Noticed a few days ago that my ammeter was staying centered while running and would show a slight discharge when the headlights were on. The stator output at wot is good, around 49 VAC, and the regulator output voltage is also good.
I checked the charging circuit today, stepping from terminal to terminal using an ohm meter. Good continuity everywhere but in the key switch between the B+ red wire terminal and the green regulator/rectifier wire terminal. When I pulled the switch plug off, the B+ terminal came with it. It was so badly corroded that it easily broke free of the switch stud.
I'm surprised the tractor would even start. I figure the reason it started and ran OK was that the current needed to operate the starter solenoid and the ignition system is much lower than the charging current needed when under additional load such as the lights. The corroded terminal couldn't handle that much current causing the slight discharge when lights were on. A new Stens switch is on order from Amazon, it is the replacement for the original Indak switch. Hopefully this will solve the problem.