First, one must determine if one or more of the tubes are bad. One bad one can cause another good one to go dark in a 2 tube fixture. (As a general rule, though it is always possible that there may be an exception, when an electronic ballast fails all of the lamps will go completely dark.)

Ballast: A specially designed transformer that regulates the voltage applied to a fluorescent lamp in order to keep the current flowing through that lamp at the level required to provide the intended lighting output.

If one tube is lit and the other is not, this could be either a bad ballast or a bad tube. Swap the tubes to see if the dark tube now lights up. If not, you have found the bad tube. BUT DON’T change out just the one tube if both have been in together the whole time. Change them both. Why? Because the so-called “good one is old and it too will also die soon. Save yourself the problem of having the light go out again next week. Change both tubes!

Easier yet is to just change out both tubes and see if it works. If it does, then you are done. If it does not then you are faced with the fact that not all NEW tubes are good. This happens to be fairly common, say 1 to 5% of the time. So at this point, you still need to determine if the old tubes were good or bad. You could try installing two more new tubes, or borrow two working tubes from another fixture and try them, or try the old tubes in a fixture that does work. There are many ways to skin this cat.

If none of the tubes light up or do not even glow at the ends then it is possible that there is no power. Get out your voltage checker and verify the ballast does receive power. In some single tube fixtures, the power passes through the end piece before reaching the ballast. End pieces can be the problem but most often they are not. But do not ignore the possibility. Even end pieces that are damaged or broken often will work just fine.

If you have power, good tubes, adequate end pieces with connecting wires that do not fall out when you tug on them gently, and it still doesn’t work, then replace the ballast.

Other problems that may exist if you have tried but failed in your efforts to repair the fixture include:

  • Incorrect ballast wiring.
  • Incorrect ballast type.
  • Incorrect Lamp or tube type.

Joe Duncanson

Copyright © 1987,2009 by Joe Duncanson. All Rights Reserved.