The ballast system in a MH fixture is a two part device. A
transformer/ballast the raises the voltage to the operating voltage of the
bulb. And an Igniter that produces a high voltage surge to start the arc
inside the bulb. Either part failing can cause the lamp to not light.

For all you who thrive on information overload check this out:
http://ceenews.com/ar/electric_long_...hite/index.htm

Have Fun
JOhn :-)

"Don Geddis" wrote in message
...
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]