Post by ChrisK on Apr 12, 2007 16:34:25 GMT -5
or Guitar Wiring Diagnosis or... as previously titled.
Brain Scanning Thru a Nostril (or Output jack).
I’ve often talked about connecting a digital Ohmmeter to the output jack on a passive (as in no internal preamp or batteries) electric guitar to “see” the values inside, and to attempt to troubleshoot things prior to surgery (taking it apart to find out for sure).
I first want to answer the question as to why this process is important. Many folk rip their guitar apart and then ask for help, describing things that they think that they see in words and terms not related to specific conditions. When one folk was asked how the wiring on a switch looked, he said it looked “like a spider.” While certainly descriptive, it was most unhelpful to those asked to suggest what said folk should do to fix things (“fix” also being subject to interpretation).
While the wiring “stuff” in an electric guitar looks complicated to the unschooled, it is really fairly simple.
The Fender Stratocaster
I’d like to start with the Fender Stratocaster since this is likely the most popular and common design in history. The basic wiring design is also used in many other guitars of similar pickup configuration.
The basic Stratocaster has three single coil pickups. These are comprised of a single wire coil wound around six cylindrical magnets. The coil’s DC resistance is typically around several thousand Ohms from the thousands of feet of very thin solid wire in the range of #42 to #45 AWG wound thereon. There are other structures available in a single coil sized form-factor including dual coils for hum cancellation, dual rows of pole adjustment screws, and so forth. There are also dual-coil side-by-side pickups (the traditional humbucker). Without getting into the variations of pickups, the basic tenants of this means of testing are valid for most.
The Stratocaster has a pickup selector switch which is really a two pole three position lever switch (as used in the Fender Telecaster and all Stratocaster’s until the early 1970’s) with two additional position detents added between the three original ones. These additional detents were first added by guitarists who filed notches onto the original 3 position switch detent mechanism to realize pickup combinations that the manufacturer never thought of (the switch was a commercially available one from the 1950’s often used in intercoms and telecom gear). For some strange reason these additional positions are known as the notch positions. In the basic Stratocaster this switch selects the Bridge pickup (an original position), the Bridge and Middle pickups in parallel (a new notch position), the Middle pickup (an original position), the Middle and Neck pickups in parallel (a new notch position), and the Neck pickup (an original position). These are selected using one of the poles on the switch. There are other combinations available on some factory guitars, and on many of the custom wiring schemes designed by folk with way too much spare time.
The basic Stratocaster also has two tone controls that are connected in variations of the pickup selection. The most common one is one tone control is selected for the Neck pickup, and the other tone control is selected for the Middle pickup. There is no tone control selected for the Bridge pickup. This is the original configuration from the time when only one pickup could be selected at a time. Note that in the Middle in parallel with the Neck position, both tone controls will be active. Also note that in the Bridge in parallel with the Middle position, the Middle tone control will be active. The tone controls are selected by the other pole on the lever switch. These tone control circuits traditionally are each comprised of a variable 250 K Ohm audio taper (logarithmic) potentiometer connected in a rheostat mode (two-wire) connected to a common capacitor of 0.1 uF, 0.047 uF, or 0.22 uF, although other values may be in use.
Additionally there is a volume potentiometer that effects local output level. This is traditionally a 250 K Ohm, audio taper component. The wiper (center terminal) is connected to the output jack.
The Test
The test is performed by using a ¼” phone plug with each of the two terminals connected to a 3 ½ (preferably a 4 ½) digit digital multi-meter. Both of these are available at Radio Shack. If you don’t have one, go buy one. Until then, don’t bother trying this test.
Insert the phone plug into the output jack. Set the multi-meter to the Ohms position. If it is not an auto-ranging meter, select the 20,000 Ohms range.
Turn the volume control and both of the tone controls to the maximum fully clock-wise position.
Set the lever switch to the Bridge pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Bridge and Middle pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Middle pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Middle and Neck pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Neck pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
(In the following table, “||” indicates in parallel.)
Measurement Reading Structure
Bridge |______| B|| Vpot Single pickup in parallel with Vpot
B||M |______| B||M||Vpot Two pickups and Vpot all in parallel
Middle |______| M||Vpot Single pickup in parallel with Vpot
M||N |______| M||N||Vpot Two pickups and Vpot all in parallel
Neck |______| N||Vpot Single pickup in parallel with Vpot
Note which tone controls seem to affect each switch position, which indicates the tone control connections on a pickup basis.
The Analysis
Now, while knowing the resistance for a single pickup in parallel with the volume pot gives us an idea as to the coil wire resistance of each pickup, we don’t know the actual resistance of either the pickup or the volume pot.
