Electrical equipment and instruments (54)

Dual tone horns

Dismantling and repairing the dual tone horns are not especially difficult. A lot of information can be found on the internet but often hidden from google in different oldtimer forums for e.g. Mercedes, Volkswagen and Porsche models from the fifties. Some people tries with the "there is  o n e  thing that you need to do right and it's very difficult. I won't say what but I can restore the horns for you for a fair price". That is b-llsh-t. I've seen restorations with step-by-step instructions where people have turned more or less a pile of rust into very good looking and functioning horns. Have e.g. a look in the "Early Early 911 register" and "Pagodentreff.de" (in German) for reference material. I don't want to copy and paste the instructions due to their copyright. The only thing that may be a bit problematic is to adjust the frequency of the signal so that it sounds as it should.

Bosch HO/FSA 12 bass and high tone horn restored

I can again be very thankful to my father who cared for the car. For me it was almost just to take them apart, clean and paint everything and then test.

I must again highlight the importance of being meticulous when you take them apart; take pictures, make markings, measure everything and see to that you know in what order everything goes. I wasn't, so I had a couple of hours of trial and error until I got them working...

The first problem was that I had taken one of the horns apart long time ago and hadn't made the temporary assembly correctly. The second problem appeared when I started this restoration work; I did both at the same time and hence mixed some things and pictures up.

Note; in the photos below I will mix pictures from both the horns. "F150" is the rear bass horn mounted on the left wheel arch, behind the radiator, and "F151" is the high tune horn mounted in front of the radiator. The difference on the outside should be obvious and they are identical on the inside except the settings of the tuning screw of the frequency. Each has its unique frequency (stamped on the central plates!) so that the two tones match each other and give them and your Ponton its unique horn sound. I measured the tuning screw settings and re-used those. Seems to work…

If you need to adjust I assume you will need a tool for measuring frequency. The long tuning screw in the centre adjusts frequency and the adjusting screw on the back of the housing adjusts the function of the contact. As I've understand it, it is normally the long pin in the center that you make most of your adjustments with.

"Before" first. "After" later…

Notes; In the first picture you can see the screws holding the cover, the were black plated. In the seventh picture I've written "black?" close to the cable connector screws M4x6. Forget that. They should all be white "cadmium". 

The tuning screw for the rear horn was protruding ~0,6mm and for the front horn ~1,1mm.

Then it was time to make four new "petrol-oil-water-ect"-resistant paper gaskets. I covered them with a thin layer of Vaseline to make them easier to hold in place while doing the assembly.

Then I did the sandblasting, grinding and cleaning. First you see the high tone horn and then in the second picture the bass tone horn. In the third picture where you have the rear side of a casing you see a slotted screw that allows one type of adjustment (don't know which actually, didn't try it) and removal of the contact support bar. 

You could of course remove all the electrical wiring and rubber gaskets but I decided just to protect them before the paint. First one or two layers of primer and then glossy black on the top. In the last picture you can see all parts having been painted black, but note that there are also some parts that do not belong to the horns!

Then the housing was assembled in terms of bracket and electrical connections. The Bakelite was cleaned white spirit, polished with Greygate's "Paste Polishing No 5" and protected with bee wax all according to a Bakelite restoration site (for restoring old phones). There are many ways to restore Bakelite. I did it this way… Maybe a layer of lacquer would have been more protective in this environment than bee wax, but... Note, that the screw and the nuts in the last picture are not black plated as they should and later would be!

The two Bosch central plates, were cleaned and covered with a couple of layers of clear lacquer. Be careful, the text on the plates is fragile and doesn't stand too much solvent.

In the first photo you can see contacts that you may have to clean. The second photo shows the correct order for the rear horn. In the front horn you don't have the Bosch plate on the central tuning screw, but the order of the washers in front of the large disc is the same. The third photo shows my "third hand" while assembling all the parts.

My Mercedes-Benz 180K, W203

Put a little amount of grease on the part of the central tuning screw that goes into the anchor and magnet. Position the tuning screw so that the bevel in its "screw" is fixed by the splines in the central pin so that you can adjust the horn by carefully turning the pin. Be careful, if you unscrew the "screw" too much it will lose the connection to the pin and get stuck.

Finally I tested the horns, connected according to the electrical diagram in the workshop manual. Doing this in the basement, even my neighbors wondered what was going on. Warn the family and wear ear protection!

Tada, the horns ready for usage! First picture shows the bass tone horn (mounted behind the radiator) and the second the high tone horn (mounted in front of the radiator). However, I wouldn't be surprised if the screws holding the cover were painted together with the complete, assembled horn and not only black plated as mine are. Well, easily adjusted if that's the case. But black they should be.