I noticed a 400W HPS light out in my flo room this am. The bulb was maybe 4 mos old, and the ballast was just replaced about 2 weeks ago. The bulb has a dime sized black spot around a small hole and some cracked glass less than an inch above the base. The ballast will not power up. The fixture works as I put my spare ballast and another bulb in it. I don't know much at all about how these things work, so I have no Idea what caused this. I am looking for someone who knows about these things to tell me whether it is the ballast that causes a malfunction like this, of if it is the bulb. Common sense tells me the ballast, but without the requisite knowledge it is merely a guess. My question is about the process, not the products, so no brand names. Just looking for the reason this happened, so I can try to avoid a repeat. Was this the fault of the ballast, or the bulb?