G&L tech resources: Wiring harnesses


Any F-100 with passive/active wiring harness is a complicated beast with a dedicated preamp on board, implemented by using a National Semiconductor LM4250CN op-amp. On January 26, 1976, still in his CLF Research Corp. days, Leo had filed for US Pat. 4,002,994, describing the idea of using an operational amp to sculpt the sound. Also the F-100E needs its own “Instruction Sheet” to help the musician navigate all the options. Going over all the items on the control panel, the large black switch is still the 3-position pickup selector for the 2 Magnetic Field Design (MFD) humbuckers. Compared to the passive version of the F-100, the small red tip “Splitter Switch” is still present but now offers 3 settings using a C&K 7411 mini-toggle: “single-coil without bass boost” (towards the neck), “single-coil with bass boost” (center position), and “humbucking mode” (towards the bridge). The bass boost is in the center position is achieved by a 50,000pF (.05µF) capacitor and/or a 100,000pF (.1µF) capacitor soldered to the switch. Note that the former value is somewhat uncommon. In most diagrams one will find 47,000pF (.047µF) instead. But for all intents and purposes, given the tolerance, these values are identical. Instead of the black tip “Phase Switch”, one finds a white tip MTA206PA mini-toggle. This 3-position switch controls the 9V battery powered preamp while protecting it with a 10MΩ current limiting resistor. The preamp is not exactly a unity gain amplifier, but close enough, with RC-circuits to affect the frequency response. Pointing towards the neck, the guitar is in active mode with treble boost, the center position selects active mode without treble boost, and when flicked towards the bridge, the battery (and preamp) are disengaged for the passive “normal” mode. It is all explained in both ad slick and instruction sheet shown below. This album in the Guitars by Leo Gallery contains 2 schematics for the F-100E and even a picture of a wiring harness taken by John Toner at G&L after he fixed an F-100E. The guitars below are a F-100 Series IE and a F-100 Series IIE w/Dual-Fulcrum Vibrato, both from the last quarter of 1981, and seem to match that picture more than the schematics. They have the same CTS 250kΩ Audio Taper with a 20pF ceramic treble bleed capacitor for the volume control. Also on an F-100E, selecting the bridge (lead) pickup bypasses the treble cut control. The pictures below show a 2,200pF (.0022µF) capacitor connecting the pickup selector directly to one half the “Splitter Switch” for this pickup. The neck pickup also has such a 2,200pF (.0022µF) cap in its signal path but it is connected both to the wiper of the treble cut pot as well as to the other half of the “Splitter Switch”. On these guitars, that treble cut is a CTS 250kΩ Audio Taper pot, not the 500kΩ usually found in the PTB tone stack as well as the referenced schematics, in series with a 22,000pF cap soldered between the wiper and ground. The bass cut consists of a CTS 1MΩ Reverse Taper pot with a 2,200pF (.0022µF) treble bleed cap. The 1,000pF (.001µF) cap in series to ground seen in the referenced schematics is absent on both guitars, just like in the referenced John Toner picture above.


F-100E (active)