I'm not a horn genius but few I've repaired the hardest part is getting it open.
Once open only thing inside is a coil of wire and the diaphragm. First thing is check coil with an ohmmeter to be sure it's ok. Continuity check either end of coil, should be less than 100 ohms. It's just an electromagnet. Diaphragm in center has a contact point. Points are closed, with power flowing through coil points open, then shut again. This makes the diaphragm vibration buzz. One coil end is the wire exiting horn, other end of coil is center contact point. This touches diaphragm point which goes to metal case.
The adjustment screw changes tension on point contact.
Ones I fixed the points had to be lightly sanded (corrosion).
To open it I used different things, side cutters, a small chisel, etc. Before putting it together I tried it on 3 volts, from lead wire to metal case, holding diagram in place.
Back together a very small hammer, turning it around in a vice peening it back together.
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