Believe that it is not a dynamo, but an alternator. As such it does not have a magnetic core but uses primary and secondary windings.
Think in this case it take a LT feed of 24v into the armature to create the spinning magnetic field and as that passes through the secondary windings that creates the high voltage output.
First thing to check is whether the is a LT feed to the contacts on the armature, then to check to see if the bushes are making a good contact.
Believe that the contacts on the armature can wear down leave a build up of metal swarf in the gaps between the contacts.