Dude,
Metal halide has a capacitor.
HPS has a capacitor and an ignitor.
if you have an old light with an ignitor in it, then you have a hps that the capacitor has been removed.
It is very simple, both of my lights are old MH lights that i took out of a shop.
when i'm out there again i will look at mine and takea picture of how everything should look.
Be careful with that light dude. those bulbs will pull all the power allowed to them, and trip your breaker or explode and set something on fire.
it doesnt have a capacitor i actually went to lowes and bought a MH just like the one i have took it apart then returned it! and low and behold it doesnt havd a capacitor i was told by somebody that it doesnt need one if its under 250 watts the light i have is only100 watts the one from lowes didnt have a capacitor so it obviously dont need one