I had a similair problem on my 82 7720. It didnt seem to charge and sometimes the batteries would go dead over night. Just so happens I ran out of ideas and it was starting to get on my nerves so i hit the volt meter. all of a sudden the voltage came up and stayed up and didnt go dead. When it would go down again a day or 2 later sometimes longer i would just push or hit the gauge again... the gauge is bolted to a circuit board and i checked to make sure the nuts were tight on it.... now i cant remember if thats what fixed it or if the board was cracked and i soldered in a jumper wire across the break but it doesnt give me trouble anymore and that was 3 years ago. The other thing i noticed is you have to blow the alternator out a couple times a day when doing beans and at least once a day under normal conditions (this is on a delco alternator) otherwise it burns out. the other thing is if you have to rebuild it get the guy whos doing it to cut off every second cooling fin on the retifier bridge this way it allows more of the dust and chaff to pass threw rather than plugging it up. These are just things if discovered after many alternators between a 7700 and 7720. hope any of this helps, Ed