How sure are you it's a MH and not HPS?

How sure are you it's 400w and not a 250w? Most (but not all) of the hood/transformer sets are only 175 to 250w at most.

If if is a 400w MH and your new bulb does not light then the I would assume your cap is bad and needs to be replaced.

Just a guess
Zandor Reviewed by Zandor on . Question about MH lighting A few days ago a growing friend gave me his old 400watt MH hydrofarm grow light with a built in ballast. So I went out and bought a 400watt MH light that fits the requirements listed on the growlight. Now when I went to turn on the light it didn't come on right away, which I wasn't worried as I've heard it can take up to 5mins. But after 10mins it never came on, same after 30mins and a hour. I checked the bulb while it was in and turned on and inside the middle part there was like a little Rating: 5