2000 Ford F-150 charging system

Hello my name is Jim, my problem is alternator. My battery light came on on dashboard of my 2000 f-150, I went to autopart store and they tested my alternator to find out it was bad (diodes)i bought a new alternator and put it in. Battery light went off for three days of driving and then came back on.I went back to the store and they checked it again to find out it had a charging problem they said check the wires to make sure all are tight if so take alt. Out bring it back and they will give me a new one. Is it posible my charging system did somthing to the previous alternator or did I get a defective one to begin with? Should I be cousios with puting in another alternator? Also have new battery.