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I've finally managed to get my 1954 D125 up and running. Everything OK except horn sounds like a dying duck ! I've fitted a 12v Ducati Stratos ignition system which supplies the lighting/horn side of things from its regulator/rectifier (the output from this is connected to the brown lead in the wiring loom. So, I presume, this is a 12v rectified d.c.supply. The horn fitted to the scooter is a new single wire one from Cambridge Lambretta fed from the lighting switch with its body earthed. Is this type of horn meant to be fed from an AC supply ? I note that 2 wire horns are also available - what are these ? Not sure if this makes sense but should I fit a horn that runs from 12 instead of 6v, and uses a DC supply ? Grateful for any help.

They ALL sound like that, makes the MOT man laugh.

Your set-up can supply 12v AC or 12v DC. AC horns use the alternating voltage to work the horn. DC horns have points to make and break the circuit. Sounds like you have an AC horn with AC output. To know for sure, rev the engine. If the horn gets higher in pitch and louder then it's an AC horn.

As noted, it's fine.

Thanks jb. I reckon the horn is AC. I bought it to replace the original horn which was way past its sell by date. Come to think of it all model D horns must be AC as thats what was supplied from the magneto lighting coil. I did the 12v conversion at a later date and, as I read the circuit diagram, the supply for the lights (and horn) is supplied from the rectifier which in turn is energised by the cdi unit, thats why I suspect it is DC. Previously I've come across info which says that an AC horn will not work from a DC supply, and vice versa. Thats what triggered off this post. The other annoying thing is that there is no way (external) to adjust the horns tone, its a riveted up unit. Didn't horns used to have an adjusting screw so that you could adjust tone ? I'll fire the bike up and see what increasing the revs does for the horn.

Give the horn a tap with a hammer sometimes the diaphragm gets stuck

Only (some) DC horns had an adjusting screw. AC horns don't need one. The horn makes it's noise by vibrating a steel diaphragm. Here is how a DC horn works:

https://www.autocurious.com/2019/02/how ... -work.html

There is also a spring in there working on the contact points. On some horns you could adjust the tone by changing the spring tension.

An AC horn has no points. It moves the plunger back and forth just by the fact that the coil's polarity changes every time the alternating current flips. If an AC horn is hooked to DC current, it emits a single click as the plunger moves all the way one way and then stays there.

Back to where we were. It's all working fine and you have an AC horn and are running on the regulated (but not rectified) AC output.

I've rev'd up the bike and it doesn't make any difference to the noise or tone of the horn. Thinking about it I don't see why it should, the cdi unit is supplying a relatively constant voltage no matter what the engine speed is, not like the original alternator which, depending on speed, produced a variable voltage output. I'll give the horn a tap but, by the sound of it, there's nothing else I can do but to live with its present croaking noise. Don't know how anybody is supposed to hear that when I'm on the road.

Just to wrap this topic up, the horn now seems to be producing a bit more noise. Don't know why, did a bit of hammer tapping but not convinced it did a great deal. Thanks to jb I now understand the electrics a bit better. Now I think about it the regulator/rectifier unit is putting out a nominal 12v AC. There is an unused terminal on the unit marked in red which, I believe, puts out the DC and would normally be connected to the battery (which I don't have of course). Thanks for all the help.