The master is a simple adjustable voltage divider. R28 is a kind of attenuate which helps to reduce the signal level a little bit (ca. 15 - 20%) and C7 is needed to decouple the potentiometer from the R28 otherwise the R28 will be parallel to the potentiometer and then the master will not work as it should. Changing the value of C7 will affect the bass/treble. We found 680 pF okay even with maximum overdrive.
Best regards, Dirk
