I can't get my Alternator to work.

I have a 1985 Buick Century. When the battery sits in the car for three weeks the car will start right up and run and drive. However the car will only run as long as their is juice in the battery. I bought three alternators, the second one and the third one I had tested at Napa and they said they both worked, however when I put the first and second alternator in I disconnected the negative cable while the car was running to see if the alternator was working. The car quite on both alternators. The third alternator I bought I didn't take a cable off while it was running, instead I drove it to Napa and they tested in the parking lot to see if the system was charging. They said and I saw the alternator was putting out 12.93 volts. Even when revved up it still only put out 12.93 volts. Someone told me that there was a dead short or inline fuse out causing this. Does anyone know how I would fix this problem so I can drive it around the country again?