I have a FS74 trimmer, which I believe is pretty much the same but with a loop handle and a flexible drive cable in the straight shaft rather than an actual driveshaft. Same powerhead and gearbox (I think). I was just looking at the manual today, which covers the FS74 and FS76, and noticed that the print date on it was 1992.
We bought the 74 in '92 or '93, I think.
Not the lightest or most powerful unit, but I have to say that it has been in near-weekly 3-season use for, what, 23 years now AND HAS NEVER NEEDED FUEL LINES OR A CARB KIT! I have to tweak the carb screws once or twice per summer. Had to replace the gearbox and bump head once each (used gearbox and head off a parts trimmer the local shop had out back). It refuses to die, despite pretty extended hard usage (rural area, lots of cutting hours). Maybe you should fix yours!