My ST-3 is out, won't operate on the harmonic winding system either, and flashing the FC dinn't change anything. I did use the generator yesterday and it worked fine. So far I'm baffled. The static (off) testing of brushes and slip rings looks good, field coil resistance is OK, harminic winding resisitance is OK, stator resistance winding is OK, my other various switches, fuses and relays OK.
Came in for late lunch, meds, and a study of the circuit diagram. The bridge diode is a modern 800V unit so highly unlikely to have failed. I'll have to start using my handheld o'scope with a 15 foot extended probe lead and see what's gone wrong with the spinning harmonic signal while I stand outside. It's cold, cloudy and windy.