I've got six 400w metal halide low bay lights in my shop and slowly but surely they have been going out one by one.I bought them used off of E Bay back in 2000 so I am not complaning but I do need some trobleshooting help.I have already changed the bulb in the ones that dont work with a known good bulb and the bulb is not the problem.I finally took one fixture down today and it has the actual ballast in it as well as a capacitor.How would I go about testing either of these to find the problem? The fixture buzzes when plugged in and the sealed tube in the bulb will start to light very faint but it will never get bright at all.They are wired on 120v if it matters.I really dont want to replace all six fixtures at the tune of $160-$180 each if I can fix mine cheaper.Any advice will be greatly appreciated.