So we're out having a nice drive in the PV when after about an hour the dreaded "AMP" light comes on nice and bright.

Head home, let things cool down a bit, then re-start to see if a thermal problem. Nope, AMP light still on.

Take off fan belt, generator spins freeley, nothing seized up. Took generator off car, cleaned up, no sign of any arcing or other problems. Brushes look nearly new. Can't get inside regulator because the aluminum screw's phillips head is totally stripped. Tried applying power from a low-amp source to the D+ terminal, tripped breaker on power supply (5A) and did not spin the armature at all. Measured resistance, looks like dead short between DF and frame, very low ohms (4-5) between D+ and frame.

Manually spinning the pulley did show a slight voltage at D+.

Any ideas? What should I be seeing for resistance inside the generator? This is a '64 with the later-style 12v generator (open in the back, with no lubricating cup).