I'll just toss this out. You may have tried it already.
I ordered a set of used stock horns for my solo bike off of E-bay, and when they arrived, only one worked.
I did what all really clever people do, who have boxes of stuff that don't work
....I took the back off, looked at it, and put the back back on it. I ran the screw in and out a few turns that adjust the tone???, and for once, this method of creating more junk, actually worked. Imagine my surprise.