I am trying to replace a ballast for my garage lighting. I removed the old one but there are absolutely no markings, numbers, diagrams or anything on it. It is a 2 lamp 120V fixture with 40W T12 bulbs. Based on the chart at Lowe's I bought a Pilips Advance AmbiStar RELB-2S40-N. Besides being a different size and much lighter, the 2 yellow wires are on the same end as the red and blue wires with a black and a white on the other end. The old ballast has red and blue wires on one end and the black, white and 2 yellows on the other end. Does that mean it is not the right one? I'm scared to try it.
If you live in the USA, that's a perfectly fine ballast for your purposes, Lori. Some manufacturers bring the yellow wires out the same end, some bring them out the opposite end. That ballast is rated to handle two 40W T12 tubes.
If you live in a country with 240V municipal power, though, that ballast won't work for you; it's built to be connected only to the 120V common in the USA.
just a note the reason this ballest is lighter is it is electronic, the old one was a "mag" or wire wound type, they were taken off the market back in 02 or so. the electronic ones are safer as far as a fire danger. on the face of your new ballast is a wiring diagram the power for the light are the white and black connect those to your incoming white and black on the face of the balest there is a diagraham to hook the toombstones to (the white things that connect to the lamp) you can cut the old wires and use small wire nuts or take a piece of wire strip an inch shove it in next to the wire in the tombstone and pull the old wire out some times it takes a few trys to do this and most just use wire nuts