I am repairing a pair of High Frequency horns on my XK150. I originally planned to send them out for overhaul but the sources I had listed, including Joe Curto, no longer repair them. I tested both horns and they do not work so I sourced a pair of capacitors to replace the original aluminum capacitors. I am using useful internet site from The Netherlands as documentation, which I recommend (www.bobine.nl/jaguar/13-electrical/lucas-high-frequency-horn-hf1748-on-jaguars-1953-to-1961/).
As a part of the repair set-up I need to check the points. This is causing a problem as when the adjusting screw is fully in or out I do not seem to get a points gap. I can only guess that the points are worn down. They look OK but not having a good condition example on hand I cannot tell.
Any help/advice gratefully received. I would prefer to retain the original Lucas horns as opposed to replacing them with Chinese copies.
Thanks for visiting my website. I’m afraid that you will have to take the horn apart (which is sometimes very difficult or even impossible and you will have to drill out the screws) in order to check why the contacts aren’t reacting on the adjusting screw. But also keep in mind that the adjusting screw is only for “fine tuning” once everything else is OK. Once you’ve removed the front, is the horn very corroded inside and check the internal wiring? See photo of a very corroded horn which is “beyond repair”. Also the capacitor may have failed.
If you need specific parts, send me a PM as I do have quite some parts left from older restorations.
Or…you can buy E-Type horns which are readily available. And…they are located where the judges will never see them. And…if they did, I doubt if they would catch the non-authenticity deduction.
Pat H
Bob, I found your website very valuable and did not realize that you were on the forum …. I should have guessed. My units are in generally very good condition. Questions I have are:
Is the capacitor polar sensitive. The original aluminum unit is not marked + and - so I assume it is not.
I assume that the heel of the lower points-contact is opened by the magnet pressing on it. Is that true?
As a mechanical engineer, I’m not an expert in electrics but as far as I understand the purpose of the capacitor is to suppress any arcing over the points. I’ve never seen any remark that these Horns are polarity sensitive.
Regarding your second question: the contacts are normally closed. When you push your horn button the magnet pulls the armature down and the contacts are opened (as the armature just touches the bottom part of the contact points). So the magnet is switched off and the contacts close again, after which the whole cycle starts again.The frequency can be adjusted (within boundaries) by adjusting the screw on the backside of the horn: you are in fact adjusting the maximum opening of the contacts.
Have you measured the resistance over the 12 V connections? It’s an indication whether the points make contact.
Bob, I have finally got a peep out of one of my XK150 horns after changing the capacitor. The second repair is on hold, I misplaced the armature return spring which is lurking somewhere in the garage. However, when cleaning the parts number plate I realized that the second horn was also a HIGH horn so I now have two highs. Can I retune it by adjustment or do I need a different diaphragm?
Regards
Mike
The diaphragm is definitely different for the HIgh and Low Note versions. Has something to do with the natural resonance frequency of these steel plates that can be influenced by choosing different dimensions for the circular wave pattern stamped in the steel. See pic below
The Low Note diaphragm has Lucas part number 703726 and the High Note version 703727. As far as I remember, one had a single mark on the outer perimeter and the other had two marks (like shown on the photo above). Cannot remember which had what marking but as you have a High Note version you will be able to look for the other one. If you cannot find one, I still must have a few, so let me know via PM.
You also know that w.r.t. to the adjustment of the horn, the air gap between the armature and coil core.differs for the low and high note: the low note requires 0.025” (or 0.6 mm) and the high note a bit less: 0.020” (or 0.5 mm).
