Check that the short brushes are touching the commutator.
Try connecting the F to the generator frame while spinning it to excite the field.
Check to see the field isn't open. It connects to the A terminal inside.
Take it to an automotive electric shop for repair, instead of tossing it. Mine has worked fine for a long time and charges at 20 amps when its needed and I get long battery life.
Loosing the tachometer is a pain.