Après moi, le déluge.
Louis XV

Fender's Telecaster was an elegantly simple instrument from its introduction through the late 1960s. The Tele's wiring was a study in subtlety:

• The control plate provides the ground surface for all the controls and is a partial shield against radio interference
• The neck pickup is obviously shielded; the bridge pickup's metal baseplate is both a partial shield and, through the pickup mounting screws, the bridge's connection to ground
• The bridge cover completes the bridge pickup shield
• The tone capacitor is isolated by the tone pot's resistance rather than acting as the lead to the pot as in, for example, Gibson's designs of the same period
• The 100 picofarad capacitor across the volume control's lugs allows extra highs to pass from pickup to output as the control is turned down so that the response is heard as more linear

In the beginning was the Esquire...

Introduced in April of 1950, the Esquire, Fender's first electric Spanish (as opposed to Hawaiian) guitar, was available with either one or two pickups and a non-adjustable neck. Perhaps fifty of these were produced, most being replaced under warranty because the necks warped within weeks. Production briefly ceased while Fender re-tooled to make adjustable necks. The single pickup version of the guitar was put on hold for economic reasons.

Late that year the two-pickup model, re-named the Broadcaster, was back in production. This quickly met with a trademark infingement claim from Gretsch who had been using the name "Broadkaster" for one of their drum lines since 1937.

The name was changed again.

For several months in early 1951 the instruments (years later nicknamed "Nocasters") were badged only with the Fender name until new headstock decals that said "Telecaster" were obtained.

The Esquire, now with only one pickup, was re-introduced in April of 1951.

I have not been able to find a schematic or wiring diagram for the earliest members of the Telecaster family.

For the first two years the twin-pickup guitars, regardless of name, did not have a tone control but had a "blend" control that allowed the neck pickup to be mixed with the bridge pickup when the selector switch was in the bridge position. In other regards, the controls were as shown in the following image for early Telecasters. In 1952 the tone control featured on the single-pickup guitar replaced the blend control.

Early Telecaster Control Plate

This version of the Telecaster controls, shown above, did not combine the two pickups. The switch selected the bridge pickup only, the neck pickup only, and the neck pickup with a .1 microfarad capacitor in parallel to ground. The .1 microfarad cap knocked off highs and some upper midrange, producing what Fender literature referred to as "deep rhythm" sound, allowing the guitar to make a muddy imitation of a thin-sounding bass (remember, this was before electric basses were in common use). The volume and tone controls were both 250 k ohm audio taper potentiometers and the tone circuit used a .5 microfarad cap as a high-pass filter to ground allowing a very dark, but still clear, sound.

"Modern" Telecaster Control Plate

Various sources date this change to the late 1950s or even late '60s: The Tele's controls had evolved in response to the rapid and innovative development of other electric guitars - and the electric bass. The .1 microfarad cap was eliminated and the switch assumed its current bridge/both/neck pickup selections. A 100 picofarad (.001 microfarad) cap was wired in parallel across the volume pot's input and wiper to retain highs as the volume was turned down; the .5 microfarad tone capacitor and 250 k ohm tone pot were as in the original.

CBS Period Telecaster Control Plate

Fender had become a division of CBS when the next variation occurred. Perhaps to use up excess parts from the tube amplifier line that CBS hoped to stop producing, the volume pot was increased to 1 meg ohm, patently too high a value for the single-coil pickups; the 100 picofarad high-pass cap now thinned the sound noticeably when the volume was turned down rather than just retaining the pickup's brilliance. The .5 microfarad tone capacitor and 250 k ohm tone pot were still unchanged.

1968 saw the introduction of the Telecaster Thinline semi-hollowbody guitar. It was soon followed by other variants, some with humbucking pickups. The dam had broken. The deluge had begun. The Telecaster was no longer the simple electric plank it had been and active circuitry hadn't even reared its ugly head... yet.