Rex. Try a test with an earth connected to a nice clean part of the dynamo body, F & D bridged, 12 volt headlamp bulb between this bridge and the direct earth. Then check the internal wiring, making sure all connections are sound and correct, insulation where it should be and that the D terminal is connected directly to one brush. The other brush is earthed, usually on the same point as one field coil lead. The other field lead goes to the F terminal. Make sure the brushes are free to move. The dynamo may be incorrectly polarised, but this does not matter for this test.
Still no good, first check that the brushes actually conduct. Then disconnect both field leads, check the field coil is open circuit between each lead and the body, but the coil itself has continuity. In other words the field current is flowing through all the field winding, and the current is not taking a short cut. Also worth checking the field coil pole piece is securely tightened against the body, as anything which reduces the inherent magnetic field will result in low output.
The armature should have more or less the same resistance across opposite commutator segments. Test the resistance between adjacent segments, here again, although the two values may be different, they should be consistent for both sets of readings. Finally check each segment to the shaft. It should be open circuit, showing the shaft to be electrically isolated from the armature windings.
Swarfy.