Hi, Graham,
In a word 'Yes'.
If your dynamo has been out of use for some considerable time, it may have lost it's residual magnetism - so wont generate any charge. To restore the magnetism, remove the 2 wires from the dynamo. Then with another length of wire, connect the 'F' terminal on the dynamo to the live side of the battery for a few seconds - be careful, the wire may get hot, as there will be quite a discharge. Having done this, reconnect the 2 dynamo wires, and start the engine. Providing the dynamo is OK, it should now show a healthy charge.
Regards,
Eddie.