How to check out a fluorescent bulb?

My eight foot light is not working, and changing bulbs does not seem to help.
How can I tell whether the problem is the bulbs or the ballast?
Thanks
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Dwight wrote:

the root cause is likely not the bulb. If you have two choices and it ain't one, it's likely the other...assuming there's only two choices...
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If it is not the bulbs, it could be the ballast, however the lamp holders themselves could also be to blame, or a connection that has become loose or corroded...
It is fairly easy to figure out:
Inspect the lamp holders, if they look damaged or melted then replace them... If they don't appear damaged then examine all the connections one wire nut at a time -- if none of them are loose or show signs of arcing, then remove the ballast and take it to the electrical supply house and obtain an identical replacement...
Odds are however that your problem is with the ballast, but it is always good to check everything else also because it only takes one break in the circuit inside the light somewhere for the lamp(s) to not light...
~~ Evan
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Also don't forget that on some fixtures with multiple bulbs, all the bulbs have to be good for any to work.
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Excellent point overlooked by earlier posters.
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That has not been the case on any of the 4' or 8' light fixtures that I have worked on in the past and some of them had up to 4 lamps being powered by the same ballast...
Sometimes there are connections within the light from the ballast that if they loosened would open the circuit to 2 lamps at the same time... But they aren't like the old fashioned Christmas lights where if one lamp goes out all of them go out... Multiple lamps fed off the same circuit on the ballast are wired in parallel so each would remain lit independent of the other, they would have to be wired in series for one lamp to kill a circuit...
~~ Evan
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Be sure the bulbs are touching all the contacts in the sockets. Those sockets do break. If that's ok, then is there a starter? In old fixtures there were these small starters that look like 3/4 inch round by 1.5 inch tall (roughly). Those go bad. If it's a newer fixture, you only have one choice left, the ballast. Of course a new bulb could be bad too..... Get a 3rd bulb and replace one at a time before buying a ballast or new fixture. If you replace the ballast, get an electronic one. They are better than the old transformer type.
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When I was in the Coast Guard we used to hold fluorescent tubes next to our 100KW Loran-C tower and light them up. Obviously we were *much* farther away than an inch or so!
The cool part was when you held the end of the tube in one hand and slid your other hand up the tube. The tube would only light above your hand so you could "silde" the light up and down the tube.
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*Assuming that you installed all BRAND NEW bulbs, you've already eliminated them as being the problem. Change the ballast.
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Dwight wrote:

Thanks everyone.
Dwight
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On 5/11/2011 11:21 PM, Dwight wrote:

process of elimination. If you put in new tubes, and it no worky, then ballast it is. (if it's got power to it)
--
Steve Barker
remove the "not" from my address to email
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Steve Barker wrote:

Unless it is the socket or the wire to the socket which is bad or came loose.
Jon
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If changing bulbs does not help, then the problem is probably either in the ballast or with wiring - such as a broken or corroded connection.
When a fluorescent bulb burns out, there is usually a characteristic blackening of 2-3 inches (or so) of one end, occaisionally both ends.
If the problem is a failed ballast, I would suggest replacing the entire fixture with two 4-foot fixtures. Preferably, one using 1-inch-diameter (T8) 32-watt 4-footers, and having a modern electronic ballast. The economics get better, the bulbs are easier to replace, and a better choice of colors and color rendering properties is available with 4-foot bulbs than with 8-footers.
More fluorescent bulbs are manufactured in 4-foot length than in all other lengths combined. 4-footers usually cost less than all other sizes of fluorescent bulbs. Most of the lumens of light produced by fluorescents worldwide are produced by 4-footers.
If you have a problem with the ballast or the fixture, then I think it's a good idea to "go with the flow" and change to 4-foot.
--
- Don Klipstein ( snipped-for-privacy@misty.com)

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Don Klipstein wrote:

Four foot tubes are also a lot easier to dispose of. Around these parts, fluorescent tubes are considered to be hazardous waste, but any tube up to four foot long can be disposed of free at several participating merchants (the local ACE hardware being one of them).
Jon
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Dwight wrote:

You already checked out the bulb, what's left?
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