The regulator/rectifier is a very standard part found on many motorcycles. It takes a 3-phase AC input and converts it to a charging voltage of nominally 13.8 VDC.
If you are seeing diagnostic trouble codes in K-Scan like P0563 (System voltage too high) testing the rectifier/regulator would be prudent.
I have been experimenting with an aftermarket OTK M261 MOSFET rectifier/regulator after an original equipment (OE) unit failed in one of my bikes. MOSFET rectifier/regulators exhibit some theoretical benefits including more efficient, cooler operation and a more constant output voltage. Although it seems to be working fine, the output voltage is a bit higher than I would like to see (so can not yet recommend it). It seems to make a constant 15.0 volts DC regardless of rpm. OTK's specification sheet says 14.5 +/- 0.3 VDC. It exhibits a very small AC content (as measured with a true-RMS meter) of about 0.4V. Conversely, the OE thyristor-based rectifier/regulators are more variable with rpm - approximately 12.6 to 13.1 VDC, with a higher AC reading of around 1.5 volts.
Twelve separate measurements must be made using a multimeter (preferably one with a Diode Test function).
Red (+) multimeter lead to positive rectifier/regulator terminal. No diode present on any of the 3 AC inputs.
Black (-) multimeter lead to positive rectifier/regulator terminal. Diode present on each of the 3 AC inputs.
Red (+) multimeter lead to negative rectifier/regulator. Diode present on each of the 3 AC inputs.
Black (-) multimeter lead to negative rectifier/regulator. No diode present on any of the 3 AC inputs.
If your meter does not have a diode test function, look for continuity (diode) and open circuit (no diode).
This test does not guarantee a good rectifier/regulator, but failing the test requires replacement.
Pin functions for OE regulator
The filter capacitor is mounted behind the number plate. It is closely related to the rectifier/regulator because there is no battery in the system.
The output of the rectifier/regulator is pulsating DC. In most vehicles' electrical systems, a lead-acid battery smooths these pulsations into a low-ripple DC output. Because the TR280i has no battery, the capacitor is necessary. I measured its capacitance at approximately 23,000 uF. A proper replacement capacitor would be rated to withstand sufficient ripple current (which makes these capacitors fairly large physically). The capacitor weighs 162 grams.
On bikes with the Easy-Start system, the filter capacitor is smaller (10,000 uF) and mounted inside the Easy-Start box. Furthermore, this capacitor is only connected to the rectifier/regulator when the fan is energized via the chassis relay.
12V bus, good rectifier/regulator @ idle, fan off
Small SLA battery next to filter capacitor