[Arran]

You may find, upon investigation, that the field coil has a break near one end, and is not burned out, hopefully it's that and not corrosion in the copper wire. These F.C speakers in the model 20 are easily rebuildable as the pole piece that keeps the field coil bobbin in place is held in with a large nut on the back of the rear bracket. Of course you may have to remove the speaker cone to get it out, but I'm not certain, the only attachment to the pole piece is a machine screw in the center of the spider. Another issue may be the audio interstage transformer, those sometimes fail, usually in the primary, but they can be bypassed just to test the set.
Regards
Arran