This is a commercial 4' T12 fixture. One bulb lights fully, the other not at all, even with new bulb. The fully lighting bulb has me puzzled. With one bulb it usually lights partially, not fully. Obvious is the ballast, but could the wiring be off with the mounts? I'd rather disassemble once only. Thanks.
Failed capacitor inside the ballast is likely.
Both the T12 ballasts and lamps are now gone or being phased out. It's a good time to convert to a T8 system. You get about the same light output. Use T8 lamps and an electronic instant-start ballast. They'll fit into the same fixture. The same lamp sockets will work, but replace them anyway to avoid future problems. Some ballasts come with the wires cut to length and the sockets already attached - makes wiring and installation simple.
Tomsic
It could be a problem within a single combined ballast, or the thing could have two separate individual ballasts, one per lamp. You have to take it apart to at least find out what is going on. If you need light in the meantime, hook up a trouble light while you get parts. I agree the new electronic ballasts are readily available, and do not hum like the old transformer-based one are prone to do.
It's the ballast. As suggested, now is the time to upgrade to a T8.
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