Generator Operation

[Joe/Ct]

I have a generator in our Model A roadster. It is set according to descriptions that I have seen here (8 amps with lights off; 0 amps with lights on). My problem is this: If I operate the horn with the lights on, the ammeter drops to negative 8, and the horn barely makes a sound. With the lights off, the horn blow loud and clear. I can have lights or horn, but not both. Any of you have this condition? What's the fix?