What could turn the Sustain pot in an old ram’s head muff into a second volume pot? Bad caps? Bad pot? Something else?
I picked up a later 70s BC239C ram’s head muff cheapish with a broken switch. Installed a 3PDT wired for TB and it worked but didn’t sound right and turning down the Sustain also drops the volume. None of my other ram’s heads do this, so I figure something is amiss. also, at maximum sustain it’s not really singing like a muff. Something’s not … engaged?
So I opened up and repaired the old switch just in case it was my TB wiring, rewired it as it was originally, and the same thing. I figure some other component(s) is knackered. Any idea where to start looking. What could lead to the Sustain pot turning down the volume?

Tone pot seems a bit off as well actually — with treble at both ends of travel and a bit muffled in the middle.

EDIT: Just pulled the board and C1 looks to be newer than the others. It’s 10uF electrolytic marked SURGE
Mine’s the same as this one. Which side should be negative on the C1 cap in this pic? Left or Right?