I have a 175 watt mercury vapor yard light mounted on a pole I'm going to have to replace, it is shorting out and now stays on 24 hours a day, despite me putting in a new photocell. I was thinking about replacing it with a 100 watt high pressure sodium light from home depot or Lowe's. I would like to use the existing mast arm that the light was mounted on. My question is, will this new light mount on the old mast arm? Are they universal? Thanks

More than likely yes. Those arms are pretty standard.

If I were you I'd go with metal halide. The color is much nicer than the pink/orange of HPS.

Okay. Thanks