M horns are constructed out of six basic components, along with some other minor parts: the manifold, bells, heads (diaphragm housings), diaphragms, nozzles, and back caps. With the exception of the diaphragms and nozzles, the construction is all aluminum. The head is bolted to the bell, with the manifold in-between, just like on a Leslie SuperTyfon. The only difference is that the three bolts attach inside of the head. Also as part of the head there is a bronze nozzle, which sits in the base of the head, sealed with an O-ring on all except the earliest of M horns. There is an exception, though. The later horns produced by AirChime and Holden (not Nathan) had the nozzle cast as part of the head, made of aluminum. This design, while simpler, wears away faster since the aluminum is much softer than the bronze.

The diaphragm consists of either two (on the #5 diaphragm) or three bronze disks bolted to a single, thick clapper disk. There is a spacer between the leaves of the diaphragm and the clapper, and loctite is usually used to keep the diaphragm assembly from rattling loose. The entire assembly is inserted into the head, with the clapper contacting the nozzle. Finally, the back cap screws on, and is secured in its final position with a lock bolt on the bottom of the head.

tips

M horns and Leslie SuperTyfon horns are arguably the most difficult (and rewarding) horns to work with, each for their own reasons. M horns have more parts than any other AirChime, and they all have to be "just right" for the horn to operate properly. The following are my ideas on M horn maintenance/restoration - follow at your own risk!!

Before continuing, here are a couple of tidbits agreed upon by most M horn collectors. An M horn, if properly voiced, should be able to sound when quilled from low pressure to high pressure. If the horn only sounds at high pressure, then something may not be right inside, or the horn may need a revoicing. Also, diaphragm pressure against the seat in each head is necessary for an M horn to work properly. If the diaphragm doesn't exert enough pressure on the seat, the seat can be replaced, or shims may be added between the clapper and the diaphragm disks as part of the diaphragm assembly. DO NOT add shims underneath the seat, as this will destroy the effect of the O-ring underneath it to create an air-tight seal, and the horn will stop functioning (and possibly cause damage to the horn). Of course, if any part is heavily worn, either it must be remachined to correct specs or replaced. Now on to my tips.

The first thing I like to do with an M horn is to take it apart and make sure all the diaphragm parts are in tact. Also, make sure that the diaphragm isn't worn down too much where it comes into contact with the housing or back cap. If it is, I've worked around this by reversing the large diaphragm leaves/disks. This puts the worn part of the diaphragm where it will not wear further. Also, the clapper may be reversed if there is significant wear on one side. Of course, it's best to replace the diaphragm assembly if it is worn too much or cracked. If just the large disks are worn or cracked, just these can be replaced if you like. Thanks to Bob Packer, I've been able to replace some cracked diaphragm disks with stainless steel reproductions, which work as well as the originals!

With the back cap off, now is a good time to check the seat in the diaphragm housing. The seat is the cast-in ridge that the diaphragm sits on towards the outside of the housing. If it is uneven or has chunks out of it, then the corresponding bell will most likely leak air and not sound right if at all. If the seat is causing problems, there's not much that can be done aside from having it remachined on a lathe. Also, if you like, remove the bronze nozzle and replace the O-ring below it if it is worn. If there is air leaking from the bell, this is another possible culprit. A little grease on the O-ring will help for an air-tight seal.

Last, examine the back cap. The two rings that press up against the diaphragm assembly should be level all the way across the cap and in the same plane as each other. One very common problem with the M series is that the inner ring has worn faster than the lower ring, causing uneven pressure on the diaphragm assembly, and which could also potentially bend the assembly when the back cap is tightened. If the rings are not level, or one is more worn, the cap can be machined back into spec. Another option, I believe proposed by Fred Berry initially, is to file the rings down. With a marker, trace along the top of the ridge which is more worn than the other. Making even passes across the center of the rings with a file, keep filing until the marker is gone, meaning that the two rings are on the same plane. However, I've found that if the ridges are excessively worn, even filing won't work, since the resulting ridges will be too wide for the horn to operate properly. At this point, the only option is to have the back cap remachined.

Now, with all repairs made, place the diaphragm assembly into the head and look through the hole in the head on the side that attaches to the manifold. I've found that the clapper disk of the diaphragm assembly should have slight pressure on the nozzle. If there is a space between the two, I've had luck adding small shims in the diaphragm assembly between the clapper and disks in order to restore correct pressure between the clapper and nozzle. The other option to resolve this is to replace the nozzle with a new one.

voicing

Finally, the repaired horn needs to be reassembled and voiced. Voicing is nothing more than adjusting the back cap to put proper tension on the diaphragm. The following is adapted from instructions given from Ken Lanovich, a true master at voicing M horns. One precaution - get a real spanner wrench, or build a real spanner wrench. A shoddy spanner using bolts that "fit", or what have you, can very easily destroy a perfectly good back cap. See the other resources page for a place to purchase a spanner if you need one.

I like to have the low bell start first, and then have each other bell start in sequence. To do this, I place the smallest bell on the manifold, running the back cap in only until the diaphragm no longer rattles. I also run bolts through the orifices for each other bell, leaving just the high bell to sound. If a little air leaks around the bolts, that's alright. Next, apply slight air pressure to the horn, but constant pressure. Nathan recommends tuning each bell at just below 20 pounds of pressure, so you have an idea of what pressure to use. If you want less play in the horn, you can always use a higher pressure, such as 30 or 40 pounds. I don't use a regulator, but rather a ball valve opened "just so". Now, using your trusty spanner wrench, tighten the back cap until the mooing sound turns into a solid tone. You can play with the positioning until it sounds good. The cap shouldn't be so tight that it's almost not sounding, but it also shouldn't be so loose that it's barely started sounding. When the cap is where you want it, tighten the lock bolt on the small bell's head to secure the cap in place.

Now, remove the bolt on the next smallest bell's orifice and attach the next assembly, also making the back cap just hand tight. Open your air line, but to a little less pressure than used for the smallest bell. The pressure should be as close to the starting pressure for the smallest bell without the small bell sounding. From here, just repeat the voicing of this bell. When complete, move on to the next smallest bell until the entire horn is assembled and voiced.

If you want to voice an M horn that is already assembled, this is not hard to do either. Remove the cap screws securing each back cap and tighten all the caps so that each bell is no longer sounding. Next, voice the smallest bell as indicated above. When this bell is voiced, move on to the next smallest bell until the entire horn is voiced properly.

There are a few things to keep in mind when tuning an M bell. The back cap is used to put tension, or pressure on the diaphragm. Too little tension will cause the bell to not sound or sound poorly, and too much tension will cause it to require too much pressure to sound, and possibly warp the diaphragm. The tighter the cap, the higher the pitch, but also more pressure is then required for the bell to sound. Also, once the horn has been voiced, it may take a couple minutes of service time for the diaphragms to seat completely. The horn may need a little tweaking at this point to get it just right. M horns can be a lot of fun to work on, and when complete, to blow.