I've been trying to revive a dead EHX Q-Tron pedal back to life, and R.G. Keen's website GEO-fex on the history of effects pedals has been a tremendous resource for me. Check out the links to schematics and PCB layouts of legendary pedals and mods on the left side. There are also articles published by R.G. Keen breaking down different types of pedal effects that are a great read if you're at all interested in working on pedals. Check this one out about the wah pedal.