In reverse order of age, had a father who listened to classical music, a mother who liked jazz, a sister who sang along to musicals while doing the dishes, a brother who liked rock, and another sister who listened to just about anything like me. I guess the younger ones in the family have a more diverse exposure and develop more flexible tastes, or maybe no taste if my bass instructor who hates my 'mujazz' has credence lol.
Wow, thanks for that picture. Brings back memories of my confusion when I opened mine over forty years ago. Am I that old? Hand me my cane so I don't fall down and hurt myself while analyzing that circuit, will ya?
Okay, taking it from the top...the top/right switch is the pickup selector and the red wire on the center pole connects to the output jack. The wiring network is rotated roughly top to bottom, so the volume controls are pictured to the right and the tone controls are to the left. With the pickup toggle flipped toward the neck, it connects the white pickup/volume wire at the bottom volume control to the red output jack wire, so the controls that are pictured on the bottom are for the neck pickup and the ones at the top are for the bridge pickup, inverted from their normal position where the top are for the neck pickup and the bottom are for the bridge pickup.
The switch at the left is the phase switch and it connects to the bridge pickup. It swaps the black and white wires, so those are the terminals that span both coils in the bridge pickup. The orientation of the phase switch is roughly either up or down. It is pictured in the down position (up when installed), meaning in-phase, and the switch is currently connecting black<->black, white<->white, or identical to the neck pickup. The red wire on the pickup is the common lead from the coils and forms a 'center tap' terminal between them that isn't used for the polarity inversion and is not connected to the phase switch. Now that we know the color coding of the pickup terminals, we can figure out the rest of the circuit.
On the neck pickup (bottom controls as pictured, top controls when mounted), the tone control pictured on the left, when rotated clockwise (counter-clockwise when viewed from the bottom as pictured), is shorting the red wire to ground and that shorts out the black/red coil in split mode. Shorting one half of the pickup converts it to single coil because the output signal from one coil of the pickup is dumped to ground rather than sent to the amp.
On the bridge pickup (top controls as pictured, bottom controls when mounted), the tone control is shorting either the black/red coil in split mode when the phase switch is set to in-phase, or it is shorting the white/red coil in split mode with the phase switch set to out-of-phase. This explains the use of the phase switch in that tone selector diagram when only the bridge pickup is generating output. The phase switch is selecting either the coil in the bridge pickup that is closer to the bridge, or the coil in the bridge pickup that is closer to the neck, giving a slight positional shift by flipping the phase switch. I wasn't aware of this subtlety before. As a teenager/twentysomething, I didn't notice the difference in tone from flipping the phase switch with only the bridge pickup generating output in split (single coil) mode.
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The difference between Jazz (bridge) with the tone at 10, and Music Man New, is only the position of the phase switch that selects which half of the bridge pickup is in use. I wonder which half of the pickup corresponds to which model? The answer likely lies in which configuration more closely approximate the position of the pickup on the bass being modeled.
As the volume controls are turned down, they shift the output wire from the pickup coil away from the output jack and toward the ground node, progressively increasing the resistance to the output jack while shorting the pickup out completely when the volume is all the way down. The brown ceramic capacitor on the volume controls looks like it is adding some frequency compensation as the volume control is turned down. I'm guessing that as the pickup is progressively shorted out, the increased resistance from the pickup coil to the output jack takes down the treble before it takes down the bass, because the capacitance of the cable between the guitar and the amp soaks up some treble. The ceramic capacitor provides a parallel path for the treble from the pickup to the output that helps drive the guitar cable better at higher frequencies when the volume is turned down.
There's also minimal difference between the Music Man Old and Music Man New. The only difference with the pickup selector set to 'center' versus 'bridge' and the neck volume set to zero is the addition of the compensation capacitor on that neck volume control creating a shunt load to ground for treble through that compensation capacitor. This additional capacitance will darken the tone similar to the effect of using a longer cable. Presumably the older Music Man has darker sounding pickups and the newer Music Man has brighter sounding pickups?
I'm not sure what I'm looking at for the last component on the tone control. There's a glass tube with copper at either end. It looks like a diode, and that has me puzzled. I remember being confused by that at the age of 18 also, before I had an engineering degree.
If that component is a diode, it's possible that the parasitic capacitance of that diode is providing the tone control compensation, although I have no idea why they would use the parasitic capacitance of a diode to provide the tone control function. The capacitance of a diode is an asymmetrical and nonlinear function of voltage, so it maybe adds some 'fuzz' distortion as the tone darkens? The effect would principally be one of lower voltage and fewer harmonics on the portion of the waveform that lies to one side of zero volts e.g. positive excursion, than on the portion that lies on the other side of zero volts e.g. negative excursion.
It also maybe indicates that if that component fails, finding an exact replacement part may be important for restoring the original tone. Diodes vary wildly in their parasitic components such as the capacitance depending on the size, shape, and structure of the silicon junction. I'm not sure, but I believe that a passive guitar pickup will never generate the half a volt required to forward-bias a diode, and certainly will never generate the several volts required to reverse-bias it, so if that is a diode, it's likely not being used as a diode in this particular circuit. The only other significant characteristic it has is parasitic capacitance, and that's the only characteristic that could feasibly provide a tone control function as wired too.
In any case, there's nothing magical about the ' 7' setting on the tone control that makes it a hard line between split/single coil versus humbucker because there's nothing isolating the tone function from the split function. That's possibly just a number that engineering communicated to the marketing group. Seven is roughly a 3/4 or 1/4 setting on a linear potentiometer, and that could be a setting where the presence of current from the second coil begins altering the tone and noise rejection toward humbucker. The transition is smooth and varies continuously from single coil to humbucker as the control is rotated.
However, potentiometers with an 'audio taper' have a logarithmically varying resistance as they are rotated, because perceived loudness is roughly a logarithmic function. It could be that the '7' setting is where the audio taper begins to have a significant effect because maybe there isn't much change in resistance from the range between 7 and 10 compared to the range between 0 and 3. Audio tapers have a lot more resistance on the counter-clockwise end of the rotation.
The interaction between the split function and the tone function could be difficult to optimally tune in the design. I wouldn't have chosen that architecture. I would have used a push-pull split switch on the tone control instead and let the musician pick the balance between tone and split. Sure, it makes the operation a little simpler and adds infinite variability between split and humbucker, but it also marries the tone to the split in a way that isn't flexible.
I'd be curious to see how this wiring varies from other instruments. I've never investigated the topic in detail before.
If you want to post a picture of your electronics, I'll take a look at that too and see if I can make some sense of it. My engineering skills are rusty after a couple decades, but this stuff isn't rocket science. I think I've encapsulated the gist of it accurately enough for our purposes.
