I'd say the klaxon Activerts where Cracking bell box's back in the late 80's-90's, Although I'm convinced that's one of the main problem's your fuses are going, In the past I've Serviced a old 9800 That had an activert attached, Because there twin tamper(lid&wall) the wall tamper tends to go after a long time I'd say yours had lasted a lot longer,
Replace the bell box
I suggest A Klaxon flashguard Xl I think it is, Very reliable bell box's Loud as well ! burglar's would be legging it
Don't Get texecom bell box's as there speaker tends to f**k up in cold weather for good, bad bell box's
Death, March 2013