duhvoodooman
August 24th, 2008, 07:36 PM
** EDITED FOR ACCURACY & COMPLETENESS**
Here's a fast way to measure the approximate (within a couple of percent) DC resistance of your pickups without having to take anything apart on the guitar. All you need is a multimeter and a patch cable. Insert the patch cable into the guitar, turn the guitar's pickup selector switch to activate the pickup you wish to measure, turn the volume pot all the way up, and then contact one meter probe to the tip of the exposed cable plug and the other to the shaft below the insulating ring. You've now completed a circuit through that pickup and by turning your multimeter to the expected resistance range (I use the 20K range setting on my cheapo Rat Shack meter), you'll be able to read the pickup's resistance.
The photo below shows how this is done. In it, I'm measuring the middle pickup of my new Xaviere XV870, which reads 5.18 Kohm. Easy!! However, keep in mind that this is an approximation. To get a truly accurate measurement of a pickup's resistance, measure across its "hot" and ground wires while the pickup is isolated from the rest of the guitar's circuitry.
WHAT'S ACTUALLY GOING ON HERE (Warning: high techno-geek factor!): You are actually measuring the resistance of the selected pickup and the guitar's volume pot in parallel. Because the pot has a much higher resistance than the pickup (250 - 500 Kohm typical vs. 5 - 15 Kohm for most pickups), the way parallel resistances combine makes the measured value just slightly lower than the actual pickup resistance. Mathematically:
For the pickup and "dimed" volume pot in parallel:
M = measured resistance at output jack
P = pickup resistance
V = volume pot resistance
M = P*V/(P+V)
For example, for a 5.0 Kohm pickup and a 250K volume pot, the expected measured resistance at the output jack would be 4.90 Kohm, just 2% low. Similarly, for a "hot" humbucker at 15.0 Kohm and a 500K volume pot, the expected value at the jack would be 14.56K, about 3% low.
IMPORTANT: The accuracy of this method is tied to how close to "full on" your volume pot can get. For a decent quality pot in good condition, this shouldn't introduce any significant error. However cheaply constructed or very old/worn pots may introduce errors by leaving residual resistance in series with the pickup, even when the pot is "dimed". For example, if in the first example given above, the volume pot only goes to 99% of it's total span when dimed (i.e. 247.5 Kohm), the measured resistance would be 7.28 Kohm--a significant error. Other elements within the pickup's signal chain, like the pickup switch, the internal wiring, the output jack and the patch cable, contribute an insignificant amount of resistance and don't introduce any meaningful amount of error.
Incidentally, applying this same logic, if you turn your volume pot down as you make this measurement, you will see the measured resistance climb quite quickly. That's because you are adding a portion of the pot's resistance in series with the pickup, and at the same time, decreasing the portion of the pot resistance that is in parallel with the pickup. Consequently, the measured resistance climbs to a maximum value at about 50% of the pot's nominal resistance range, and then falls again, ending at zero (or very close to it) when the volume pot is all the way off. At that point, there is zero resistance to ground for the guitar signal through the pot, which is exactly why the volume shuts off completely!
(OK, major geek detour ends here.)
This technique is also a good way to check for a broken wire or connection in your pickup signal chain through the guitar. If you have an open circuit in the pickup, the measured resistance will be that of your volume pot. By the same token, a "dead short" will show as zero resistance or very close to it.
http://duhvoodooman.com/miscimages/musical/measure_pup_res.jpg
Here's a fast way to measure the approximate (within a couple of percent) DC resistance of your pickups without having to take anything apart on the guitar. All you need is a multimeter and a patch cable. Insert the patch cable into the guitar, turn the guitar's pickup selector switch to activate the pickup you wish to measure, turn the volume pot all the way up, and then contact one meter probe to the tip of the exposed cable plug and the other to the shaft below the insulating ring. You've now completed a circuit through that pickup and by turning your multimeter to the expected resistance range (I use the 20K range setting on my cheapo Rat Shack meter), you'll be able to read the pickup's resistance.
The photo below shows how this is done. In it, I'm measuring the middle pickup of my new Xaviere XV870, which reads 5.18 Kohm. Easy!! However, keep in mind that this is an approximation. To get a truly accurate measurement of a pickup's resistance, measure across its "hot" and ground wires while the pickup is isolated from the rest of the guitar's circuitry.
WHAT'S ACTUALLY GOING ON HERE (Warning: high techno-geek factor!): You are actually measuring the resistance of the selected pickup and the guitar's volume pot in parallel. Because the pot has a much higher resistance than the pickup (250 - 500 Kohm typical vs. 5 - 15 Kohm for most pickups), the way parallel resistances combine makes the measured value just slightly lower than the actual pickup resistance. Mathematically:
For the pickup and "dimed" volume pot in parallel:
M = measured resistance at output jack
P = pickup resistance
V = volume pot resistance
M = P*V/(P+V)
For example, for a 5.0 Kohm pickup and a 250K volume pot, the expected measured resistance at the output jack would be 4.90 Kohm, just 2% low. Similarly, for a "hot" humbucker at 15.0 Kohm and a 500K volume pot, the expected value at the jack would be 14.56K, about 3% low.
IMPORTANT: The accuracy of this method is tied to how close to "full on" your volume pot can get. For a decent quality pot in good condition, this shouldn't introduce any significant error. However cheaply constructed or very old/worn pots may introduce errors by leaving residual resistance in series with the pickup, even when the pot is "dimed". For example, if in the first example given above, the volume pot only goes to 99% of it's total span when dimed (i.e. 247.5 Kohm), the measured resistance would be 7.28 Kohm--a significant error. Other elements within the pickup's signal chain, like the pickup switch, the internal wiring, the output jack and the patch cable, contribute an insignificant amount of resistance and don't introduce any meaningful amount of error.
Incidentally, applying this same logic, if you turn your volume pot down as you make this measurement, you will see the measured resistance climb quite quickly. That's because you are adding a portion of the pot's resistance in series with the pickup, and at the same time, decreasing the portion of the pot resistance that is in parallel with the pickup. Consequently, the measured resistance climbs to a maximum value at about 50% of the pot's nominal resistance range, and then falls again, ending at zero (or very close to it) when the volume pot is all the way off. At that point, there is zero resistance to ground for the guitar signal through the pot, which is exactly why the volume shuts off completely!
(OK, major geek detour ends here.)
This technique is also a good way to check for a broken wire or connection in your pickup signal chain through the guitar. If you have an open circuit in the pickup, the measured resistance will be that of your volume pot. By the same token, a "dead short" will show as zero resistance or very close to it.
http://duhvoodooman.com/miscimages/musical/measure_pup_res.jpg