I've been asked to look at a CFL floor lamp. It's many (5-10) years old and one day stopped working; it simply didn't turn on. The fluorescent bulb (four short (5 inch?) tubes extending from a plastic base with a single four pin cluster) was taken to a store and it did work there, but the bulb's whereabouts is currently unknown.

Randomly probing between pairs of the four holes in the lamp socket at random shows between 5 and 15 VAC, but nearly zeros DC.

Is this lamp likely to have user-serviceable parts inside, like ballast for example? Or is the only course of action to buy a new CFL bulb and see what happens?

business end of a CFL floor lamp CFL floor lamp socket click for full size

update: based on helpful suggestions from @ I've opened the base of the floor lamp and there's a whole circuit board full of components!

There's a small fuse(?) labeled F1. My low quality DVM reads 0.7 Ohms when I touch the probes together and the same 0.7 Ohms when I touch them to either side of the fuse. Yes it's still in the circuit, but it at least suggests the fuse might be okay.

click images for full size

circuit board in a CFL floor lamp

circuit board in a CFL floor lamp circuit board in a CFL floor lamp

circuit board in a CFL floor lamp circuit board in a CFL floor lamp pair of BUL128A transistors

  • That is an electronic ballast. They probably removed the case to make it smaller. On the few I have looked at to try to repair the transistor that charges the cap to provide the proper voltage is usually blown. You may be able to find a replacement online by searching for ballast and the lamp type.
    – Ed Beal
    Feb 16, 2020 at 19:47
  • @EdBeal I've added a photo, there are two BUL128A transistors (several Amps and several hundred volts). Repairing factory PCBs might be above my skill level; I'd probably reach for a propane torch and a hammer (humor) but I'll give it some thought.
    – uhoh
    Feb 17, 2020 at 3:57
  • Those and possible a BJT that triggers them, the problem when the output stage fails with a short sometimes it will take out the zener that is the voltage regulator , that diode will have a .6v break down voltage forward bias and its zener voltage reverse bias , look for blisters on the transistors . I can’t make out the component far left next to the white wire is it a damaged part ? I see light in the middle but it may be a label, sometimes they can be repaired especially if you like the lamp and cannot find a ballast.
    – Ed Beal
    Feb 17, 2020 at 4:17
  • @EdBeal the lamp lives in a nearby city, I may visit the lamp again this weekend and take a closer look, thanks!
    – uhoh
    Feb 17, 2020 at 4:24

2 Answers 2


There are several different sizes of lamps that fit that pin out. The ballast is probably in the base and will have a list of compatible lamps if that is lost, many times when a lamp goes bad it can take out the ballast, so the ballast probably is bad, the voltages you mention are two low to strike the lamp.

  • The ballast may be in the base (normal) but some very inexpensive ones are in the top of the fixture not the bulb. Your lamp did not light at the store so it is probably bad (not uncommon) electronic ballasts some times have protection that keeps the ballast from failing when the lamp fails but not always, I would find out how much both the lamp and ballast cost and make the decision from there if you like that fixture enough to repair.
    – Ed Beal
    Feb 16, 2020 at 2:35
  • 1
    Oops I thought it said it did not work. Yes the base of the fixture itself , there doesn’t look to be enough room it the plug in area for even a simple cap and inductor ballast.
    – Ed Beal
    Feb 16, 2020 at 3:03
  • I've discovered a circuit board in the base and added some photos. Wow what a monster! Now I see what you mean, thanks!
    – uhoh
    Feb 16, 2020 at 3:37

Honestly, if it were me, I would get a 12VDC power supply... bring 12V up to the cobra head, and then put segments of high-density LED strip where that reflector is now. I'd use double density (120 LED/metre) strip material, and hand-cut them to a custom length and solder on #24 wires to each strip.

If you want to make it dimmable, add a 12V dimmer module. Use a UL-listed power supply; no cheap Chinese there. You could go with a wall-wart power supply and just run 12V into the lamp base. All the dimmer and LED strips are readily available on Amazon for a few dollars.

  • You know I really might just do exactly this, thanks!
    – uhoh
    Feb 17, 2020 at 3:54

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