Altette Soundings!

Having browsed the Internet and watched various YouTube videos about Altette horn repair, it seemed to me that they are regarded with some trepidation. (YouTube video 1951 Vincent Rapide, Part 38).

This is worth watching as it describes visually what I am trying to say in this Altette guide.

I therefore hope that this description of my efforts may help some other enthusiast to pluck up courage and delve into the Altette horn.

I had the beaten up remains of a late Altette Horn HF1234 12 volt horn that I planned to use on my PB, however I understand that this type was fitted to very late TA/TB/TCs – not sure about this detail, but seems possible, so I thought that ‘Totally T-Type 2’ might be interested in my attempts to get this horn working.

Ed’s note: The HF1234 wascertainly fitted to the TC. The 1937 TA was fitted with HF934/2.

This type of Altette has a cast iron body and is a simplified version of the earlier horns of the same name, shape and style.

After dismantling the horn, I found that the points were not connected correctly i.e. leads broken and the insulated Tufnol terminal plate was cracked, and there was a flat cupped washer with a rubber insert rolling around on the inside of the casting without any apparent use?

The shims were non-existent.

The chrome bezel on my horn was extremely rusty and corroded, so I purchased a new bezel, new fixing bolts, Tufnol terminal plate and a set of shims from ‘Taff The Horn’. After reading through Taff’s website and the paperwork he sent to me with the parts, I realised that what I thought was going to be an easy job might be more difficult.

I decided to retain the rusty bezel and after de-rusting and resurfacing the pitted steel, I painted it chrome colour, just in case that it might be useful in the future (more of which later).

I then downloaded Lucas Equipment Workshop Instructions from the Internet www.mg-tabc.org/library/Altette_Horn.pdf  

This has all the technical information that you are likely to need when fixing these horns.

After de-rusting the cast iron body and un-seizing the fixing and adjusting screws on the rear of the body, I was ready to start rebuilding and here begins some of the conundrums. The Lucas Workshop Instructions did not “exactly” identify my particular model, therefore it is important to get to understand the principles of operation and then apply the knowledge to your particular horn.

On this particular Altette there are just 3 screws on the back, 2 small screws are side by side and they secure the points assembly to the base of the cast iron body, and 1 larger screw that “levels” the points (it does not “adjust” the points gap).

Picture 1 showing rear of Altette and the location of the screws referred to in the text.

The first check I carried out was to establish if the coil was any good by checking its resistance.

Connect your ohmmeter across the terminals and take a reading; according to the Lucas Workshop Instructions, our 12 volt version should read between 0.70 and 0.75 ohms. Assuming all is well, turn your attention to the inside of the horn casing.

The large screw on the reverse controls the “level” of the points, not the adjustment of the points gap; by turning the screw in or out you will see the points assembly tilting left or right, the object is to get them level within the body of the horn. At this point I realised what the cupped washer with the rubber insert was for. It fits on top of the adjusting screw and provides a cushion to prevent the screw damaging the electrical connection (I think that’s correct, otherwise I’m stumped!).

Picture 2 showing the cupped washer with the rubber insert.

Next job is to clean the points, these are clamped together with their appropriate flat springs. However, you will notice that there is a triangular Tufnol area with 2 tiny brass rivets attached and when this “ear” is pressed down with your finger, the points will be forced open and it’s possible to insert a strip of emery/wet and dry to clean the points.

Do not force the points open with a lever, just use finger pressure. If you break the points it’s game over and you will need new points – if you can find them!

Now it’s time to think of the shims! I did not have a clue as to “how many” I would need, so I opted for 3 thin ones (Taff supplied various thicknesses). Start with say 2 thin ones on the horn body, then fit the diaphragm. If you dismantled your Altette and shims were fitted, start your rebuild using them as they are a good starting point. Make sure you impregnate shims with Vaseline, don’t use gasket cement of any kind.

Bearing in mind how flimsy the shims are, it’s a good idea to use two 3/16 BSF studs screwed into the horn mounting holes on each side of the horn and fit the shims over them, this helps locate the shims and later the diaphragm.

Pic 3 – 3/16 BSF studs help locate the shims.

Having fitted the shims fit the armature, this has an aperture on two sides and it’s an easy matter to sit it into the larger magnet core face. Fit the diaphragm, making sum that the diaphragm is the correct way around; my opinion is that the convex side faces outwards i.e. the bulge around the rim of the diaphragm faces outwards (see next pic).

