Drago’s margin bell is finally working 100% again (probably for the first time in decades, actually) – after examining every millimeter of the rather Rube Goldbergish margin bell system of my Underwood #5 and finding nothing specifically faulty. The problem in the end was probably one of design, as the flippy arm (#20 in illustration above) that powers the bell hammer is operated mainly by gravity to come back down and ring the bell after having been flipped by arm #24.
In Drago, it appeared that the gravity-assisted flippy action of lever #20 just wasn’t aggressive enough to cause the bell hammer to bounce hard enough against the bell to ring it clearly. My solution? Add a bit of spring-powered assist. Luckily, there’s a stopper bar attached to lever #20 about an inch ahead of the pivot point that makes for a handy mount point for a spring.
The other end of the spring is mounted to bar #28. Choosing which spring to use is trial and error – too strong, and it causes the line lock to engage early (which should clue you into the baroque sensitivities of the entire margin bell/line lock system). This thin, loose-coil spring worked wonderfully to add just enough oomph to the return stroke of the bell hammer.