Need help with Prestolite horn repair/rebuild

[Rickster]

Bought a HI and LO Prestolite horn at swap meet at Mopar Nationals last week. Of course, couldn't test them right away but got them cheap. After cleaning exterior, tried plugging in horn to friends 70 Road Runner to see if either would work. They didn't.

Found a post on another forum that explains method to disassemble horn and clean contact points inside. Still not working though. Getting power to terminal and coil, ground is good also (testing with alligator clips to battery so no relay issue).

Anyone have a similar problem? Any other advice?

Thanks

[dutch]

have you tried turning the small bolt while having power on the horn?  worked for me.  Mine both had some hesitation after I rebuilt them.  Never had the problem again after that

[Rickster]

Yes I did turn the tone adjusting bolt but it didn't make a difference except take some pressure off the contact switch. Still not working. Do you know how the contact works? With the points touching in a closed position, it seems to be completing a circuit. At what point in the process do the contacts separate?

Any thoughts?

[dutch]

I cant remember. Someone? 

[FY1Cuda]

Sometimes the horns respond to a few sharp raps.

[burdar]

I can only offer two suggestions. 

1. Drill out the rivets and take the two halves apart to inspect.(probably just rusty and in need of a good cleaning)

2. Soak them in EvapoRust to remove any rust that may be inside.

[Rickster]

Thanks, burdar and FY1Cuda, for the suggestions. I do have the horn apart, it has been cleaned of rust, and the points have been cleaned with a piece of sandpaper.

Maybe these are never going to work, but I'm still curious, if anyone knows, how these things work mechanically. Specifically, when do the points separate and what makes the metal diaphram vibrate to create the sound in the trumpet?

It's one of those simple pieces of equipment that shouldn't be hard to figure out, but everyone takes for granted when they hit the horn button.

Any ideas appreciated.

[burdar]

Did you get this figured out? 

I just rebuild one of my Sparton horns over the weekend.  It looks basically the same as the inside of a Prestolite.  All it took was sanding the contacts to get it working again.  Since I've had mine apart now, I'm pretty sure I know how they work.

I assume when the horn gets power, the flow of electricity makes the contacts bounce open and closed.  Since the center diaphragm touches the springy half of the contact, the diaphragm moves in and out as well.  The cone then amplifies the sound of the diaphragm.

Two things could be happening.  1. The diaphragm could be damaged so it doesn't spring back and forth any more. 2. The contacts aren't bouncing open and closed when power is supplied to them. 

I'd try hooking power up to the contacts while the horn is apart.  You should be able to see them open and close rapidly.  Then make sure the center diaphragm is still springy.  Sand blasting it will ruin it.

[Rickster]

Burdar, thanks for checking back and giving some insight to the mechanics of the horn.

I was able to get one working. Adjusting the tone screw properly did the trick.

The other one is still dead. Either have a grounded or cut wire internally. Not getting proper voltage. Planning on drilling out rivets and seeing if salvageable.

Ditto on the the advice to not sandblast diaphragm. Luckily read that on another post before I took these apart.

Thanks again

[shawge]

Is that a paper or thin cardboard gasket?  What did you use to replace it?

[Rickster]

I followed advice on another forum and cut out my own gasket using the back page of an old magazine (of course, an old mopar magazine works best  )

[dutch]

I don`t think it matters what kind of paper is used.  I used a heavier weight writing paper and it works like a charm.

[burdar]

There were two thick gaskets on my Sparton horns...one on either side of the center diaphragm. I used an old non corrugated cardboard Christmas box. I just finished the second horn today. That one needed the knob adjusted a little.