To get back to the original post, on my 1971, you can't sound the horn if the headlights are on main beam! Not sure what, need to investigate!
Arwyn
Likely a high resistance connection in a shared circuit such that the extra current from the headlamps increases the volt drop (E= I x R) depriving the horn from a great enough voltage to allow function. A voltmeter would prove this and a rewire would cure it.
ETA: just checked the wiring diagram 1969-1973 and it shows the horn and rad fan sharing a fused supply taken directly from the starter solenoid in turn close to battery positive. It was common practice at the time for the horn to receive this direct supply bypassing the ammeter. My Velocette is wired so giving the added advantage of proving the dynamo is working as it shows current flowing to battery to compensate when horn operated.
Advice remains the same regarding voltage checks but also confirm wiring is ex factory as above, and if so check this dedicated wiring to horn fuse. The electrical path from battery neg to chassis is also common to headlamps and horns as well of course so worth checking including possible shared ground connection. Finally although unlikely, it could be a near dead battery with the headlamps taking most of remaining current when engine not running or at idle.
