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With the advent of complex coil interconnection circuitry, some nomenclature commonly used within this context has to be revised to describe phenomena previously non existent.

If those terms are not made clear, it is impossible to understand the following elaborations.

For example, in a basic two pickup system consisting oft two single coils, one at the neck and one at the bridge, one may find a 3-position selector switch, one for neck, one for both and one for bridge position.

Now this may be classically called pickup selector switch, which, since it has no secondary function, also serves as source selector switch.

For instance, a Les Paul type guitar traditionally has a pickup selector toggle switch, maybe adorned with some coil splitting functionality restricted to one physical coil cluster. Coil selection (and thus interconnection) is equivalent to the source selection.

A classical Strat has three single coil pickups and is equipped with a three-position switch. Each position selects one coil. Later models have a 5 position switch that also allows for intermediate settings and combinations of adjacent coils. Still the coil selection (and thus interconnection) is equivalent to the source selection.

Even later connection schemata arose that made complex interconnections possible like series and non-adjacent coils. Those clearly exhibit a distinction between coil (= pickup) selection and switch position. A switch position begins to select a fixed sound-scape. This is a performance selected feature rather than a recording session feature, which is the approach Dan Armstrong has taken with his Superstrat switching scheme. Lester Polfus (see later under “voicing”) has explored a similar path with his Les Paul Recording guitar. Similarly, one tinkerer modified his L-2100 with three three-position switches that allows for all imaginable interconnections – not necessarily a performance approach.

Scarcely however people have gone the way to the end by also incorporating individual “voicing” caps into those positions or dummy coils where needed.

In the case of the modded L-2100, the previous pickup selector switch (the one closest to the neck) loses its direct coil cluster fixation and becomes a source selector switch. The center switch (series/parallel) maintains its original functionality, but complex coil interconnection, beyond fixation to a cluster of adjacent coils, is transferred to a newly introduced  three-way switch, best called parallel pair selector.

In this case, any pair of coils, possibly spreading over non adjacent coil clusters, may be selected (such as inner pair or outer pair), which if source selector is in both mode will be a humbucking (parallel) pair, but in solo (neck or bridge mode) a single coil. See the reference chart later on.

For clarity, a distinction has been made between coils belonging to one physical cluster of adjacent coils (hereafter native mode) and two physical units (hereafter cross mode).

According to the previous differentiation between performance and recording switching schemes the scheme presented in the following is positioned in-between. It is experimental and work in progress. Ideally, there should be a “P”, a “J” and a “S” position and little more.

The widely used nomenclature „single inner“ resp. „single outer“ is a misnomer, since it indicates single coils, but it is actually a hum canceling pair (for both mode at least).

This nomenclature stems from the fact that one coil out of physically stacked up pair is used together with one single coil borrowed from the pair across, hence the name cross-mode.

It is replaced by the terms inner pair and outer pair, referring to the physical arrangement of 2+2 coils at the neck resp. bridge side. "Inner" and "outer" makes only sense in reference to this arrangement. Nevertheless, true single coil operation for all four coils is possible, it comes into the bargain. Refer to the pickup selection chart for more information.

Some users of the L-2100 insist on using it passive, with volume and tone pots dimmed. Why? We want to understand this, because there may be things we can optimise into the bargain. This also builds some foundation for any alleged voicing later on.

Some aside to technical explanation is inevitable.

Every pickup basically is a resonant R-L-C circuit (winding resistance, inductance and winding capacity), which forms a higher order resonant low pass. Guitar Letters explains this well.
Such a circuit on its own will exhibit some ringing at the cutoff frequency, sometimes substantially dependent on its Q.
Although this may be perceived as extended treble range, the ringing is usually not appreciated. Think of a wah pedal halfway back at a fixed position – good for an effect, but not for always.

Technically this peak can be modified by two factors:  a parallel cap and/or a parallel resistance.
An additional cap will add to the winding capacity and shift the peak towards lower frequencies. An additional load resistor will bring the Q down and with it the ringing.

Our pickup always exists within  a practical electrical environment, in the vicinity of a volume pot and a tone pot at least, so a certain load resistance is always there. Humbuckers are more sensitive to load than singles, that´s why you always see recurring pot values of 500k versus 250k in single coil environments.

A passive treble cut pot, when at 100%, presents some more load, say 250k, which totals in 120k together with the volume pot for a single coil. Turning tone down just a little may be enough to remove the hump and thus the ringing. (Remember: that´s what many players say they like...).

Turning it down completely will bring the tone cap in parallel to the winding capacity and create an artificial resonance at a ridiculously low value, which is unfortunately beyond good or bad to be useful for stock cap values. All passive guitars work like that.

Now we have not yet plugged the guitar into an amplifier in our mind experiment.

