How can I fix an excessive ripple in my charging system?

i have a 1990 mercury sable v-6 3.0. my charging system light came on today so I went to advance auto to have it tested. The battery and starter are fine. unfortunately when the alternator was tested there was an excessive drop when it was loaded from unloaded. It has an external regulator. My question is:is it just the regulator, just the alternator, or both?