Hi Mike; as ever, there's always the question of whether the amp matches the schematic but ... this is the preamp schematic for both the V2 Thunderbird and the Superbird with the tremolo upper right in red, the reverb on the far right in magenta, and the fuzz on the lower left in green.
In this schematic, there's a tap at the DC rail 'E' powering the relay, on/off footswitch, and 1/2 a 12AX7. There's a constant voltage on the relay coil keeping the contacts in the fuzz/off position. Switched 'on', the footswitch allows the current to run to ground de-energizing the coil, and the spring in the relay throws the contacts. Also note the provision at the top edge for a bridging cap in the circuit for a model not built with fuzz ... the Thunderbird?
Fuzz off; the signal from the channel 2 input jack loop-de-loops through the relay contact arms and continues to the next gain stage.
Fuzz on; when the coil is de-energized, the contacts move putting the 12AX7 in the circuit. I don't know exactly how the fuzz effect is created ... but this is how the relay puts that tube in/out of the circuit.
Opening the footswitch and drenching all three switch bodies with lubricating-type contact cleaner will help get rid of corrosion on the contacts and lubricate the spring. If it's the switch and short of replacing it, this would clear the balky on/off. If cleaning / replacing the switch doesn't do it then chances are the relay contacts are burned and/or the spring is weak. Finding one that throws contacts, that's easy ... finding one whose pins line up with the layout on the circuit board ... that's another matter.
