Vintage 6v Motorcycle Horn Repair

Thread Starter

DanielHowden

Joined Nov 8, 2019
65
Hi, part of my job is restoring motorcycles. On this project I am trying to get this 1973 6v horn working again. It seems to me that the 6v modern battery is too strong on the electro magnet and is not allowing the plate to move away from the pull to create the noise.

I am wondering it there is a way, electronically, to reduce the power to the horn. I did connect a small bulb in series but this took too much power.

Can anyone suggest how I can achieve this lessoning in power please?

Thanks, Daniel.

Here is a video and some pictures.

https://photos.app.goo.gl/AFT1b4Ws1CkCkYqw9

PXL_20231123_102700708.jpg

PXL_20231123_102710713.jpg
 

Ya’akov

Joined Jan 27, 2019
8,973
On it’s face, your theory—the battery is “too powerful”—doesn’t really make sense.

A 6V PbSO₄ (Lead Acid) battery will have three cells in series, at 2V each, and then some number of these 3-cell clusters in parallel for more current.

When you say “too powerful”, there’s a problem. If the horn is designed for 6V, and use a 6V battery, there is no room for “too powerful” since every case of 6V is equally “powerful” in the casual sense.

Technically, power would be measured in Watts (W). This is the product of the voltage and current. While a new battery may well be capable of producing more power in W, the load (horn) plays a key role in how much current and so power will be consumed.

You can determine the current (A, or I) in a circuit if you know two other things: voltage (V) and resistance (R, or Ω). The formula is simple and if you examine it just a little you will see the relationships among the terms. To get current, we use this form of Ohm’s Law:

\[ \mathsf{I = {V\over R}} \]
You can see from this that current (I) in amps is directly proportional to voltage (V) in volts and inversely proportional to resistance (R) in Ω. This puts the load in charge of current draw*. The battery can be ready to deliver more current than is needed but it can’t force that on the load—unless the voltage is increased, in which case calculating the current with the same formula will give a higher number.
*The exception to this are non-ohmic loads which is important but not for this problem.

So, I don’t think your theory is a good one. I suspect the switch that is supposed to disconnect power to the coil forming the electromagnet is the source of the problem. The horn works because as soon as the electromagnet attracts the diaphragm it also operates a switch through which the coil gets power.

The switch is NC (Normally Closed), so if the horn is not energized otherwise, it is possible to power the coil. But, when the coil gets power one of the effects is to turn that power off. This causes the oscillation that translates to the sound.

I am not family with the particular design of the horn in your photos, and the angle doesn’t show the mechanism well. A shot more from the side might help.
 

Ya’akov

Joined Jan 27, 2019
8,973
Brilliant post, thank you - how I show my ignorance!

Let me takes some more pics and I will upload.

Thanks, Daniel.
All of us are ignorant until we learn. Never worry about ignorance, it is so easily cured by knowledge. It is foolishness you need to guard against, and so far as I can see you have nothing to worry about on that front.
 

Tonyr1084

Joined Sep 24, 2015
7,772
What I know of horns: The diaphragm pulls toward the electromagnet (EM). That opens a contact point and the diaphragm relaxes, thus making contact again. This happens at a frequency dependent on the diaphragm size and weight. I've seen those contacts burn up over time. Cleaning and polishing the contacts seems to restore horns I've tinkered with in the past. Other things that cause horn failure (in my experience) has been sand/dirt getting into the diaphragm. In your pictures I didn't see any contacts. You spoke of a switch. I'll have to review the pic's again but I don't see any contacts that would make the horn work.

Now: you said
It seems to me that the 6v modern battery is too strong on the electro magnet and is not allowing the plate to move away from the pull to create the noise.
As has been pointed out - that's a false premises. A battery has a capacity and a force. The force is the voltage and the capacity is how much current it can deliver in a dead short and how much current it can deliver over a given period of time (rated in Ah or Amp Hours). The way it works is the voltage pushes against the resistance of the horn. You can have a battery with a capacity of 100 amp hours and 250 cold cranking amps. But that doesn't mean it's going to force that much current through the device. The amount of amps going through the horn will depend on the voltage and the resistance. I'll assume the resistance of the horn is on the order of maybe 2 ohms. 6V divided by 2Ω = 3A. That's 18W (6V x 3A = 18W). It doesn't matter what the battery is capable of doing, the horn is going to draw 3A in my example. If the horn resistance is 1 own then the amperage would be 6A (6V ÷ 1Ω = 6A) or 36W.

I had a 72 Chevy Nova with a single horn that went bad. I replaced it with horns from an Oldsmobile. Then out of curiosity I drilled out the rivets on the horn and exposed the inner workings. There was a lot of sand AND the contacts were burned. I polished the contacts and cleaned out the sand then riveted the horn back together and it worked fine. Installed it under the back bumper where it couldn't be seen and wired it into a switch that could handle the amperage. It was fun to blast that back horn and watch people behind me flip off the car BEHIND THEM. Until I started a fight. So I stopped doing that. But I ramble.