Adding a cable adds more capacity and more load resistance dependent on the pot´s settings, so that there may be a sweet spot of all settings that sounds good. Of course passive circuits also introduce  other imponderabilia that can wreck the signal.

That´s where the active mode enters. However, this way has not been gone to an end with the L-2100´s stock circuit. Dropping all load factors – too much brightness, too much ringing and that consistently. Since the built in amplifier is configured for some treble boost even in the normal mode, letting alone the treble boost option, this can only worsen things for some.

Happily, some small capacitors and resistors before the buffer can tame any of those things and thus introduce all of the merits of passive mode without its drawbacks. It so happens the L-2100 already has a 1nF cap in parallel by definition (this is the one soldered onto the bass pot).

This, to my taste, is too hefty. See later in the section on voicing.

Our extended switching circuitry offers some unassigned positions for that into the bargain.
Since this procedure is really that easy and cheap, it makes me wonder if the ones who devised the circuitry did only have their sales numbers in mind, saving caps worth cents... It brings more money to sell another instrument rather than have just one that does all you want...

In 1982 Leo Fender filed a patent (US.-Pat. 4,319,510) for splitting humbuckers that do not have all leads accessible, also known as three wire humbuckers.

This is in fact astonishingly simple. As different to shorting out a coil entirely, a capacitor shunts frequencies out. Choosing the capacitor´s size wisely, 50 Hertz hum cancellation remains intact. However, higher frequency hum (such as from switching electronic appliances) will not cancel. Making the capacity smaller will work better, but let increasing amounts of low frequency content through.

On the L-2100 basses, Leo was probably trying to achieve a pseudo single coil sound, which is less boomy, and in the verge of trying different caps found the bass boost attractive. How the term OMG came about is unknown, since this is recent social platform lingo.

By shorting some of the treble content out, bass content appears louder. Although just cutting and not boosting, this mode was called „bass boost“. All is relative. Unfortunately, together with the fixed 1nF voicing cap, this mode sounds pretty dark. Maybe interesting for some retro sounds, but not for contemporary sounds.
Again this is no real coil splitting, it is a humbucker wired pseudo single coil. There is no coil shorted out with a hard wire bridge.

OMG mode lead to many complaints by musicians, because hum suppression is, like in all single coil circuits, inferior. Moreover, Leo could have at least the center position (both) humbucking, which was recognized by the DIY community^[1], but never cured by the company. Conversely, they decided to drop OMG entirely on later modes, such as in my specimen of the bass.

My personal opinion is that the series wired pickups are powerful enough the way they are - without "boost", and that it is somewhat awkward to generate a bass overload first and then use treble boost to remedy that. The cat bites its own tail.

When working with new modes it became obvious that the volume drop between serial and all other modes was a nuisance, requiring a permanent re-adjustment of the volume pot.

The previous owner had made a feature out of this flaw by calling it the „rock switch“ - volume boost. Hmm. Rather make a small, adjustment on the volume pot if needed.

I found the idea great to equalize the volume levels by adding a series resistor where normally the wire bridge between yellow and black goes. Extensive listening tests in both active and passive mode and also some PC simulation did not indicate any change in tone.

Of course, OMG does not harmonize with that, but this was deemed questionable anyway as elaborated above. There is no need for pseudo humbucking if real coilsplitting is at hand.

However, by bypassing the dropping resistors with a cap, some partial "boost" of high frequencies can be achieved. Again, this is no boost, but treble content gets less suppressed. Why making half hearted attempts towards single coil sound when a brighter sound can be achieved another way while maintaining full hum canceling functionality?

In a nutshell: the newly devised switching scheme keeps the original serial/parallel switch, except that the range of available parallel modes has been expanded, determined by the setting of the parallel pair selector configuration switch. Dependent on the setting of the source selector switch, all four individual coils can be addressed in true single coil mode too.

The parallel pair selector switch physically resides in the space, that was previously occupied by the active/passive switch, whose functionality moved onto a push-pull switch linked to the volume pot knob.

Note: treble boost mode has been dropped as mentioned earlier.

(click on the picture to load larger image)

Pickup Selection Chart: The parallel pair selector switch allows for three positions: inner pair (lever towards the neck, pairs one of the neck with one of the bridge coils, here referred to as cross mode) parallel (lever centered, pairs physically adjacent coils, here referred to as native mode) and outside pair (lever towards the bridge, pairs one of the neck with one of the bridge coils, here referred to as cross mode)

When both is selected on the source selection switch, those positions are hum canceling. Since the coils are arranged N/S - N/S starting from the neck, the cross-mode arrangements xS - Nx (inner pair) and Nx-xS (outer pair) are hum canceling as well as the native N/S arrangements.

