At work I have a large sign at the front of the building which contains a large ballast and 6, 6' fluorescent bulbs. Everything was working fine last week, but this week only two of the 6 bulbs light up, even after replacing every bulb. Does anyone have any clue what could be happening?

The ballast is most likely defective, however, any one or more of the lamp sockets may be defective also.

Signs are subjected to all the weather, so water can get in and short out the sockets and wiring, which can cause the ballast to fail.

Be sure to check each socket and the entire wiring harness to all the sockets for any evidence of arcing or burning.

Very possible that any one or more of the lamps are also defective if the ballast or sockets failed.

I should also mention that every socket (including those where the bulbs are not lit) is receiving power, which makes the situation even more strange. I guess this leaves me wondering: is there any reason that voltage would be getting through to the sockets, but resistance (due to water damage in ballast?) prevent sufficient current from being drawn by those affected bulbs?

The power you notice at the sockets is from the output of the ballast, and is irrelevant. As I said, check for any burnt sockets or wiring, and if all OK then replace the ballast.

Thanks, tkrussell.

-Brad