Had been experiencing battery being drained and 'Low Battery' warning when starting the car, and replacing the battery did not fix the issue. Always less than 12v when the car was started.
I can see many people just get the alternator replaced, but being inclined to try and fix the problem myself, I decided to give it a go.
After hearing that an alternator replacement involves removing the radiator etc, and then also replacing the accessory belt, I determined that I would instead access the alternator brushes and regulator without removing the alternator from the car.
The alternator I have is a Valeo SG12B098, and there are many YouTube videos on taking apart Valeo's and checking the brushes.
By tying the vacuum hose to one side, and working around the air con hoses, I managed to remove the black plastic cover on the alternator (2 x 7mm nuts), and then remove the brush pack (3 x 7mm nuts).
Turns out my problem was the brushes were caked with carbon, and no longer touching the slip ring.
By tapping and carefully moving the brushes from side to side, I eventually got them unstuck from the carbon, so they could move in and out freely. Reinstalled the brush pack onto the alternator, reinstalled plastic cover, started car - bingo, 14.1 volts, and less than an hours work. Obviously, if you have extremely worn slip rings etc, this won't work for you.
Cheers,