Horn

[Mervyn Hyde]

I will give it a try then. So, I after a film cap with up to 50uF and (say) 7volts? Or 50V?

Still no idea why my electromagnetic switch is not working. It should work with or without a capacitor, at least momentarily.

[Jim Alton]

Just had a client come in, an auto electrician . What is occurring is one circuit is firing at a different frequency the net effect is to double the frequency of the horn, back to 12v in parallel .
So 6v in series with horns ON 12V IS NOT THE WAY TO GO!

Yep.

One of these horns isn't a constant resistance--it's a make or break circuit.

So, when the contacts are closed on both horns each passes all or most of the 12 volts to the other horn. Meanwhile, each horn is trying to make and break the circuit at its own frequency which has the same effect on the other horn as its points opening or closing.

That's kind of like doubling the frequency (although it doesn't exactly double--it's more like the sum of the two frequencies). Doubling the voltage probably raises the frequency, too.

If it hadn't become necessary to rewind at least one of your horns it might be possible to put a high amperage resistor in series with each horn (with the horns wired in parallel). I don't know the resistor value.

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Let me confess that--although I have Bachelors and Masters degrees in Electrical Engineering--I couldn't figure it out until you related the automotive electrician's findings. I've worked mostly in software and [aerospace] systems engineering.

I thought I could give a more complete explanation, but it turns out this is hard to explain.

[Mervyn Hyde]

You know more than I do Jim, for sure. My knowledge of auto electrics is sourced in crystal radios, home built shortwaves, and wiring a race car at one stage. I am currently working on the low tone (RH) horn, on the bench. I refurbished and cleaned the LHS earlier and it works fine, but is lonely all by itself. Both are 6V. I have completely dismantled and cleaned it and read all the info that is available and I believe it is set up correctly. I do understand how it works now. Always a good step. The only suspect component is the old foil and wax capacitor. If I momentarily 'jump' across that then I get the movement in the electromagnetic system needed before tuning.

[Martin Benade]

Apparently they can work fine in series if they feel like it, from what I have heard. Left up to my understanding, I would have said that was impossible.

[Fred Winterburn]

Merv, I would go with 63 or 100V rating for the cap, or higher if there is room inside for a physically larger capacitor. However the very nature of what the cap is doing will reduce the peak voltage when the contacts open, so a lower voltage rated cap might survive without problems. I've been using a 6V rated, 10,000µF electrolytic capacitor to pulse a relay coil on my 12V car for many years and it is subjected to a kickback from the solenoid (OD solenoid) of close to 100V when the relay contacts subsequently re-open. Anyway, I did not expect the capacitor to last, but it has held up for years. I've found that capacitor voltage ratings are much more forgiving than most semiconductor voltage ratings which absolutely must not be exceeded. Fred

You know more than I do Jim, for sure. My knowledge of auto electrics is sourced in crystal radios, home built shortwaves, and wiring a race car at one stage. I am currently working on the low tone (RH) horn, on the bench. I refurbished and cleaned the LHS earlier and it works fine, but is lonely all by itself. Both are 6V. I have completely dismantled and cleaned it and read all the info that is available and I believe it is set up correctly. I do understand how it works now. Always a good step. The only suspect component is the old foil and wax capacitor. If I momentarily 'jump' across that then I get the movement in the electromagnetic system needed before tuning.

[Mervyn Hyde]

Fred these seem on the money in terms of specs. Size might be an issue however. https://au.mouser.com/Passive-Component ... itance%7c1

[Fred Winterburn]

Those might be fine if they will fit. One of the filter capacitors I use in the CDI is a 10µF film type, automotive rated, 63V. Epcos, B32522C106K. They are about 1.8cm tall (not counting lead length), by 0.875cm thick, by 1.8cm wide. You could always put a couple in parallel if there isn't enough room for one big cap, but a single 10µF might work well enough. Or, don't be afraid of using a modern electrolytic capacitor. They live a long, long time. Vibration inside a horn rather than heat would be the most likely cause of failure. Fred

Fred these seem on the money in terms of specs. Size might be an issue however. https://au.mouser.com/Passive-Component ... itance%7c1

[Mervyn Hyde]

I will order and test a couple of electrolytic caps then

For example: https://au.mouser.com/ProductDetail/Vis ... A4M1NFA%3d

Or this may even fit better https://au.mouser.com/ProductDetail/Cor ... AJFw%3d%3d

Or these are local http://www.altronics.com.au/p/r4797-lel ... capacitor/

[Rudy Bernhard]

From page L71 of the Porsche Workshop Manual:
The horns are the standard diaphragm, magnet and contact type known as the "Wagnerian Hammer". The electromagnet attracts the diaphragm which in turn opens contacts that interrupt the current to the electromagnet. The resulting vibration gives the frequency of the horn. A condenser connected in parallel with the contacts suppresses the arc which would normally occur and cause rapid damage.

When you connect two 6 volt horns in series and run them on 12 volts, you have twice the resistance in the circuit. V=I*R, so the current I would be 12/2R=I, or 6/R=I, the same to the horns as you get with 6 volts in parallel, as they came from the factory (6/R=I for each horn). They should not be damaged due to high current, as the current does not change. In series the frequency for both horns is determined by the contact on the horns that opens first, and breaks the circuit for both horns. In parallel the horns run independently, and can be tuned. If you wish to tune 6 volt horns, use a 6 volt source.

12 volt period Bosh horns were used in the Mercedes of the time, look identical, except for the label indicating a 12 volt horn, and can be swapped out with your original horns and left in paralled, as original. You will get two tones.

I just picked up a set of 12 volt horns from Julian Hardy, a registry member who restores Porsche and Mercedes Bosh period horns. They were very well done.

[Mervyn Hyde]

Nice Rudy. If I ever get this 6v one working on the bench, the next question regards how it will operate parallel with the other working one. A journey ...

[Rudy Bernhard]

Mervyn,
Good point. my 12 volt discussion was for those (GASP!!!) folks that are running their car with a 12 volt battery (myself included). It would also apply to trying to tune two 6 volt horns using a series circuit and a 12 volt battery. And i have no intention in starting the 12 volt discussion here...

[Mervyn Hyde]

Not too many Porsche 'problems' beat me, with enough time, thought and support. However, this Bosch Low Tone (original) horn seems to have done it. It wouldn't respond to adjustment and after cleaning the points, I decided to disassemble it. The high tone horn had responded well to cleaning and adjustment.

Read every possible article and post on the horns and found that my tin foil capacitor was not working (less than 1uF). These are not obtainable. So I went for a film capacitor of the original rating (up to 130uF). Reassembled the horn as per the two useful guides available and soldered the new capacitor into the circuit. Checked with ohmmeter and capacitance meters for continuity. But, I still cant get the necessary pulsing action through the electromagnet with the points. The capacitor carrier and the points are set correctly I believe (diagram)

Any suggestions welcomed.

[Martin Benade]

Try knocking the horn on the workbench with the power connected. Occasionally they just need you to get their attention and it may start making some kind of sound.

[Mervyn Hyde]

That has been done Martin ... I am writing to some folk who know these horns and they may have a solution. I also found a man here in Australia who has a bunch of these in various states of repair and he is going to give me one of his to play with and use for parts.

[Jay Darlington]

years ago there use to be a gentleman that rebuilt horns on here, is he still available. I can't remember his name.
Jay D.