Hi, I had a similar experience......an additional check is to put some current directly through the field then spin the Dynamo with a drill and see if the armature can power (say) a headlight bulb.
The thing about the f and d check is that it relies on the residual magnetism to get the armature to put some "starting" current through the field to get things going. If your brushes have too high a resistance then sufficient starting current does not happen, putting some power directly through the field is a better test of the 2 coils.
It's ok to put a 6v battery straight across the field, for short periods at least, with a resistance of a 2.9 ohms only about 2 amps will go through the field.
If you can get a bulb lighting up then you can look at the brushes and reg side of things....
Edit: my Dynamo had not run for over 30 years, when I eventually got it going it would run for a couple of hundred miles then stop charging
, cleaning the brushes (which were not THT dirty) got it going again but only for another couple of hundred miles
, replacing the brushes with new ones sorted it permanently, maybe the brushes degrade over time