I have a 175 watt mercury vapor fixture illuminating my driveway. Something happened a few months ago and now it stays on 24/7. I've changed the photocell but the problem persists. It actually went off once during the day but only that one time. I don't think it's a ballast problem, because it stays on. I'm about to install a new fixture, but before I do, is there anything else I should check? Should I try changing the bulb? Since they're not made any more, I would hate to throw it out if it's fixable. Thanks

Does it work without the PC? The feed is a black to the PC and the switch leg is the red to the ballast, simply trace it out and see if there is a short that the ballast is getting power somehow.

When the photocell is removed, the light does not work. I'm going to have to take it down to get a good look at it.

Then sounds like a defective photocell is the problem.

The photocell was actually replaced twice, with two different kinds. Once I take it down maybe I will be able to figure it out.