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Fixing Irregular Pedal Throw
My expression pedal had all of its action in the last 1/4 of it's travel. I recalibrated it according to the instructions on Behringer's website but that didn't fix the problem so I opened the pedal up and took a look inside. The expression pedal part of the pedal looks like this: 
I removed the three screws so I could examine this little circuit board and got this. The yellow rectangle highlights what makes the pedal work. 
There is no potentiometer used with the expression pedal but rather an optical light sensitive control. Here, the red LED light shines and the white receptor on the other side of the slot adjusts whatever the pedal is set to adjust based on how much light it receives. The thin strip of plastic that is attached to the bottom of the pedal and which moves through the slot between the LED and the receptor determines how much light gets to the receptor. This piece of plastic is clear at the end, and gradually more opaque as it gets closer to the pedal. When the pedal is all the way down (fig. A), the most opaque part of the plastic strip is between the LED and the receptor and the least amount of light is being recepted. When the pedal is all the way up (fig. B), the least opaque part of the strip is between the LED and the receptor and the most amount of light is being recepted. More light equals less expression (volume on 0) and less light equals more expression (volume on 10). 
What does this all mean to you and fixing your expression pedal? If you move the LED you can adjust how the receptor gets its light, and thus how the expression pedal expresses itself. Did you notice in the photo above that the LED is not directly in front of the receptor? Truth be known, I opened the pedal up just after I got it and when I saw that those weren't aligned correctly, I moved the LED a bit to the left so that it was more in front of the receptor. Ouch. There goes my expression pedal out the window. That's why all the action happened in the top 1/4 of the pedal's throw; I had changed how the light from the LED was getting to the receptor. If your pedal isn't usefull in its lower sweep, move the LED a little bit to the left. If there is too much happening the first part of the pedal's throw, nudge the LED to the right. 
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