Diaphragm fitted – convex side facing outwards

Screw on the large lock nut, ensuring that diaphragm sits on the shallow ledge machined into the armature and fit the fixing bolts around the diaphragm and tighten them and then tighten the large lock nut.

Fit the points pushrod and its locknut into the armature and screw the pushrod in until you feel it “bottom” – do not over tighten. Hopefully you are now ready to make your first attempt to achieve a sound from your Altette.

Make sure that you use a 12v battery for testing and not a battery charger. I used a cheap 12-volt burglar alarm battery, being convenient and lightweight – at my age I find car batteries a bit too heavy for me.

Rig the 12-volt battery with a lead to one of the 2 terminals on your horn. Using the second lead from the battery gently touch the second terminal on the horn, if you do not get a sound do not hold the lead in contact – ‘Taff the Horn’ warns of this, over and over again – as you are likely to melt the points. At this stage they are probably closed i.e. no gap therefore “no sound.”

This is the point at which your patience is demanded, gently turn the screw half a turn and then touch the lead to the horn terminal. If no sound, try another turn and so on, always making sure that you do not touch the terminal for too long.

Assuming the you do not get a sound from the horn, dismantle the whole assembly, then fit an additional shim under the diaphragm and repeat the whole process!

Hopefully, after carrying out this exercise a couple of times i.e. adding shims, suddenly you get a strangled sound from your horn. Perhaps rather weak but nevertheless “a sound”. Now it’s a question of gently screwing the points push rod in or out until a reasonable sound is achieved, not forgetting to adjust the locknut up tight, as the points rod will quickly unscrew when the horn works. This surprised me as the operation of diaphragm is quite violent and unless the locknut is tightened you may lose it.

The points adjuster and locknut have a very fine thread and not easy to reproduce or find.

So, at last “It’s Alive” and now it’s a further exercise in patience. Find your bezel and make sure that it is still “round” and that there is no old paint or rusty bumps on the inside of the rim.

Also, find the orientation that the bezel was fitted; originally the Lucas logo was at the top, however some bezels have lost their surface finish and who knows where the logo was?

I found that my original bezel had been assembled for so long that it had taken its shape from the casting i.e. it fitted beautifully when using the fixing bolts that hold the horn to the bracket, however turning it one sixth and it did not fit!

Once again ‘Taff the Horn’ insists that the fit is close but not binding – remember that the diaphragm should be held firmly and softly by the shims and not clamped by the bezel in any way.

Once you have sorted your bezel it’s time to fit shims i.e. remove all of the perimeter fixing screws and place your chosen number of shims on top of the diaphragm (I ended with 3), fit the bezel and perimeter screws and tighten them carefully.

Go back to your battery rig and touch power to the terminal as before, do not be surprised if you get no response! Revert to the points adjuster and carefully go through the adjusting sequence again, you should soon get your sound back, it might sound puny and weak; it’s up to you to adjust until you get the sound that you think adequate.

Do not expect the mellow tone of the Twin Windtone with its High and Low horns, the sound from the Altette is harsh but insistent.

Assuming that all is well, fit the Tone Disc and the large dome cover nut, making sure that points locking nut is “locked”. Then try the horn again, it may sound fine — but then again it may not, so remove the Tone Disc and play with the adjuster until you can achieve a sound that suits you.

You can see from all of the above that every time that you tighten or fit components the points adjuster is affected i.e. the points have been opened or closed. The correct air gap between the Armature and the Magnet core face is 18 to 20 thou on our 12 volt Altette, but because it’s under the diaphragm it cannot be seen.

The Lucas Workshop instructions explains, how to measure the gap using a dial gauge. I did not want to spend even more time setting up a “rig” with a dial gauge to establish the gap and then find that I had to alter it to get a sound.

‘Taff the Horn’ suggests using an old bezel to avoid damaging your new chromed bezel whilst carrying out adjustments. Bearing in mind the number of times that the perimeter screws are fitted and then removed to fit shims etc that’s not a bad idea. By now, you should have a horn that works!!

I hope that this long explanation of my experiences with the Altette will help someone else to attempt to repair their rusty horn.

Please note that I am not an auto electrician or an expert in any way and also, I have no connection with ‘Taff the Horn’ other than being a satisfied client of his and appreciating his efforts to educate me in the operation of Altettes.

Russell Dade