I'll look for contacts in your photos.

Otherwise, I've seen horns that are powered from a magneto. The horn tone would vary with the engine RPM's. Those horns don't have contacts internally.
 

Tonyr1084

Joined Sep 24, 2015
7,772
After re-reading the original post you said the coil pulls the diaphragm in and doesn't release it. That suggests there is something that alternates the current to the coil. There must be a device to generate a pulsed signal to make the diaphragm vibrate. It would be conceivable that this unstated device is not producing pulses but rather just simply turning the EM on and holding. In other words, the horn might not be the issue. This is speculation only. How is the horn connected to power? A full schematic diagram would be highly beneficial.
 

LesJones

Joined Jan 8, 2017
4,172
It may be the same as old motor horns. these use the same principal as a buzzer. The coil pulls the diaphragm towards it. That causes a set of contacts to open breaking the current through the coil and the process repeats. I think I remember that there was an adjusting screw to adjust the contact gap. This adjustment may have changed over time. If the contact adjustment has closed up the contact may never open to break the current. It is many decades since I dismanteled this type of horn sow yours may be a different design.

Les.
 

Thread Starter

DanielHowden

Joined Nov 8, 2019
65
Thank you for all the input. The first picture that I posted shows all the parts to the horn that I have disassembled. They go together in the order that they are laid out from right to left.

I am wondering after reading the comments if there is a purpose to the post I have marked A. Is A working differently to B and would there be an effect if A and B were connected by the disc on the diaphragm?

PXL_20231123_1344566651.jpg
 

Alec_t

Joined Sep 17, 2013
14,208
Is A working differently to B and would there be an effect if A and B were connected by the disc on the diaphragm?
If the diaphragm were metal but insulated from the horn casing it could be used as a NO switch which closes when the diaphragm is pulled against the solenoid poles. The switch could then activate a relay or transistor to turn off the solenoid current. However, you would then expect to have an insulating gasket and insulated fixings, none of which are shown.
 

Ya’akov

Joined Jan 27, 2019
8,973
After re-reading the original post you said the coil pulls the diaphragm in and doesn't release it. That suggests there is something that alternates the current to the coil. There must be a device to generate a pulsed signal to make the diaphragm vibrate. It would be conceivable that this unstated device is not producing pulses but rather just simply turning the EM on and holding. In other words, the horn might not be the issue. This is speculation only. How is the horn connected to power? A full schematic diagram would be highly beneficial.
Given no evidence of a switch that I could see, and the symptoms described, I wondered if there was some kind of AC of pulsating DC driver missing. But, I couldn’t find any horns that were designed like that.
 

Tonyr1084

Joined Sep 24, 2015
7,772
Given no evidence of a switch that I could see, and the symptoms described, I wondered if there was some kind of AC or pulsating DC driver missing. But, I couldn’t find any horns that were designed like that. {the word "or" has been edited in from "of".}
I've seen moped horns that operate on the principle of a magneto on a flywheel. Each time it generates a pulse of energy the diaphragm moves. Higher RPM's makes a higher tone horn. As the motor speed slows down the tone drops and so does its amplitude. An AC signal would just serve to hold the diaphragm in unless the diaphragm itself is magnetized.
That Horn will not "Beep" using just plain-ole 6-Volts DC.
A Schematic of the entire Motorcycle/Scooter is required.
I agree. There must be something else driving this horn, whether it's a module or some pulse picked up from a generated source such as a magneto.

Parts A & B are just parts of the iron core that make up the horn. If it were likened to a motor - parts A & B would represent the two poles of a simple motor stator, the part of the motor that doesn't turn.
I'm thinking that the circular item must vibrate or spin on the plate?
I'm sort of wondering the same thing. But I see no shaft or bearings or bushings. It kind of reminds me of a "Choke" I've seen in some loud speakers. Just a simple focusing aluminum plate milled with tapers and holes to act similar to a megaphone.

I must agree, there's more to the story than we're seeing. Or being told of. "SOMETHING" must be operating that coil.

All that parts A & B do is conduct a magnetic field to the diaphragm. Each time it's magnetized it pulls the diaphragm in. When the magnetic field collapses due to a lack of electrical energy the diaphragm relaxes, sending a pulsed sound wave. Aside from the two plugs on the back side of the housing and the two insulated copper wires that are soldered to posts inside the housing I see nothing else that would make the horn act nothing more than like an electro-magnet.

A full schematic of the horn circuit (at the very least) is needed.
 

MisterBill2

Joined Jan 23, 2018
17,777
What I do not see in any of the photos is the switch that opens the circuit as the magnet pulls in the diafram. So something is missing. I have repaired a few carhorns over the years and in every case there is that interrupter contact, operated by the same electromagnet. So something is missing. My guess is that it is the switch for the groud side of the coil, which we do not see in any of the photos. It will be found below the coil connected to the connection that does not go to the black wire. nly a very small bit of it shows in two photos. THAT is where the problem is. It is in the portion that we never see in any pictures. I hope it was not removed in the cleanup process.
 
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