How to repair a Nanni 12V alternator

This time I have a alternator from a Nanni marine diesel, it is the same Valeo model as on a lot of Volvo Penta engines. The charge light is on and the alternator gives no voltage to the batteries so I'll like to find out why it doesn't work. Regarding the voltage regulator it starts charging at 850 rpm. and reach it's max output at 2000 rpm. At 850 rpm idle the voltage must be around 13.5-14 Volts and the charging Amps must be around 2-3Amp on a near full battery. If the voltage is over 15 Volts at 2000 rpm. the rectifier is overcharging and is then faulty. 50A rectifier from ebay : 42 US Dollars