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Adding a Knob to Limit/Control the Expression Pedal
One of the reasons I got the X V-Amp was because I liked the idea of being able to control aspects of its many effects with a foot pedal. The largest annoyance I had with the system though, was that dynamic adjustment of an effect's Speed was only possble through the expression pedal. I wanted a knob too. Here's how I added one to the pedal. The knob I added serves two purposes: dynamic adjustment of an effect's speed when the pedal is set to contol speed, and a bonus of allowing the pedal when set as a Volume control to adjust the minimum volume point when the pedal is pulled back. After opening up* the X V-Amp, you'll see that the expression pedal area 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). 
Varying the amount of light that hits the receptor is what increases or decreases speed or volume. It should make sense that putting a 'dimmer' control on the LED itself would serve the same purpose as the plastic strip attached to the pedal. Depending on how much the knob of the dimmer is turned, the LED will shine more or less brightly as more or less power is fed to it and the receptor will get more or less light accordingly. Below you can see the three circuit board points that the dimmer control will connect to. Figure A shows the path that the pedal's power takes to get to the LED. Figure B shows where you need to sever the connection between the power source and LED. Do this with a small razor blade or even a straight pin or needle. Figure C shows wires attached that you connect to the dimmer so that power can get to the LED on your terms. 
Here are those same points with wires connected and the dimmer potentiometer with its wires connected. The value of the pot is 1k and there are two 100ohm resistors in parallel (50ohm roughly) crossing the power-in and power-out legs of the pot. 
After drilling hole for the control and tucking away the loose wires the pedal is ready to be closed up. 
All done. 
USAGE To control effect speed, make sure that the pedal is assigned to control effect speed and pull the pedal all the way back. This will normally set speed to zero because the LED is shining full on the receptor. Now the knob can be turned to dim the LED and thus increase or decrease the speed of the effect. To set a minimum volume level, make sure that the pedal is assigned to control volume and pull the pedal all the way back. This will normally set volume to zero because the LED is shining full on the receptor. By turning the knob up, you can set the volume level that the pedal will stop at when it is pulled all the way back. Now you can easily switch from full volume to a custom volume level just by rocking the pedal all the way up and then all the way back. *NOTE: If you remove the backplate entirely from the pedal by unscrewing it from the wires that connect to the circuit board, DO NOT put power to the pedal for more than a few seconds for testing your changes. The back plate is a necessary heat sink that helps the chip screwed to it to keep cool. Letting that chip float freely off the back plate with power to the pedal can result in opperation malfunctions. I had some odd malfunctions until I figured this out but no pemanent damage that I am aware of. |