OK, sounds like you have the "what would be expected" 69/ earlier alternator/ regulator. Your regulator should look something like this:
It is NOT recommended that you use an original type 69/ earlier MECHANICAL regulator with electronic ignition. However most replacement 69/ earlier regulator are actually all--electronic. You can tell by looking at the bottom. If there are two large wirewound resistors on there, it is the old mechanical type. If there is not, you have an electronic replacement.
First order of business, if you have not, is to arm yourself with a few necessary tools. Go down to Radio Shack and buy a bag or two of test clip leads. If you don't have one, spend 35 bucks or so and get a multimeter. The auto parts stores often have models which are somewhat optimized for automotive use. Also get a 12V test lamp
Now make a few basic tests. Charge up the battery, start and run the car at a fast idle to simulate a low or medium cruising RPM, IE down the road 30-35 mph
Measure battery voltage right at the battery. A good system, warmed up, should show between 13.8---14.2V. Limits are not less than 13.5, nor more than 14.5
I'm guessing "not." If you DO show this, turn on the headlights, heater blower, etc, and remeasure. I'm guessing it CERTAINLY will drop at this point.
Now move your meter to the alternator output stud, the great big stud with a big black wire. This is NOT ground, it is the battery charge lead.
See if there is a substantial difference in that reading compared to battery. If the reading differs from the battery by no more than a volt or two, consider this "OK" for now.
Now unhook the green field wire at the alternator and hook a clip lead from the exposed alternator field terminal to a battery source, such as the battery stud (big stud) on the start relay.
Recheck battery and alternator stud voltage, and if the voltage has not come up above 13--13.5, you probably have a bad alternator.
If the voltage HAS come up some (this will now be unregulated) you may have a bad regulator or something bad in the field wiring.
So if you DO get charging in the above test, hook the field back up. Unhook the two regulator wires, and hook a clip lead between them, IE hooking the IGN terminal wire off the regulator to the green field wire.
Repeat running the engine, check if battery voltage goes up. If not, Check the field wiring. You should have near battery voltage at the clip lead connection with key on/ engine off. Recheck down at the alternator field. You should have battery voltage there as well
If the above test results in charge, remove the regulator, clean the ground area around the mount, tighten the regulator back down and retry. If this does not cause charging, replace the regulator.
To boil this down into simple terms
1---Hook battery voltage directly to alternator field to check if alternator can charge.
2---Check that the alternator stud is actually electrically connected to the battery
3--If neither of above results in alternator output, repair or replace the alternator
4---If the above tests result in charge, check regulator field wiring, reground the regulator, and last, replace the regulator if no change
What do you have in the way of wiring diagrams or shop manual?
If you DO buy another alternator, get a middle 70's "squareback" and then you can either use it with the old style regulator (69/ earlier) by grounding one field, or add one more wire and convert to the 70/ later regulator. These squareback design alternators produce more output at lower RPM
Squareback alternator:
