Electric Guitar modification

What about the best Embedded System? Right! A system *without* any controller or DSP :-) This is about modifying the circuit in a electric guitar; a Fender Toronado to be more precise. No worries, no code, just wires, some passive electronics, and increased sonic possibilities.

Guitars?

Embedded controllers are everywhere. and sure you can say that again for contemporary musical equipment. Walk into a music shop and see the countless devices for (home-) recording and music production. There's a dazzling amount of knobs or menu's on this stuff, all made meaningful by embedded (signal) processors. Synthesizers and stage piano's, sound shaping gear, effect processors, automated mixers, sequencers, modelling amps, self-tuning guitars, electronic drum kits.. There's also loads and loads of pc/mac software to complement, augment or replace this equipment.

Still, there's also pure and simple electricity to be found in electric guitars. Circuits that remain virtually unchanged throughout a history of 50 years; rock solid stuff so to say. Coils, wires, switches, potmeters and an occasional capacitor.. It's a pleasure to tamper with these circuits and customisation has become in fact a kind of tradition for certain people in the search for 'ultimate tone' or personal bliss. So yes, this article is on guitars.

Modification

Below is a picture of the modified functionality of the controls on a Fender Toronado as I want to propose here. Essential is that the mods are made without changing the appearance of the instrument and that full undo remains easy. Click 'factory' to see the original functionality.

Toronado Controls

The Toronado is a modern instrument - at this point again out of production - in the vein of the Jazzmaster and the Jaguar. Some would consider these instruments (plus the Mustang and derivatives) a collectve third strand in the classic Fender family next to the Telecaster and Stratocaster. The Toronado is distinctively Fender but brings in typical Gibson-like features as a slightly shorter scale length (mensure) and dual humbucking pickups. The internal wiring is also in the vein of Gibson; a tree-way switch (S1) allows selection of either the neck pickup, the bridge pickup, or both pickups in parallel. Both pickups are equipped with separate volume and tone controls. In the mid position of S1 all of these controls interact. This classical 'Gibson' wiring is found on many other dual humbucker instruments and can be found on many places on the internet, for example at GuitarNuts. Likewise, the modification I want to propose is not limited to the Toronado but can be applied to all guitars with a similar setup of pickups and controls.

Change control

The modification was result of the desire of the owner of the guitar (Tom van Nuenen; yes a relative ;-) to obtain more sonic variance from the instrument. This mainly involves the ability to have brighter Fender-ish single coil sounds (by 'coil tapping' the humbucker pickups), to have even 'hotter' output (pickups in series) and to have this typical slightly auto-wah kind of sound coming from an anti-series circuit. The out-of-phase effect benefits from the ability to combine a humbucker with a single-coil pickup.

Furthermore, the original three-way switch S1 on the guitar (throwing direction perpendicular to the direction of the strings) can be too-easily switched by accident under certain strumming conditions. I wanted to have a more solid toggle switch with prefferably less lever height and with a trowing action parallel to the strings. The toggle should quickly change between two different main sounds for live performance like as in a rythm-lead switch. I thought it would be nice to have separate and independant volume controls for these two positions. The 'standard' sound, given by S1 in the direction of the neck, is given by both pickups in parallel. The 'custom' position (S1 towards the bridge) gives either single pickup or both in series or in anti-series as determined by an extra 4 position rotary switch S4.

I came up with an arrangement as shown in the flash figure above (click 'Modified' if needed). Notice that the two volume pots are the controls nearest to the strings and that these are pull pots for coil splitting of the humbuckers. There's only one single and overall tone control pot.

Schematic

See the flash image of the modified schematic below (it can be zoomed and then panned). Click here to get it in PDF format. I've not yet seen this exact arrangement elsewhere, but many other people will sure have had similar ideas. Use is made of a 3 pole double throw toggle for S1 (no mid position, so 'on-on') and a 3 pole, 4 position rotary switch for S4.