Note: In solo mode (e.g. neck only), out of the selected pair, although both coils are active, only one will contribute to the output signal - the one residing in the source selected (neck in the example), hence a true single coil. This one is subject to hum as all single coils are. Determining which one out of the two adjacent coils is currently active is slightly awkward and is not always in direct relation to the switches´ lever position due to the fact that one has been eliminated out of a cross-pair. Logic has been optimized in so far that the lever points towards the inside of the guitar body in inside pair mode and towards the outside in outside pair mode. Thus, only if bridge solo is selected, the parallel pair switches´ lever points to the correct single coil, but this is reversed for neck. This is inevitable and a small problem.

 Humbucker pairs in cross mode may have an slightly inferior hum suppression capability due to a greater “magnetic aperture”

	Download  serial mode chart (click on the icon to download)
	Download serial options chart
	Download native pair parallel mode chart
	Download outer pair parallel (= cross-) mode chart
	Download inner pair parallel (= cross-) mode chart

• The threading of the 4pole on-on-on switch I used (Knitter Switch MTA 406 PA) appears slightly shorter than the C&K variety, so it needed to go slightly deeper into the wood. The usage of any machine operated drill was avoided at all cost, since this eats itself into the wood way to fast. A few turns by hand were enough.

Note: knitter switch MTA 406 PA datasheet information is erroneous on the pin layout. The layout is equivalent to the ubiquitous C&K 7411 and Multicomp 1M46 switches. WARNING: I just received two switches from them that DO adhere to the Datasheet. Measure before you solder!
Update: refer to a special document on those switches for more information.

• I looked for  a good quality push-pull volume pot with a solid 6mm shaft like the other pots on the L-2100. There exists one from Alpha, but this seemed unavailable. The one chosen is a CTS 250k PUSH/PULL 3/8", which fits just perfectly inside the cavity. It has a very smooth volume reduction over its travel and also exhibits the same friction to motion as the others do. Its thread too, is a bit short, but using a bigger diameter lock washer that sits on the pot body rather than the small mounting rim works well.

• It also comes with a 1/4” knurled shaft. Unfortunately, dome knobs for knurled shaft pots are not available hereabouts. Fortunately, you can buy small brass sleeves that slide over knurled shafts and make 1/4” solid shafts out of it. The dome speed knob´s bore was increased to 6.5 mm with a stock steel drill and this works great.

• The ceramic treble bleed cap was replaced by a good quality cap on the go.
• The push-pull switch activates passive mode when pulled, but this is a matter of  taste.
• The barrel output jack was replaced by a new one since it permanently interrupted the signal when the slightest sideways pressure was applied onto the jack^[2]. Removal was easy with a screw extractor (the type that has reverse „threads“ on it). The jack does not even get destroyed by this. It just screws out. Unfortunately that the new jack appeared to be no cure. Signal went intermittent when you use a long flange plug and apply sideways pressure. So using angled plugs and folding the cable back under the strap is paramount if the kind reader insists on keeping the barrel jack.
  
  The barrel plug is just inferior. A disgrace for such an expensive instrument. It is beyond comprehension why they used it, probably to distinguish themselves more from other manufacturer´s products visually. Internet forae are full of complaints about this nuisance, and many owners have decided to replace it by a conventional jack:
  

(click on the picture to load larger image)

Soon before long, the barrel jack was replaced by a standard jack together mounted onto a metal football shape mounting plate. The procedure is described here[3]. An oval (a.k.a. football) jack plate together with a standard Switchcraft short ¼“ jack was mounted (a Gibson style square jack plate appeared almost too big, extending into the beveling) – end of discussion and end of signal problems. Folks doing the same before worried that it may be a problem to drill a large enough hole without slipping and damaging the guitar finish. Not at the least. I used a stock metal step drill in a slow running cordless electric drill to enlarge the hole to 20mm just at the rim, yielding a somewhat conical bore. A 20mm Forstner bit now fitted perfectly into this bore, so no danger of slipping. The wood is soft enough that a very slow drilling speed can be used. Some more gentle drilling and the hole was through. No chipping, nothing, a razor sharp cutout. A five minutes procedure and painless.

• Another questionable omission was the unscreened electronics compartment cover plate. This is ridiculous, since the compartment itself is paint screened.
  A piece of self adhesive metal foil was applied to the plastic plate. A small strip of  copper foil was folded and glued over the cavity´s edge and soldered onto a ground point so that it makes contact to the cover´s foil, at a location near the output jack where the back plate is screwed onto the body for good contact.
• The wire to the preamp that does the treble boost was be removed.
• The preamp itself got wrapped inside a piece of self adhesive tape to prevent contacting anything.

The cavity is now quite crammed, but serviceable since I left enough slack on the wires.

