Hi guys this is my first post asking for advice and as a retired mechanic I feel somewhat awkward so help a old man out if you can.
I have a 77 F150 with a 300 engine.
I noticed one day it was not charging and I verified this with a dvm, the alt was a cheap rebuild and the regulator looked to be original so I put on a new regulator and rebuilt tested alt but still not charging.
All the wiring looks ok so I'm at a loss of where to look for the problem.
Any ideas would be welcome.

 

I had the opposite problem a while back, overcharging. Was getting good readings and bad ones on my voltage regulator, turned out to be rotted cables on my battery and a voltage regulator. Cables on battery were replaced about a month before I bought truck along with alternator, starter and solenoid. 8 month old cables were rotted inside the sheilding. Not saying thats the problem but check that your wiring is okay and not rotted. It cost the po a lot more than it cost me.lol

 

I had the same issue with an old tbird. like yours the wiring looked ok but it wasn't. I peeled back the harness cover and about 5" back there was a split wire. no clue how it could have happened. fixed the wire and the charging problems went away. I'd start tracing the wires.

 

Does "rebuilt tested alt" mean that you rebuilt the alternator and tested it to determine that it was working properly? If so then there could be a wiring problem as others have suggested. You may have to trace the system with a meter to determine where the voltage disappears. Watch for hidden corrosion, especially if you live close to the sea. I've seen corrosion inside insulation and connectors.