Since the original post was about ballasts in signs, I thought about some of the signs I've worked on, and some of the responses I've seen in this thread. A lot of signs will have more than one ballast. It's not uncommon to work on a sign that has 3 or 4 six lamp ballasts. I always look at the lamps first (that is, after checking the line voltage), and will replace all lamps if the sign has been opened (with the owner's permission, of course). While the lamps are out, I inspect the sockets (always HO's in outdoor signs), especially in the bottom of the signs if mounted vertically. If suspect, I'll open the raceways to look for burned up or pinched wires. If the ballast is singing a pretty tune, or is extremely hot, or the label or paint is off color, suggesting extreme heat, I'll replace the ballast. Only in these cases and if replacing lamps, and the sockets and wiring look good, but still no lights, will I replace a ballast. They should last at least 5 years, and do sometimes last a lot longer.
I wasn't aware that the voltages at the sockets could be metered. Thanks for the tip, iwire. It's printed and will be on my truck tomorrow.
