400W metal halide doesn't work at full brightness?

I have some standard 400 watt metal halide "high bay" type fixtures in my shop. I just put new bulbs in them all. For some reason, one of them only lights up at about half brightness. Does anybody know why this would happen? Could it be a bad bulb? Or is it something wrong with the ballast?

They are 22 feet in the air so it's not very easy to get up there to switch out the bulb...

Two possibilities come to mind:

Maybe sometime along time ago, someone put a 250W ballast in that fixture.
or
Maybe the new bulb isn't really 400W as you think.

I doubt it's a 'bad' ballast.

Anyway you have to get up there and take a look...

If it's a Constant Wattage Autotransformer (CWA) type ballast which are common on probe-start MH fixtures 175 watts and upward, I'd check the capacitor. CWA ballasts have a capacitor wired in series with the lamp that is also used to control crest current. Get or borrow a multimeter that has a capacitor check function (Fluke 117 for example), remove the capacitor from the ballast case and check the value against the label ( ~24uf should be right for a 400 watt probe-start if I remember ) Replace if it is considerably lower. I've run into a few instances where a fixture will be dim with a good working bulb and it turned out to be a bad capacitor (Currently have a Cooper-Warrior flood light that needs a new capacitor but is out of reach unless I can borrow a scissor lift)

My church uses metal halide fixtures for the multipurpose room and has a bi-level system that dims them by switching capacitor values on CWA ballasts. The "low" setting is 17uf for the 400 watt and a 7.5uf is paralleled to bring the total to 24.5uf on "high"

Well, I rode the forklift up and put another new bulb in it, same problem. I guess I'll just change the fixture out, used ones aren't very much on craigslist. It's really hard to keep going up there to play with it...

Never thought it would be so hard to change a light bulb.:shakehead

Number21 said:

Well, I rode the forklift up and put another new bulb in it, same problem. I guess I'll just change the fixture out, used ones aren't very much on craigslist. It's really hard to keep going up there to play with it...

Never thought it would be so hard to change a light bulb.:shakehead

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Make sure you check the input wiring, many of these ballasts are multitap (120v, 240v, 277v, 480v) and I've seen people miswire them causing dimming, flickering, strobing, ect.