I would think a bad alternator would be low on the list, although it can in rare cases.
Pull the wires off the field connections one at a time, checking each time that the charging drops out. Then switch the two field wires and repeat. (This will eliminate the possibility of a grounded rotor "at one end."
Next, take your meter, with the engine running, stick one probe right on the battery neg. post, the other probe right into the regulator case. You are looking for a very low reading, the lower the better, zero would be perfect. Anything over .2 (two tenths) of a volt is too much, and shows the regulator is not grounded to the battery neg.
Now do the same thing on the 'hot' side. Stab one probe into the battery positive post, the other as close to the IGN terminal as you can get at the regulator. The dark blue at the ballast resistor is good for now. Same deal as above, you are looking for the "lower the better"
If these two checks are OK, and you don't think the battery is bad replace the regulator.
