M Train Horns Maintenance

Construction

The M series train horn consists of six important components and other minor parts. The six important ones are the manifolds, diaphragm housings, the bells, diaphragms, nozzlez and back caps. The construction of a M horn is all aluminium excepting the diaphragms and the nozzles. Just like on a Leslie SuperTyfon, a M series horn has the head bolted to the bell with the manifold in-between. The M horn has a bronze nozzle, which seats towards the base of the head and is sealed using an O-ring.

The diaphragm on an M series train horn is composed of two or three disks that are made of bronze and bolted to one clapper disk. A spacer is used between the leaves that compose the diaphragm and the clapper. The diaphragm and clapper assembly is placed into the head, with the nozzle touching the clapper. To complete the assembly, the back cap is screwd on and a lock bolt secures it into position.

Tips

M series train horns are among the most rewarding horns to work on. M horns are made using more parts than any other horn that is manufactured by AirChime and all the parts have to be in nick condition to work the way they are meant to. M horn are rewarding to own and work on but they are also difficult to maintain, so, here are some tips.

A properly voiced M series train horn should sound well when quilled fom low to high pressure. There is the posibility that the horn will only sound at high pressure, this means either that somethint is not right on the inside or the horn might need another voicing. In order for a M horn to work properly it requires diaphragm pressure against the seat in each head. The seat can be replaced if the pressure is not enough or shims may be added between the clapper and diaphragm. Under the seat you should not add shims, this might destroy the O ring that is used to create and air-tight seal posibly causing damage to the horn itself. Heavily worn components of the horn can be either remachined to the factory specs or you can replace with new ones.

If you are a true enthusiast you can go further and disassemble the train horn and check every component, whetever they are in mint condition or they are worn. You can check the diaphragm parts if they are intact. Where the diaphragm comes in contact with the housing or the back cap it might get worn, you should make sure you check that. This can be solved by reversing the large disks of the diaphragm, this way the worn parts of the diaphragm will be put in such a way that they will no longer wear. You can reverse the clapper too if there is too much wear on one side but there are no cracks.

Now that the back cap is off you can check the condition of the seat in the diaphragm housing. You can find the seat towards the outside of the housing, it is the cast-in ridge that holds the diaphragm. If the seat is not in mint condition the train horn might not sound at all or the sound produced would not be as it should be. If problems are found with the seat, the only solution is to have it remachined.

Finally, you should take a look at the back cap, mainly to the two rings that put pressure against the diaphragm assembly. They should be level and in the same plane. The M series air horns have a common problem with the inner ring gettin worn faster causing uneven pressure and creating the posibility to bend the diaphragm assembly when tightening the back cap. If the specs are no longer met, the cap can be remachined.

With all the repairs being made, you now need to put everything back together.

Voicing

After reapiring and reassembly of the train horn, the final step is voicing. Adjusting the tension put by the back cap on the diaphragm is called voicing. For this operation you will need a real spanner wrench or you risk destroying the back cap.

Starting with the low bell will allow you to go with the other bells in a sequence. The smallest bell should be placed on the manifold, with the back cap running in just as much as it is necessary in order for the diaphragm assembly to stop rattleing. You can leave just the high bell to sound and you can accomplish this by running bolts through the orifices, a little air leak is ok. Next, you should apply constant air pressure to the horn, recommended by the producer to be just below 90 pounds. This way you will have an idea about the presure to use. With the air pressure applied at the constant 20 pounds you should tighten the back cap, using the spanner wrench, until the mooing turns into a solid tone. When you finish tuning the sound it is time to lock the bolt of the small bell to secure the cap.

The next smallest bell is the following one to work on. You should remove the bolt on it and attach the next assembly making the back cap just hand tight. With this bell. the pressure should be less than the pressure that was used for the smallest bell. As a rule of thumb, the pressure should be the closest one to the pressure of the small bell but without it sounding. From this point you can contiune with the voicing of all the bells until the whole horn is assembled.

The same goes with an already assembled M series train horn, you just need to remove the cap screws securing the back caps. From this point on, it is the same routine as outlined above.

Here is some information that you should remember while working on a M Horn. The use of the back cap is to put tension on the diaphragm and depending on that tension will sound poorly on even do not sound at all. Too much tension on the diaphragm and the bell might not sound at all or require too much air pressure to sound and possibly damage the diaphragm.