[rotorwrench]

The Sparton and other FoMoCo horns all work with the vibrator principle. The coil is inergised and vibrates the diaphragm by opening & closing those contacts really fast. In slo-mo the coil pulls the core to a point that the points open which momentarily relax the core and close the contacts whereby the process repeats itself many times a second. The contacts can be cleaned and some minor adjustments can be made for wear by tweeking them a bit. The diaphragm may also have an adjustment for its movement allowance.

Since the construction of these units vary from manufacturer to manufacturer and year to year, I don't know if there was ever enough information published to cover them all. There are a few threads on this board about horns & horn repairs. You might try a search on the subject and see what comes up..