The indicator lamp circuit is only used for initial excitation, so once the alternator is up and charging, it shouldn't need to rely on a functioning indicator lamp circuit.
Is it in fact not charging when the light is on? I doubt it could be a worn pulley unless you're running very high electrical loads.
As for printed circuit boards, I can't say as I've ever seen a faulty one on a Ford tractor. I've seen my share of malfunctioning electronic instrument clusters, but nothing related to the indicator lamp circuits. I'd suspect the bulb holder before the PC.