Check it is the alternator first ....
Using a multimeter/voltmeter it's very simple.
You can get new regulator boards for some makes, often it is the board that is faulty,
New alternators aren't as well made as the old ones, even branded ones.
So true.
When I was looking for an alternator for my V8 D2, I only found reconditioned alts. I settled with a £90 150A Bosch alternator from eBay for a Range Rover P38.
You can get a new rectifier (diode pack), regulator and brushbox easily; worn-out copper slip rings on the shaft are harder to replace (I have seen one tutorial explaining the process) - the slip rings wear out if you get grit and oil between the carbon brushes and the slip rings - acts like cutting fluid! A multimeter can easily be used to check the integrity of the rotor and stator windings, the regulator and the rectifier (also, if battery is draining overnight it can be due to a faulty rectifier); electrical cleaning fluid can be used to clean oily, dirty windings. I had a lot of oil leaking down there at the vacuum pump and coating the alternator with oil - all gone now, nice and clean - a new copper crush washer on the vacuum pump feed stopped the oil spraying out.