• Pickup height. While not being a new feature, this subject belongs to the changes made to the instrument: during slap playing, the E string easily thunders into the pole pieces, with an according sharp snap from the amp. With the pickups fully down and the spiral spring underneath fully compressed, they still protrude about 10mm from the body. The remedy was to shim the neck slightly so that all strings are elevated somewhat. This of course requires raising the bridge again to restore the previous action.
  

* Maybe I expected to have a Precision sound or a J-Bass sound at hand, and while it surely crosses into that territory (like any similarly built pickup in that position will...) I found there is no such a thing like THE XY sound. There are several web sites that try to carve this out and they had to admit that they failed, because over the years so many models with different body and neck woods resp. shapes and surely differently wound pickups had been made. This can´t be generalized.

Now that all active methods to eliminate hum failed, I find myself back at the roots. While it is true that screening won´t help against magnetic interference, it will certainly help against  electrostatic interference. And true, some of what I hear, sounds like a harmonic of mains (transformer originated) frequency, without the fundamental.

I opened the pickup cavities - and they are bare wood. Agreed, the guitar was never intended to be used with single coils, but even in humbucking modes it appeared vulnerable to higher frequent noises. To cancel those the "aperture" of the two coils is too big. People report that with screening they made even humbuckers quieter.

I have plenty of copper foil at home, but there is no space in those cavities. So I bought a small bottle of Humbrella screening paint invented by a German guy and allegedly preferred by luthiers. This does not consume any noteworthy space.
Three layers with drying time in between is recommended. With a small brush I even poked a little into the tunnels that go from the electronics compartment to the pickup cavities. The wires should be screened too. (See my hints further down on working with the paint)

MFD pickups from underneath: (click on the picture to load larger image) (picture taken after cavity painting) Take a look at the pickup´s underside. There is a massive copper base plate with two green wires attached. One comes from the coil, one leads to the switching assembly. As stated above, the method Femto has devised to switch the coils from the ground side was ruled out previously due to the base plate grounding problem, but it looks this can be remedied no problem by adding a separate wire to the base plate. Soldering is no witchcraft, just make sure the extra wire is secured against pulling strain. So his method may prove useful in some respects after all.

Painted pickup cavities: (click on the picture to load larger image)

A few hints for using the paint: stay away from contacts. While the paint is not super conductive, it is low ohm indeed. stay away from cables. They advise not to rely on the paint´s adhesion to plastics, but it adheres to cables like hell. While this does not harm the cable itself, it might make contact somewhere else. the paint adheres greatly to rough untreated wood. From lacquered wood you can scratch it away with the fingernail after drying. Be aware that it stains blank wood immediately. Rough up lacquered surfaces as recommended, but be aware that steel wool debris will go flying to all magnetic parts. Better use fine sanding paper if you have the pickups in the vicinity like I had.

Reassembling the pickup cavities: (click on the picture to load larger image)

The pickup cavities have two holes drilled to hold springs to push the pickup up (left arrow). A piece of foam pushes back (the foam was already a bit deteriorated and was renewed). I had to widen those holes slightly, because the springs are really tight in there. After painting with conductive paint and re-inserting the springs prior to assembly, I inserted small strips of copper foil between the spring´s windings (bottom arrow). Once the spring pushes down and gets compressed, the copper foil makes contact to the paint. Since the foam pads are right beside, those push down too. The springs themselves work against the bare copper baseplate, which by itself is grounded. Note the piece of copper foil on the right. I wrap this around the piece of wire that grounds the bridge (and strings), which was already a bit recessed in the wood.

After assembly, I inserted a small strip of copper foil in a crevice right beside the pickups to contact the paint and measured some 50 Ohms against ground. This is absolutely perfect.

Repainted electronics compartment: (click on the picture to load larger image)

The electronics compartment is insufficiently screened. There is conductive paint inside properly, but incomprehensibly, they failed to screen the back cover plate. This is simply a matter of applying foil to the plate, which conveniently contacts the rim of the compartment - but not on this guitar (see the pictures above). G&L did not paint the rim with conductive paint. But this can easily be done with the newly acquired wonder paint. Make sure to paint copiously over the already existent paint a little down the side walls to ensure connection and stay away from all electrical connections inside. Of course I also painted the cavity I drilled in order to replace the dreadful barrel jack as explained above. Insulate the jack inside! The arrow points towards one of few places where I spilled the paint (picture taken after drying before any further steps. No sweat, it does not adhere to shiny lacquer.

As expected, hum did not go down, but its higher frequent components did. The effect is subtle, but noticeable.
Let me know if any of this is useful or appeals to you.

Comparison serial / parallel (native) modes: Neck PU (serial first, parallel on second phrase) Note how little difference there is sonically between serial and parallel modes when volume-equalized and when a buffer is behind to avoid tone sucking. A valid argument to ditch the serial modes if one wants.