For resistances in parallel, the resulting resistance is calculated by:
Reff = 1/(1/ResA + 1/ResB + 1/ResC + ….. 1/ResN)
This method is the easiest to use for more than two resistances in parallel. Since calculators are everywhere (such as here www.1728.com/resistrs.htm), we’ll use this equation.
The Bridge position measurement is equal to 1/(1/ResB + 1/ResVpot).
The Middle position measurement is equal to 1/(1/ResM + 1/ResVpot).
If both the Bridge and Middle pickups had their own volume controls, the effective resistance of both of these circuits selected in parallel (four elements) would be
1/(1/ResB + 1/ResVpot + 1/ResM + 1/ResVpot).
We already know the resulting Bridge position resistance as well as the resulting Middle position resistance, but when both pickups are selected (three elements), we are seeing a resistance that is equal to 1/(1/ResB + 1/ResM + 1/ResVpot).
Therefore, by taking the calculated effective resistance of two parallel measured pickup circuits each with its own (well, the) volume control, and subtracting from it the actual measurement of these two parallel pickups with only one volume control, we get
1/((1/ResB + 1/ResVpot + 1/ResM + 1/ResVpot) - (1/ResB + 1/ResM + 1/ResVpot))
Or
1/(1/ResB + 1/ResVpot + 1/ResM + 1/ResVpot - 1/ResB - 1/ResM - 1/ResVpot)
Which reduces to;
1/(1/ResVpot)
which is the actual value of the volume pot. Due to the limited resolution of even a 4 ½ digit multi-meter, this procedure should be performed on the Middle and Neck in parallel position as well and the volume pot results averaged.
Knowing this value, we can derive the actual resistance of each pickup.
1/(1/Bmeasured – 1/ResVpotAvg) = Bactual
1/(1/Mmeasured – 1/ResVpotAvg) = Mactual
1/(1/Nmeasured – 1/ResVpotAvg) = Nactual
Here is a test based on a Fender Highway 1 Stratocaster and a check/proof thereof.
Note that this procedure can also be used on the Fender Telecaster as well (a two pickup guitar with a common volume control and a common tone control).
Now that we can “scan the patient” and know what’s inside (mostly) without starting surgery, we can therefore determine a lot of “ills” prior to surgery.
Brain Scanning Thru a Nostril (or Output jack).
I’ve often talked about connecting a digital Ohmmeter to the output jack on a passive (as in no internal preamp or batteries) electric guitar to “see” the values inside, and to attempt to troubleshoot things prior to surgery (taking it apart to find out for sure).
I first want to answer the question as to why this process is important. Many folk rip their guitar apart and then ask for help, describing things that they think that they see in words and terms not related to specific conditions. When one folk was asked how the wiring on a switch looked, he said it looked “like a spider.” While certainly descriptive, it was most unhelpful to those asked to suggest what said folk should do to fix things (“fix” also being subject to interpretation).
While the wiring “stuff” in an electric guitar looks complicated to the unschooled, it is really fairly simple.
The Fender Stratocaster
I’d like to start with the Fender Stratocaster since this is likely the most popular and common design in history. The basic wiring design is also used in many other guitars of similar pickup configuration.
The basic Stratocaster has three single coil pickups. These are comprised of a single wire coil wound around six cylindrical magnets. The coil’s DC resistance is typically around several thousand Ohms from the thousands of feet of very thin solid wire in the range of #42 to #45 AWG wound thereon. There are other structures available in a single coil sized form-factor including dual coils for hum cancellation, dual rows of pole adjustment screws, and so forth. There are also dual-coil side-by-side pickups (the traditional humbucker). Without getting into the variations of pickups, the basic tenants of this means of testing are valid for most.
The Stratocaster has a pickup selector switch which is really a two pole three position lever switch (as used in the Fender Telecaster and all Stratocaster’s until the early 1970’s) with two additional position detents added between the three original ones. These additional detents were first added by guitarists who filed notches onto the original 3 position switch detent mechanism to realize pickup combinations that the manufacturer never thought of (the switch was a commercially available one from the 1950’s often used in intercoms and telecom gear). For some strange reason these additional positions are known as the notch positions. In the basic Stratocaster this switch selects the Bridge pickup (an original position), the Bridge and Middle pickups in parallel (a new notch position), the Middle pickup (an original position), the Middle and Neck pickups in parallel (a new notch position), and the Neck pickup (an original position). These are selected using one of the poles on the switch. There are other combinations available on some factory guitars, and on many of the custom wiring schemes designed by folk with way too much spare time.
