If you have 1 lamp lit only on a five lamp ballast, you either have a bad ballast,(but you've replaced it and odds are slim that a new ballast would the same exact malfunction), or you have secondary wiring that is going to ground.When I prepare to replace an outdoor sign ballast, I cut the secondary leads 6" or so from the ballast, remove all the lamps,and use my Extech digital insulation tester (around two hundred bucks from Grainger a few years ago,although they no longer carry it, but I saw Mitchell Instrument still does),and check all the secondary leads to ground.
I would bet that there is a wire either one of the yellows or one of the browns that is burnt or pinched and most likely in the bottom part of the sign.If that yeilds no answer, try using a continuity tester on each pair of leads, (the end of the H.O. lamp stuck into the sockets one at a time will provide a "jump" between the two recessed double contacts in the socket),mark by each socket which pair of wires goes where,and compare that to the label on the ballast to ensure it is wired correctly.
I hope my methods are understandable, please let us know what the results are
Chuck
