750 charging problem

[oldbike80]

I'm looking at buying a low-mileage 1979 750F, but it won't charge. When it is checked with a voltmeter, the voltage from the charging system is the same as the battery.  For a 12 volt system, I'm told the output from the charging system should be 13.5 to 14 volts. What could be the problem?

[Re-run]

Could be a number of things. But when you cheked it, was the bike at idle or revved up a bit? I do not know the dohc but the sohc needs to be revved over 2K before it charges much. Otherwise, it could be dirty connectors, a bad regulator or mis-adjusted one if it is mechanical. The stator could be having issues too.
See if you can download a manual from someone or at least the charging troubleshooting steps and start there.

[DesignBuild]

The DOHC is pretty much the same, it has to be revved up a bit before you get a higher charge (at least mine is like that). I would suspect the regulator, i had a similar problem to this and ended up replacing both the R/R and the battery. But re-run is right, it could be any number of things.  

[Polymer-2]

If it turns out to be the RR then be sure to check the rotor with an ohm meter. Should be 4-5 ohms. A shorted rotor often kills the RR.