MM RD-100 troubleshooting

I just bought an RD-100-EVM. A little rough on the outside, but everything works. Except for the distortion channel. The LED lights up on the pedal, but no change in sound. Any diagnostic hints?

All the switching is done using a number of CMOS switches (4066 chips). They are plentiful and cheap so replacing them all MAY fix the problem.
If you have test equipment and troubleshooting skills you can just walk through the switching stages one at a time and check for proper swith operation.