Silver knob Concert bass help

[Kustomsunn]

Hey Guys, I need some help. I have a Sunn concert bass silver amp made in 74'. The distortion does not work. Yes I grounded the RCA footshitch I cleaned and tested the pots, the boost works but the distortion does not. Did serious testing of components and found nothing out of the ordinary. Really stuck here, any one have any advice?? The rest of the amp works well and makes me sad the distortion is not! HELP!!!!!

[lovikpoy]

so does the handle have to come off as well? or just the knob?

ive seen it done on 240s and its gravy. you could do it at a red light