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This web page describes the function of the Mesa/Boogie FU-3 footswitch.

The schematic for this device is part of the Mesa Mark IV schematic and is available at:
http://www.mif.pg.gda.pl/homepages/tom/schematics.htm

The first four footswitches are momentary contact footswitches. The final two footswitches are latching footswitches. The first four footswitches act like radio buttons - stepping on any one of them deactivates the other three. The final two switches will not turn off until you step on them for a second time.

The fourth footswitch, LD-EQ, selects the LEAD channel and turns on the EQ, regardless of the status of the EQ footswitch. When the LD-EQ footswitch is selected, stomping on the EQ footswitch has no effect until the RHY1, RHY2 or LEAD switch is selected. The LOOP footswitch acts independently of all other switches.

This series of photos depicts the status of the FU-3 when the following switches are selected:

RHY1: (Yellow LED)

RHY2: (Orange LED)

LEAD: (Red LED)

LD-EQ: (Red / Orange LEDs)

EQ: (Orange LED)

LOOP: (Yellow LED)

For this series of photos the cycle is repeated with EQ on and LOOP off:

RHY1:

RHY2:

LEAD:

LD-EQ:

Kelley, here are some additional photos that show dimensions of the footswitch and its storage in the back of the amp head:

Additional notes that go with the schematic, including pin-outs for the footswitch, are located here.

A copy of the Instruction manual, which explains how the switching circuit works, can be located here.