I've owned an MXR Phase 100 for about 5 years now, and consider it one of the best phase shifters around. However, the sound of a phaser whooshing back and forth without regard for the rhythm of a piece of music can get a bit tiresome. The answer is foot pedal control, and this mod shows you how to do it.

How it works

The amount of phase shift is controlled by the amount of current flowing through the LEDs in three dual opto-isolators. A Darlington transistor pair controls this amount of current; in this mod, we break the connection between the LEDs and the controlling transistors, substituting a pot for the transistors to give us manual control over the phase shift. Mounting this pot in a footpedal gives us footpedal control.

The modification

Look at the foil side of the circuit board, with the wire connections on the bottom; the upper left hand corner should be marked P100. Figure 1 shows a detail of this section of the board. Cut the trace between points A and B as shown to disconnect the opto-isolators from the driver transistors, then connect up points A and B to the circuit in figure 2. Work fast when soldering, especially point B - you wouldn't want to burn out that opto-isolator!

With S1 in position 1, you will have the standard phase shifter sweep. In position 2, the potentiometer will control the phase shift sweep. If you mount this pot in a footpedal, don't worry about noise pickup on the line to the pedal, since this part of the circuit does not carry any audio signal. Good luck - I hope you enjoy being able to control your Phase 100 as much as I do.