I have to ask the obvious since it's your first machine: have you set the right margin, and if so, does it stop the carriage when it reaches it? Assuming the margin has been set, there could be a number of things going on: one of the components in the mechanism could be bent out of shape, the gong spring (or some other part) could be missing, or the actuating lever might be seized. 

​You need to remove the back cover of the machine to get at most of those parts. I'd suggest doing that and cleaning all of the pivot points of the bell's levers. If while doing that the problem doesn't make itself obvious, you'll have to post a few pictures so I can see if everything is there.

Same problem but wrong typewriter. I will post this photo of an SM-3 anyway on the excuse that the machine may have a similar mechanism:


When I find a spring which fits between those attachment points I will know for sure but right now the bell only rings intermittently, while it sounds clearly if I flick the lever mounting the small toggle.

The bell hammer at "rest" should not be in contact with the bell...but just a tad off the surface.  Had this problem with my 1957 Silent Super until I formed (bent) the bell hammer arm a bit so if hovers off of the bell surface just the smallest amount.  My bell rings and vibrates now, but it is very quiet.  Maybe part of the Silent" part S-C was aiming for...??? he he he    One day I might try a bell from a different machine.  Think I have 2 parts machines now...
 

Cape, 
You can unscrew the bell from the chassis, clean the surfaces where it contacts the chassis then screw it back down tight. You can also turn the bell here and there and it may affect the volume a bit. If you remove the bell, give it a good cleaning all over, as anything on the outside or inside will reduce the volume by preventing vibration. Also, you can adjust how much the hammer hits. It will be loudest if it just kisses the bell then comes back to repose. If it is really close to the bell, it may actually strike it more than once, interrupting the vibration. It's a real hit an miss of trying out tuning tricks. Good luck.
Phil Forrest

Hi CC

Not sure if Phil intended a pun when he said striking the bell is a real hit-n-miss operation, or if it's just my sense of humor. Anyhow, just looking at one of my Silent-Supers, the bell sits awfully close to the underside of the rear frame rail, maybe close enough to touch at times and disturb the resonance. On trick that I'll often try on a dead sounding bell is to insert a close fitting lock washer between the bell and its support. This reduces the area of contact between the bell and the supporting structure allowing the bell to resonate more freely.

I know a couple of deaf people, and as I get older, my hearing is getting poorer, so a thought has been playing in the back of my mind for a while now. I have often wondered if some sort of pop-up flag or easily visible indicator that is activated by the bell trigger could be attached to the right hand end of the carriage. The flag would have to re-set itself as the carriage was returned so would be ready to pop up again at the end of the next line. All the best,

Sky