Faulty horn - how can I repair it?

[Fazerider]

If you can get it open in a non-destructive manner, then you'll understand how it works pretty well immediately.
The current flows round the solenoid creating a magnetic field which pulls the iron core. That is attached to the diaphragm and the contact breaker. Once it's reached a certain point the contact breaker cuts the supply to the coil, the diaphragm springs back until contact is made again.


There are several things that can go wrong:
The contact points can get dirty in which case a bit of emery paper and contact cleaner will do the trick.
The separation point of the contacts is quite critical, you may need to dig some sealant away first, but there should be an adjustment screw there.
Less easy things to fix would be a burnt out coil, or (what usually happens to old horns) the rim of the diaphragm rusts away until it can't provide the springiness needed for the return half of the noise-making cycle.