How can u tell when a T12 flourscent light is about to go?? or the ballast?

There will be heavy end-blackening, and sometimes the blackened end will begin to turn red as the cathode becomes incandescent (and this usually happens just before the lamp is about to go).  The lamp will also sputter a bit.  When a ballast is going, it will usually either A. begin cycling by cutting out until it cools off due to it's inherent thermal protection if it's a newer ballast, or B. if it's an older ballast, there will be a very distinct burning smell that will be created as the ballast overheats and melts the insulating material enclosed in the ballast case.

thanks man.  one other ques- in my basement there is a old florescent light (from about 1987)when you turn it on the ends of the bulbs are red and the light doesn't come on. to get it to work you have to tap it a little and it then it comes on. the bulbs are the correct wattage. i think it has either a rapid start or magnetic ballast. thanks

Yeah it happens to me very often! Still don't know why this occurs... I think, during the life of a fluorescent tube, as the cathodes get thinner and thinner, the resistance rise, so they need to be hotter and hotter to lower the resistance enough to allow the capacitor in the ballast to shoot the current for starting the lamp... Hope this helps!

On a rapid-start fixture, if the ends glow red and you have to touch the tube to get it to come on, it could be 1) a poor ground/earth connection  2) poor ionization caused by a dirty tube (washing tube and reflector will help) 3) marginal ballast or lamp. The OCV of some rapid start fixtures seems to be just barely enough to light the lamps....if things aren't just perfect, you can have these problems. I see it happen more on U-bent F40 lamps than anything else.

An electronic T12 ballast can overcome these issues, or a good quality, older commercial rapid-start ballast. Although I've also had those big older ballasts actually instant-start the lamps from time to time if the conditions are just right.

Sounds like he's having the same issue I am, the air is probably moist, or I noticed if it's REALLY cold out the lamps will just be sitting with a dim glow and striations until you turn it off and back on (LPF RS ballast)

to solve the problem which I have fixed in some fixtures in my old house...

1. clean the tubes...
2. check the grounding wire (the rapid start needs grounding connected to the reflectors (if the reflector is painted white, I usually chip a bit of paint under the grounding screw and eyeloop hole for better connection)

Sometimes if basement is cold, you can use plastic tube protector covers to keep them warmer while its off, its those plastic tubes with some kind of rubber caps on the ends...