poking around online will yield the .pdf service manual. I believe 'cosky' is in the file name. There's also a '79 650 service manual floating around on the intrawebs that won't be far different from your nighthawk except for the CV carbs.
The 650s have the same charging system that 82+ CB750c, CB900s and some others did, the same basic electromagnet setup that sucks in the same ways. I'm sure it was just fine when new, but now that it's approaching 30, it's losing some functionality. don't we all?
TwoTired is really the man for methodical, logical testing to pinpoint what's going on, but until he's on the scene here, I'd like to point you toward the electrosport troubleshooting diagram. That, in conjunction with what Mr. Cake said above should point you in the right direction. I'm going to give some model specific direction, please don't take it as condescending when I tell you where some stuff is, because when I was new at this (ok, and still am...) people would say "Take off the alternator cover..." and I would go "Where's that?" So... bear with me.
Start the bike and get a voltage reading at the battery at idle while it's warming up. Write the voltage down and let us know in your next post if you can. Once you've got it sufficiently warm, bring the rpms to 3k. Report that voltage. Now to 5k. Report that voltage, too.
Now kill it and let it cool off for a bit 'til you can touch stuff again.
Take off your alternator cover (on the left side by your shifter, should have 3 or 4 metric size 8 bolts holding it on. You'll be face to face with your rotor and brushes. The brushes are in the cap there, with the stator. Test the ohms of resistance between your brush and the black and white wires that plug into your regulator rectifier @ under the left side sidecover. Should be pretty low. Might wanna write that down and include that in the data you provide us in your next post.
Test ohms of resistance between the rotor's sliprings, then the screws or whatever raised bumps are on either side of the rotor that the windings are soldered to. Both should be close to 5-8 ohms of resistance. Anything less can indicate an internal short- though some folks here have had rotors that tested good and still didn't produce power right. Someone may correct me on those numbers but I know it shouldn't be lower than 5.
If the sliprings look really worn/dirty/gross, clean them up with a scotchbrite pad and some contact cleaner. The VAC your alternator produces depends on the kind of connection you get between the brushes and the sliprings. For that matter, take a good look at those brushes and make sure they're not worn down past the wear marks. Check the spring pressure and see if it's got enough oomph to make a good connection with those sliprings. if there's a lot of crap and graphite dust in there from slipring wear, see if you can clean that out, too.
Check ohms resistance between the yellow leads. Should be about .8 for all of them.
let us know what you get.
You're from maine, yeah? Near the ocean much? Make sure your grounds and connections are clean and tight, I would figure on there being a hefty amount of corrosion, seeing your pictures. See if you can find
this stuff, Green Ranger turned me on to it and the stuff works really well.
If the problem's not at your rotor or r/r, we might have to do some gremlin chasing.
