My 1991 with external regulator is not charging at all. I checked with volt meter an there is no power at the main post on the back of the alternator. Not sure if the fuse link is not working. I'm not sure how to find the problem. thanks

 

You should have battery voltage on that terminal with the engine off, unless you have a battery isolator.
Besides the connection on the alternator, there are 2 other connection between the alternator and battery. One near the right frame rail and one right at the battery. Check those out.

 

The alternator is connected to the battery through a dark green fusible link and an inline connector in the 6 gauge wire. Likely a diode in the alternator shorted out and blew the fusible link.