[Arran]

Mike;
 I would unwrap the field coil, and see if maybe there a break near one of the ends, like where the coil wire attaches to the leads, if it does not look like the winding burned out it's either corrosion, or a mechanical break in the wire. Bob Anderson had a go at repairing a pie pan speaker for a Philco 70 or 90 (same speaker used in each I think) and he only had to rewind part of the coil to make it work again. 
Regards
Arran