We have a 2-bulb 4 foot fluorescent light in our laundry room. I don't like it. It buzzes. It flickers when it turns on. One day, it didn't just flicker for a second or two -- it flickered for about 10 seconds before coming on. The next time I turned it on, one bulb only came on partially (it was very dim) and it seemed that the buzzing was louder than usual. Turning it off and on didn't help. Pfft. Time to replace the bulbs? Or worse?
I soon learned that there were two possibilities; either (1) the bulbs needed replacing, or (2) the ballast needed replacing. There was also the possibility of a faulty starter, but it sounded like starters are only found on older fixtures (mine is 20 years old and looks reasonably modern).
So, first, I removed the cover, took out the bulbs and went to Home Depot to get replacements. My bulbs were 34W 4 foot T12 "energy saver"s. As I understand it, wattage is usually 10 times the length in feet; so normally one would use a 40W bulb with a 4 foot fixture. But, I wanted to get an exact match since I didn't know better at the time. Note that the "T" number is an important one---it tells you the diameter of the end of the bulb. The number is in terms of 1/8ths of an inch. So "T12" means 1.5 inches and "T8" means 1 inch.
I had someone help me find 34W 4 foot T12 bulbs at Home Depot and turned in my old bulbs for recycling (my Home Depot accepts fluorescent bulbs for recycling). I took them home, installed them in the fixture, flipped the switch... and groaned when neither bulb came on completely. Both bulbs turned on very dimly. Even worse than before! Did I get the right bulb? Maybe I should have gotten 40W's? No, I was pretty sure I got the right bulb. And, I had earlier noticed that the old bulbs showed signs of age---they were both very dark at one end. It was time to try replacing the ballast.
I opened up the fixture and found that the ballast did look old. It was large and heavy. But, the wiring looked clean. Two red wires went to one socket. Two blue wires went to another socket (at the same end). Two yellow wires went to a socket at the other end and short white wires connected the remaining socket to the yellow-wire socket. I was a bit puzzled about how to remove the wires. I couldn't yank them out of the sockets. I opened up a socket and learned that they have one-way securements---you can't take them out from the same side that you put them in. I cut all the wires and to get the end piece out, I had to pull through the back of the sockets which required me damaging the sockets. So, I made a note to buy new sockets in addition to a ballast on my next trip to Home Depot.
After I dismantled everything, I took the ballast and one socket to Home Depot. Someone helped me find the right ballast. I'm glad I had someone help me because I didn't notice that my first instinct was to buy a T8 ballast, whereas I needed a T12 ballast. I didn't realize the bulb diameter would matter, but it apparently does. It's also important to make sure you get a ballast for the right size fixture (4 foot in my case) and correct number of bulbs (2 in my case). It's also worth checking that the bulbs you use (34W T12, in my case) are listed on the ballast---they were. After finding the correct ballast, I got two packages of fluorescent sockets to replace mine---they matched the one I had brought exactly. Btw, I was happy to discover that the new ballast was the same size as the old ballast but a lot lighter. It also had similar notches which are used to secure the ballast to the fixture.
Installing the new ballast and sockets was relatively easy. I first hooked up two sockets with the left over white wires. These were not provided by the new ballast. I then secured the four new sockets, being sure to press them in as far as they would go. I then hooked up the bulb wires, yellow, red and blue. Finally, I made sure the light switch was OFF and used the left over wire nuts to connect the power wires. Then, the moment of truth came... I turned the switch, and... light! One bulb came on quickly; the other was a bit slow, had a dim part in the middle and had a slight oscillating flicker. But, it gradually went away. And, after putting the covers back on and turning on the light on again, I don't notice any difference between the bulbs---they look good as new. Also, one thing I didn't notice was a buzzing noise. No buzzing! Why didn't I do this two years ago when we bought the house! Ah, well, better late than never.