I had a look at my 620 an my trusty manual: it shows an external voltage regulator is mounted remote of the alternator, so it would seem likely that this, along with other components, is fried. It should be a black box bolted on the inner guard next to the battery.Also check all of the fuses and fusible links, as an incorrectly jumped battery is the fastest way to blow every major fuse on the truck. Be thankful that it actually runs, too! ;)