The basic Stratocaster also has two tone controls that are connected in variations of the pickup selection. The most common one is one tone control is selected for the Neck pickup, and the other tone control is selected for the Middle pickup. There is no tone control selected for the Bridge pickup. This is the original configuration from the time when only one pickup could be selected at a time. Note that in the Middle in parallel with the Neck position, both tone controls will be active. Also note that in the Bridge in parallel with the Middle position, the Middle tone control will be active. The tone controls are selected by the other pole on the lever switch. These tone control circuits traditionally are each comprised of a variable 250 K Ohm audio taper (logarithmic) potentiometer connected in a rheostat mode (two-wire) connected to a common capacitor of 0.1 uF, 0.047 uF, or 0.22 uF, although other values may be in use.
Additionally there is a volume potentiometer that effects local output level. This is traditionally a 250 K Ohm, audio taper component. The wiper (center terminal) is connected to the output jack.
The Test
The test is performed by using a ¼” phone plug with each of the two terminals connected to a 3 ½ (preferably a 4 ½) digit digital multi-meter. Both of these are available at Radio Shack. If you don’t have one, go buy one. Until then, don’t bother trying this test.
Insert the phone plug into the output jack. Set the multi-meter to the Ohms position. If it is not an auto-ranging meter, select the 20,000 Ohms range.
Turn the volume control and both of the tone controls to the maximum fully clock-wise position.
Set the lever switch to the Bridge pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Bridge and Middle pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Middle pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Middle and Neck pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
Set the lever switch to the Neck pickup position. Vary each tone control all the way down to the fully counter-clockwise position and back to fully clockwise. Note if the Ohms reading changes to (and stays at) a different steady-state reading (which could indicate a bad capacitor or faulty wiring). Write down the Ohms reading for this position as well if any short-term resistance variations are induced by quickly turning each of the tone controls.
(In the following table, “||” indicates in parallel.)
Measurement Reading Structure
Bridge |______| B|| Vpot Single pickup in parallel with Vpot
B||M |______| B||M||Vpot Two pickups and Vpot all in parallel
Middle |______| M||Vpot Single pickup in parallel with Vpot
M||N |______| M||N||Vpot Two pickups and Vpot all in parallel
Neck |______| N||Vpot Single pickup in parallel with Vpot
Note which tone controls seem to affect each switch position, which indicates the tone control connections on a pickup basis.
The Analysis
Now, while knowing the resistance for a single pickup in parallel with the volume pot gives us an idea as to the coil wire resistance of each pickup, we don’t know the actual resistance of either the pickup or the volume pot.
For resistances in parallel, the resulting resistance is calculated by:
Reff = 1/(1/ResA + 1/ResB + 1/ResC + ….. 1/ResN)
This method is the easiest to use for more than two resistances in parallel. Since calculators are everywhere (such as here www.1728.com/resistrs.htm), we’ll use this equation.
The Bridge position measurement is equal to 1/(1/ResB + 1/ResVpot).
The Middle position measurement is equal to 1/(1/ResM + 1/ResVpot).
If both the Bridge and Middle pickups had their own volume controls, the effective resistance of both of these circuits selected in parallel (four elements) would be
1/(1/ResB + 1/ResVpot + 1/ResM + 1/ResVpot).
We already know the resulting Bridge position resistance as well as the resulting Middle position resistance, but when both pickups are selected (three elements), we are seeing a resistance that is equal to 1/(1/ResB + 1/ResM + 1/ResVpot).
Therefore, by taking the calculated effective resistance of two parallel measured pickup circuits each with its own (well, the) volume control, and subtracting from it the actual measurement of these two parallel pickups with only one volume control, we get
1/((1/ResB + 1/ResVpot + 1/ResM + 1/ResVpot) - (1/ResB + 1/ResM + 1/ResVpot))
Or
1/(
Which reduces to;
1/(1/ResVpot)
which is the actual value of the volume pot. Due to the limited resolution of even a 4 ½ digit multi-meter, this procedure should be performed on the Middle and Neck in parallel position as well and the volume pot results averaged.
Knowing this value, we can derive the actual resistance of each pickup.
1/(1/Bmeasured – 1/ResVpotAvg) = Bactual
1/(1/Mmeasured – 1/ResVpotAvg) = Mactual
1/(1/Nmeasured – 1/ResVpotAvg) = Nactual
Here is a test based on a Fender Highway 1 Stratocaster and a check/proof thereof.
Note that this procedure can also be used on the Fender Telecaster as well (a two pickup guitar with a common volume control and a common tone control).
Now that we can “scan the patient” and know what’s inside (mostly) without starting surgery, we can therefore determine a lot of “ills” prior to surgery.
