My real recommendation is to take the alternator to a local rebuilder. Let them tackle the repair. Tell them what it's doing, tell them how much money you have to spend.

Most alternators have 6 diodes. Individually, they are not that expensive. Most rebuilders like to replace all the positives (3) or all the negatives at a time (so they match), plus replace the bearings, turn the stator, replace the brushes, and springs.

So, it's about $25 of parts, and 2 hours labor. So, bearings, brushes, and a complete checkout out should be about $100. (unless I'm in the dark ages)

If you only want the bad diode replaced, then ask them about just that repair. (that was the $30 I was talking about, and that is taking the uninstalled alternator to a shop and working out exactly what you want for a repair)