[Chuck Schwark]

Many times the "open" is a corroded base terrminal where the primary and secondary windings connect to the lugs.
Philco is notorious for this condition.

Also, if the "open" is only a few windings into the coil, un-winding the wire until you come to the break ans re-soldering the "good" end to the lug should get you by without a replacement or total rewinding. The few turns off one winding will not effect alignement enough to notice.

Obviously, if the break is more than a few turns into the coil, then you should replace or re-wind.

Chuck