I had exactly that same generator, that I got in about 1982 and used intermittently. I installed it on a bike about five years ago and id did exactly what you described; worked for a half mile or so, then started screaming bloody murder and not generating any power. I was not able to diagnose and fix the problem, and took the dynamo off.

Just yesterday, however, I had an almost similar problem with a newish Shimano dynamo hub. It was fine when I left the house, but a half mile later it started making a crazy vibration, and a fairly loud noise that went sort of like bbbbbbbbbbbbbbbbbbbbbbb. I ignored it until I got to the train station. I took the wheel off, found nothing wrong, and put it back together, and the problem had gone away. This leads me to wonder if the problem might have been electrical in nature; perhaps some kind of a short circuit was putting undue strain on the dynamo.

Have you posted your question in the electronics forum? The real dynamo geeks hang out there.