There have been a lot of requests for this sort of tutorial, a "how to change a knob into an expression pedal" sort of thing, so I decided to photo the process of doing an expression pedal mod to a ModFX philtre... keep in mind you can add expression pedals to just about anything doing the steps below, just disregard the opening of the case stage and the removal of the jack, provided you can find space. In general this works best for pots at around 10k (that's the value of EXP pedals)... if you really want to do it to something else you're going to need to roll your own expression pedal. I'll try to post other EXP mods I do, and you should too, to provide more examples. So, here goes...
OK, first step, open up the case. If you aren't using a modfx pedal, just open up the case all the way so you can get at the pots, PCB and other innards... You will also need to drill a hole if you are doing something else. This is a tad tricky with the ModFX pedals. You need to unscrew all of the nuts on the back, pull off the knobs (they slip off), use a needlenose to unscrew all of the nuts on the 4 pots, and unscrew the single philips head screw on the back. Then it should come out pretty easily.
Now we're going to need to get that pesky footswitch jack out of the way... the only way to do so is to saw it out of the PCB. If you aren't using a modfx pedal, just make sure you have drilled a hole for a jack to go for the expression pedal jack.
Be careful... slowly cut down on either side of the jack to the very end. You can use a hacksaw if you want, I use a hobby saw here.
Crack that jack out of there. You won't hurt the circuit, I promise.
Here are the first wires you're going to want to do... The trick here is to look at the PCB and find the FIRST soldered point where the pot's connection hits. Just follow that trace until you see the first glob of solder. Then, make a wire from that point to the UNSWITCHED side of a switched stereo 1/4" jack. The unswitched side is easy to tell, just plug a cable in tthere and the side that doesn't pop up is unswitched. IMPORTANT!
On expression pedals, the TIP conductor is the CENTER lug from the pot. Always remember that. The other two can be wired either way depending on what "direction" you want the EXP pedal to go. If your pedal does not have a PCB at all, and has wires going to the pot, it's even easier... just disconnect the wires from the pot and make them go to the unswitched side of the jack instead.
This is the dangerous and tedious part-be CAREFUL to ONLY cut the traces you're needing to, use small strokes with your cutting utensil. You need to literally cut the PCB traces. I use the same saw here I did to cut the PCB originally... screwdrivers work too though. The idea here is to make it so the switched jack switches between the pot on the PCB and the pot in the expression pedal depending on if there is something inside of the jack... So, you need to make it so that it's possible for the pot on the PCB to be connected to nothing. NOTE
This step can be omitted if you are willing to let it be so that both pots work in parallel... Basically so that if you turn the pot on the PCB all the way down it will work the same way... however, I think that's sort of unprofessional, because if the pot on the PCB is anywhere but all the way down the expression pedal's sweep will be messed up. Plus, this way you can put the expression pedal in and pull it out and make the pedal switch between two settings, which can be kind of fun. NOTE
Sometimes the pot is connected to more than one spot on the PCB... If you think about it, if each lug was connected to two spots and you only cut the trace to one spot, then the pot would still be connected and still be doing something in the circuit while the expression pedal was plugged in, and the exp pedal would only be doing half of what it's supposed to be... luckily this is somewhat rare, regardless you should cut all traced going anywhere and remember to solder wires from EVERY spot that the pot goes to to the unswitched side. That way every place is switched. YOu can get by without worrying about it most of the time because if it IS connected to more than one spot, it's usually only one of the lugs... Just be careful!
After cutting the trace it should look something like this... make sure there is NO copper left. Make sure you haven't cut any other traces too... that's a mistake you don't want to make, but if you do, just replace the trace with a wire.
There's an image missing here... It's an easy step though, just solder wires from the lugs of the pot to the switched conductors of the 1/4" jack. If you are just doing a pedal with wires to the pot, make wires from the pot to the switched conductors. That way if there is nothing in the jack, the connections from the spots on the PCB just connect right to the pot, and if something is in there it lifts up the switch and connects to the EXP pedal instead.
Screw those jacks back into the bottom of the enclosure... this is what it will look like in the end. Pretty good from the outside if you ask me.
And you're done. Don't forget to test it along the way... you can even test it the first time when you put those first wires from the places on the PCB to the jack without cutting any traces, just put the pot all the way down. Enjoy!
If you do this mod, please take pictures so we can have a sort of resource because pedalas come in all shapes and sizes.