps because small caps should be measured with a cap meter,not ohmmeter,but you have to assume they are good if they are not shorted.
    Bigger electrolitic caps will read a curved line ( resistance will be variable depending on the charge),because the ohmmeter's battery will be charging it until its full,if the meter is analog,the needle will first go to 0 then will start going up to max/infinite.If is a digital,numbers won't stop moving until is fully charged.
     
  3. You can't check very easily if you've already soldered.

    If you know the actual value of the pots and the DCR of the pickups, you can determine the DCR of the circuit at the output, as
    1/([1/R[SUB]Pickup 1[/SUB]]+[1/R[SUB]Pickup 2[/SUB]]+[1/R[SUB]Volume pot 1[/SUB]]+[1/R[SUB]Volume pot 2[/SUB]]) for a two-pickup bass, or
    1/([1/R[SUB]Pickup[/SUB]]+[1/R[SUB]Volume pot[/SUB]])
    for a one-pickup bass, assuming the volume pot(s) are turned up all the way.
    You don't need to account for the tone pot's resistance, because the capacitor should present a very high impedance path for the DC of the Ohm meter. The tone pot can be tested easily with an Ohm meter, since you are only using it as a variable resistor, so the resistance can be measured between the unused terminal and the wiper.

    If you are trying to check the volume pots while they are wired up, you have to account for the fact that there are resistances running parallel to them that will alter the resistance you are measuring for. It's probably not worth your effort to determine the pot values with an Ohm meter, when you can just observe their behavior as your play the bass.
     
  4. wow, seems more difficult than I expected, guess I'll just check my connections and if I dont have a bad solder joint I'll pick up new pots and caps again and start over.
     

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