FH - that horn again

 In a post a while back I wrote that given that I couldn't take the horn to pieces to clean the points and that as it worked, I would just go ahead and fit it. The trouble is once I'd wired it up, pressing the button had no effect. It just didn't work. I had it on the bench, checking the resistance of the coil with the multimeter and driving it with a bench top power supply but it was having none of it. It would have to come apart. 

The difficulty with these horns is that the armature is retained in the diaphragm by a nut which is extremely tight. It's very difficult to remove, mainly because it's shallow and you have to be careful to hold the diaphragm to avoid it getting damaged. The way I resolved this was to make a wooden clamp that I could put in the big vice and hold the fragile diaphragm against the body of the horn whilst undoing the nut with a closely fitting tube spanner. With a 400 mm tommy bar, which I bent, it eventually came undone.

So that's the horn in bits. Once you've loosened the centre armature nut, you can remove the diaphragm leaving the armature in place. With the diaphragm off, you can slide the armature out from underneath the points assembly and that is about all the dismantling that you will need to do.

The point spring is very strong and it's helpful to use a jewellers screwdriver inserted between the moving point and the fixed point such that you can get a small sliver of wet and dry paper between the contacts to clean them.

Clean all the rust off with the wire wheel and Emery cloth. It's impossible to set the horn up properly if there is still rust on the body and inside the diaphragm and you won't be able to tighten the armature adequately if the nut is still stiff and rusty.

When you reassemble it, if you align these slots in the armature with the peg on the outside of the horn body you will have the armature in the correct relationship with the points assembly. When you have it in the right position you can tighten the nut up again using the same clamping mechanism that you used to undo it.

By the way, that peg is important. You'll note that the diaphragm has an inclined slot which engages with the peg? Well, the diaphragm can be rotated such that the slot causes the diaphragm to move up and down in relation to the body of the horn. This is how the armature air gap is set on these later HF1441 horns. The earlier Altette horns use a big grub screw and a nut on the back of the body for the same purpose. The diaphragm is held in place after setting with a clamping ring around the outside.

The easiest way to test the deflection of the diaphragm is to set the horn up on the bike with a magnetic DTI base sitting on some suitable surface, in this case the pillion footrest mount which on a PRF framed Ariel is quite flat. To test the deflection, back out the horn adjustment screw fully so that the points will not open when the push button is operated. Don't forget to undo the lock nut before you attempt to move the points screw. 

When you press the horn button with the screw all the way out, you will hear the diaphragm deflect with a click. If you don't hear anything it's likely that the deflection is insufficient or non-existent so use your hand to increase the deflection of the diaphragm by turning the whole diaphragm anticlockwise slightly.

According to the Lucas instructions the deflection is supposed to be 11 to 13 thousandths of an inch.

When you've got the deflection correct, you can replace the clamping ring to hold the diaphragm in position. Then it's time to adjust the horn points setting: undo the locking ring and adjust the points using a small screwdriver. Turning the screw clockwise opens the points further for each deflection of the diaphragm.

You have to fiddle a bit with this screw to get the tone you want. Don't be afraid to turn it half a turn each way or more if necessary and test the horn each time until you get a tone that's consistent and doesn't sound like a dying parrot. When you fit the tone ring you'll hear it change substantially.

When you're happy with the sound you're getting from your rebuilt horn, don't forget to do up the locking ring, put the tone ring on and do up the acorn nut. I made this tool to turn the locking ring: