I have a 1975 Script Phase 100 (the one with the "Tropical fish" capacitors), a 1976 Script and a 1977 Block. The 1976 Script and the 1977 Block while having slightly different layouts on the PCB board (really just different IC's and maybe some caps - I don't have them in front of me right now) sound very similar. However, my 1975 Script has a similar problem as yours. There is a bit of a volume drop when in use and the only setting that seems to work is position four on the selector knob (the two big ovals with the arrows pointing outwards) and that setting, while very smooth and silky, is a bit anemic. The speed control also goes from rather slow to ultra fast.
Anyway, I spoke with Richard Lingenberg
about it. He fixed up my Mu-Tron III and did a fantastic job (not to mention a lightning fast turn around time). He said he could fix the Phase 100 and I am going to send it to him when I resolve my flat broke situation. He knows this gear very, very well and in my opinion is the go to guy for this kind of work. Check out the link I provided and give him a call. He is a great guy and his work is top notch. He can also customize the pedal for you (true bypass, external 9Volt power connector and so forth) and the repair costs are very reasonable,