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Taking Control of Your MXR Phase 100

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

Figure 1

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!

Figure 2

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.

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Polyphony - Copyright: Polyphony Publishing Company


Polyphony - Jul/Aug 1981

Donated & scanned by: Mike Gorman


Maintenance / Repair / Modification

Gear in this article:

Guitar FX > MXR > Phase 100

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Feature by Matt Richards

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