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Discussion Starter #1
What causes the horn to produce the LOW tone (101.03.65.126.00 HO/FGD 12/1) or the HI tone (101.02.65.127.00 HO/FGD 12/5)?

I bought a box of horn parts ……. Previous person disassembled multiple horns and put all parts in one box before giving up on the project.

The main body with the coil still had the Bakelite connectors so can identify them as 12/1 and 12/5. I don’t think the front grill matters. That leaves the Diaphragm / Armature plate (plate next to “cup that holds the coils), the Megaphone plate (plate under grill) and; or something else that causes LOW / HI sound.

I see two ways the Diaphragm / Armature plates and Megaphone vary: (1) thickness of metal and (2) shape. By shape I make reference to the height of the “cone” above a reference surface.

Here are three examples
  • Diaphragm / Armature observed thickness: 0.51, 0.50, 0.60 mm
  • Diaphragm / Armature observed “height of cone”: 0.60, 0.70, 1.52 mm
  • Megaphone observed thickness: 0.60, 0.61, 0.66 mm
  • Megaphone observed “height of cone”: 7.90, 7.95, 8.11 mm
The item that is most unlike the rest is the 1.52 mm height on one Diaphragm / Armature plate
Is this the answer?
Diaphragm height.jpg
Megaphone height.jpg

And which is LOW and which is HI? When assembled I want the correct Bakelite 12/1 or 12/5 to the correct sound.
 

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Hi Carl,

Here is what I could distil from an italian article ( 1960) about the difference between high and low tones and the adjustments of the horns. Maybe this can help you.

> The more the membrane is rigid, the higher the tone.
The steel membranes have a variable thickness between 0,4 and 0,7 mm ( in general 0,5 mm) in function of the desired tone. At equal dimensions and shape, a thicker membrane results in a higher frequency.
A 100 mm x 0,5 mm membrane has a frequency of 290~340 Hz, a 100 mm x 0,7 mm membrane has a frequency of 380~420 Hz .

> Additionally, for each basic frequency 300 and 500 Hz, there would be a determined membrane diameter for an optimal movement of the surrounding air.
> The Diffusore ( megaphone) is fixed at the membrane center and works as an amplifier of the vibrations of the air. It serves also as an exciter of sought after high harmonics.
The catalogs show only one and the same megaphone for both the low and high horns !
They are no determing factor for the high or low tone difference I think.
> The capacitor is basically mounted for preventing sparks to occur between the contact points and prolonging that way the life of the points. Smaller horns ( f.i. on motos) often don't have a capacitor at the expense of a long life.

Btw, the adjustable screw at the back is for controling the contact points: more precisely for obtaining a correct interruption of the current in the coil. "
This adjustment has nothing to do with the pitch of the tone.
They advice to better not making this adjustment with the horn mounted in the car for avoiding short circuits, fuse blowing etc.

The adjustment is done on the bench with the horn connected to the battery ( notice the switch !)
1671683



Here a picture of the adjusting screw with spring, please note that only minimal adjustment is allowed, normally only 1/8 th of a turn :
1671684


Rgds,

Thierry
 

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Discussion Starter #3
Thank you very much. The statement "A 100 mm x 0,5 mm membrane has a frequency of 290~340 Hz, a 100 mm x 0,7 mm membrane has a frequency of 380~420 Hz . "; I think probably provides the answer.
 

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Discussion Starter #4
More confirmation. Just looked in 2000 (cast iron) Spare Parts and found they have two diaphragms listed .......... for the two different tones. The HO/FSA horns are very similar to the HO/FDG horns.
Bosch 2L horns.JPG
 

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Many Many years ago (50?) with 101-1300 Spiders, we discovered that Normales had both horns tuned the same, while Veloce's always had a low and high tone. This was NOT true with 100% of the cars worked on, but enough to conclude that maybe this was intentional. Further, ALL original 64/65 Spider Veloces had high and low tone horns, BUT some Normales were high/low as well when new from the same time period. Over time, horn replacement obviously might change original build.
I once restored these for customers and also had boxes of parts. The Bosch horns were designed to be worked on and repairable. The big issue was moisture migrating past seals and gaskets to cause corrosion. Diaphragms with ANY rust hole is history. AS I mentioned to you Carl, I paint those being restored, in and out, with the 2 step Rustoleum after media cleaning to bare steel. Paper gaskets can be made from yellow mailing envelopes. After cut out, soak them in melted canning wax for waterproofing. Canning wax mixed with beeswax for a gummy feel works best. You want them air and moisture tight internally.
 

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I think Terry Rushbrook suggested this.. WORKS well!
 
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