Toronado Schematic

Note how the two volume potentiometers are wired; alternative signals are entered on the wipers of the pots so that an independant sum can be taken from the joined CW end-points. As in the original circuit these two potentiometers are 500kA (audio taper) but replaced by types having an additional pull/push switch. These switches are used to short one coil of the humbucker pickups to provide single-coil sound (without hum suppression). The diagram tries to illustrate that the coils for shorting are the ones that are not associated with the adjustable poles. The pickups are mounted such that these adjustable pole pickup 'halves' are farthest away from each other. This may not be immediately evident because of the metallic pickup covers. Covered pickups like these tend to be a bit more microphonic by the way. That was the case on this particular Toronado. An effective remedy is to soak the pickups for 20 minutes in a hot bath of parafine mixed with some beeswax. This is called 'potting'.

Sonic variety

Main toggle S1 and rotary switch S4 basically act in concert as a 3pole, 5 position selector. The rotary switch could also have been 4 postions (for 6 postions in total) allowing for example an additional anti-parallel mode of both pickups. However, I consider this the poorest sounding combination so I left it out. As it is, the current arrangement provides a total of 16 different and usable sounds as listed in the table below (N and B for S1 means toggled towards Neck resp. Bridge):

	Description	S1	S2	S3	S4
1	Bridge and Neck humbuckers in parallel	N
2	Bridge single coil and Neck humbucker in parallel	N		up
3	Bridge humbucker and Neck single coil in parallel	N	up
4	Bridge and Neck single coils in parallel	N	up	up

5	Bridge humbucker only	B			4
6	Bridge as single coil only	B		up	4
7	Neck humbucker only	B			1
8	Neck as single coil only	B	up		1

9	Bridge and Neck humbuckers in series	B			2
10	Bridge single coil and Neck humbucker in series	B		up	2
11	Bridge humbucker and Neck single coil in series	B	up		2
12	Bridge and Neck single coils in series	B	up	up	2

13	Bridge and Neck humbuckers in anti-series	B			3
14	Bridge single coil and Neck humbucker in anti-series	B		up	3
15	Bridge humbucker and Neck single coil in anti- series	B	up		3
16	Bridge and Neck single coils in anti-series	B	up	up	3

Some of these combinations might seem to be equivalent, but they are not as the neck and bridge pickups are really different in their sonic output. This is not only because of their different position but also because of their different sensitivety. The bridge pickup is significantly 'hotter' than the neck pickup, with a DC impedance (resistance) of about 4.2kΩ per winding on the neck and 8.4kΩ per winding on the bridge. When taken out completely, you could still discern both pickups for their intended position by this resistance, even though a simpler tell-tale is that the pole distance (following string distance) is discernably larger on the bridge pickup than on the neck pickup (approx. 11mm versus 10mm).

Parts

The images below show some of the replaced and new parts. Above left is the original 3-way pickup switch that was taken out and replaced by a 3pdt toggle switch (S1). The one I found to be the most suitable is shown at the top-center. It is made by MEC Pickups (Germany) and it combines a full format lever and 12mm bushing with a subminiature body.


Original pickup switch	MEC 80020/C 3PDT toggle	Lorlin CK1031 3x4 rotary switch

CTK 500KA pot with SPST switch		MEC 500KA with DPDT switch

An example of a suitable rotary switch for S4 is shown at the top-right. This one is from the CK series of Lorlin UK. It can take the position of a potentiometer, but being plastic, the threading of the slightly short bushing is easily destroyed. A bit of delicate wood working was needed to make it fit.

The push-pull pots I used are from CTS USA (picture below left) having the desired simple SPST switch. However, the bushing length was too large - even after adding some thick washers - and I needed to take that down with a small metal saw. This also got rid of the splitted and krnurled shaft end that doesn't go well with the screw-on knobs of the Toronado. All this is rather tedious and I would now propably prefer the push-pull pot in the picture below right, although it has pins rather than solder lugs and a full DPDT switch.

These particular parts were obtained from Vox Humana and Banzai Effects.

Wiring

I used a discarded RS232 serial cable to get all wires from the S1 toggle switch into the control cavity. This cable has 9 color coded wires and a aluminum foil together with a naked wire which provides adequate shielding.

The controls cavity itself was given better shielding by applying aluminum tape to its entire circumference. With a proper star-based earth this effectively silences invading hum. Keeping wires a bit untidy helps retracing them and is generally a good idea to minimize interference.

Finished

The guitar was given a complement of Shaller straplocks and the bridge saddles were replaced by GraphTech stringsavers. Strung up with D'Addario 0.009, properly adjusted and intonated, cleaned and polished-up, the guitar looks, feels and sounds great!

Mixed Mode