Just had same problem with another brand 4 bulb fixture. It's the ballast, no doubt. There should be two ballast inside fixture. Take out the bulbs. Slide out reflector. Look inside and you'll find 2 ballasts. look at the wires coming off the ballast and follow the ones going to the 2 working bulbs. The other one will be your bad one. Carefully cut the wires so there is a least a couple inches of wire left on ballast. two screws will hold ballast in place. Remove them and ballast will come out. Now look on line or call company and order a new ballast. Unfortunately this will cost you about 60 bucks. When new one comes match up wires and re-splice and bam, your bulbs will fire up. That's why it was in the yard sale.