Debug MH light hood?

I've got a metal halide lighting hood over my reef aquarium. One central
250W MH bulb, and two adjacent PC actinics. Two separate power cords: one
for the PC bulbs + fan, and one for the MH bulb. The MH power cord goes to
a ballast, which says that it is 400W/120V/4A.

One day recently my MH bulb would no longer turn on. Sometimes it takes
(the ballast?) a few minutes to "power up" after I turn the switch on.
This time, I had to turn it on and off a few times before it would light up.
Then everything was fine for a week. And then suddenly it wouldn't turn on
again at all.

I figured the MH bulb must have burned out (although I don't know how to
verify this). It was about 9 months old. So I bought a new 250W bulb.

Sadly, it still won't turn on, with either bulb. A voltmeter seems to suggest
that the MH housing in the hood is still getting power. If I'm reading it
correctly, it's registering about 270 volts where I screw in the MH bulb, when
I turn on the ballast.

Any ideas what to do next? Do lighting hoods, or ballasts, wear out?
Is there some other test or repair I could try? I'd hate to have to buy a
whole new lighting hood...

-- Don
__________________________________________________ _____________________________
Don Geddis http://reef.geddis.org/
If we ever discover a race of alien beings on another planet, I bet it would be
surprising just how many of them are missing part or all of a tentacle.
-- Deep Thoughts, by Jack Handey [1999]