I have seen and read that Jerry had his Phase 100 sweep pot modded to a pedal control. When I got one I quickly realized how useful and fun this would be. There are many times you'd like to change the control without taking your hands off the guitar. Here's how I did it using a commonly available Ernie Ball pedal and two stereo jacks. When the cable is unplugged it leaves the pedal sweep knob working. - (Caveat: my pedals are mounted vertical. It might be more challenging to try this with the jack on the side.)
MXR Phase 100
Same size project box (backplate only used)
Ernie Ball Jr Volume pedal (250K passive pot)
On/off stereo jack (Mouser 1/4" Stereo 2DPDT - P/N 161-0084-E)
bits of wire
Total cost not including the MXR was about $75
Since the script logo 100 I have is vintage I didn't want to drill it for an AC adapter, nor for this mod so I bought a same size project box which awesomely enough, was the same width, but a little short leaving room for the AC adapter wire at the bottom. I was able to drill out the hole for the jack on the new backplate and the pedal is just deep enough for the jack if we bend the pins over a bit (and insulate the box with tape just in case.) I think there'd be room to mount the jack on the side of the MXR if your pedals aren't vertical.
I could not find an exact match for the sweep pot value, which appears to be about 400K, to fit the Ernie ball pedal. I tried a 500K pot as a test but the sweep range was too short. As it turns out, the capacitance added by the long stereo cable seems to work out just fine with the stock passive 250K EB pedal with the bonus of not having to mess around with removing/replacing/redrilling it.
Here's the schematic:
I simply wired the on/off stereo jack into the MXR, drilled the backplate and attached the jack, and drilled the mounting board on my rack to match the location of it so I could run the stereo cable from the back. The screw holes on the new back don't line up exactly, but the zip ties keep it all together. Then I removed the stock Ernie Ball mono/tuner jacks, added a stereo jack, and wired the tip and ring of that to the pot in the footpedal. It would probably be smart to ground the sleeve at the MXR end for shielding. Works wonderfully on Candyman, preserves the vintage pedal, and the MXR knob works when you don't want to use the footpedal!
Many thanks to all here.