Martin, a trick I've used on old horns is to turn the tone adjustment screw while someone holds the horn button down. Amazingly enough the two steps have to be done together for results.

The above is assuming all the dirt dabber nest have been removed.

RonR


1951 3600 with Clark flatbed, T5, 4.10 rear
1970 340 Duster
1990 5.0 V8 Miata (1990 Mustang Gt Drivetrain)
1964 CJ5