Folks-  For a Metal Halide fixture- If the bulbs are not coming up to full intensity/output, would it be the ballast that is bad, or the capacitor?  I know the bulbs are good because I've tested them on other fixtures.  I have 2 fixtures that are exhibiting this behavior.  1 is a 175W and the other is a 400W. 

Thanks

In autoregulator (CWA) system, lamp that not comming to full output, caused by a bad capacitor. Otherwise (With the lamp intact), this can be cause by the use of a smaller wattage lamp on a higher wattage ballast, or generally mismatch between the lamp and the ballast.

Could also be the ballast wired on the wrong tap although it would have never worked well. It could also be (but very, very unlikely) a ballasted fixture used to replace a non-ballasted (remote ballast) fixture and the remote ballast was never removed or bypassed. It can happen and did happen at my place of work. Lamp would warm up almost to full output with the ballast being very noisy and all of a sudden would dim down to clear mercury vapour stage and quiet down. Was M59 stuff.

What ballast type is this abut? CWA or HX?

It could be a ballast defect as well - either wrong air gap (but that would have tobe since new, so should never pass the QC check) or some short in the winding or so (then it would be unusually hot).


However as was stated by others, the wrong wiring and in the case of CWA the capacitor degradation I would see as the more likely scenario as well...

What if the ballast have been dropped and hit a hard surface, and its potting was insufficient to fill well the voids ? Can the gap close from the impact ?

What if the ballast have been dropped and hit a hard surface, and its potting was insufficient to fill well the voids ? Can the gap close from the impact ?

Normally the gaps are set using hard paper spacers and then by a press (or a hammer in some red neck production shops) the iron parts are pressed together to make sure the final gap is really the thickness of the spacer and nothing more (so to keep it well under control).
So If the gap is the problem, I would rather guess for the spacer just be missing at all.

Well.. turns out that the ballast and/or starter is also bad.  This fixture serves as a floodlight to light up one side of a church steeple.  There is another identical one on the other side, that works just fine.  I haven't been up on the roof in a few years when I last changed the bulbs.  One side works fine, the other- with the ballast in this topic, is the one that was not lighting up to full intensity.  I went back to the building last night and got up the guts to go climb up the roof, broke about 10 roof tiles in the process... (sucks being close to 350 lbs) got up to the lights about 150 feet off the ground, and after i replaced the bulbs, it turns out that this fixture is now dead.  I thought the lamp had gone EoL but it actually looked practically new, so I put it in the other fixture and after 10 min of running it was at full intensity.  So, I'll need to change out the ballast/capacitor/starter in this bad fixture.  Thing is though.. I'll probably throw in a 150 watt HPS ballast and the large base 150W HPS bulb.  I'll do the same for the other side when that ballast gives out.  One side will be yellow/orange, and the other side of the steeple will be the crispy white   I'm done dealing with metal halide.  That or I'll let them burn out this next time around and then hopefully by then there will be some better LED replacements out that are of the 175-250 watt range which won't need cooling fans inside the bulbs. 

At a hard to reach place like that go for MV (on MH ballast without ignitor), or top grade HPS that is expected to do years without trouble

You know.. I never considered MV.. That's not a bad idea.  What would be your recommendations on ballast and lamp?

If the fixtures are of the M57 or M59 variety then you only need a H39 or H33 respectively mercury lamps. No need to modify anything ballast wise. Of course, you'll need a new ballast for the one that is not running up the lamps. All probe start MH ballasts are compatible with the equivalent wattage MV lamps. For pulse start, you need to deactivate the ignitor for MV (disconnect and cap the neutral on the ignitor).

If the fixtures are of the M57 or M59 variety then you only need a H39 or H33 respectively mercury lamps. No need to modify anything ballast wise. Of course, you'll need a new ballast for the one that is not running up the lamps. All probe start MH ballasts are compatible with the equivalent wattage MV lamps. For pulse start, you need to deactivate the ignitor for MV (disconnect and cap the neutral on the ignitor).

Well, the question is, why the ballast have failed in the first place. Because the wrong thing may as well be the fixture itself (unfortunate position causing the ballast to overheat,...) and in that case it will fry even any other ballast.
So without a good answer for the "Why the ballast failed" question I don't think it make sense to continue much further...

Probably driving the MH lamps all the way to EoL?  The MH wallpacks I have at 1 building that I am slowly converting over to HPS maybe only lasted 10-15 years before the ballasts fried.  I attributed this to the bulbs always going EoL/exploding instead of being changed out right at 10,000 hrs before going EoL

Probably driving the MH lamps all the way to EoL?  The MH wallpacks I have at 1 building that I am slowly converting over to HPS maybe only lasted 10-15 years before the ballasts fried.  I attributed this to the bulbs always going EoL/exploding instead of being changed out right at 10,000 hrs before going EoL

That is quite unusual that they are failing like that. Could the fixtures be filling with water or something?

10 years is the rated life of Magnetic ballast. If it is heating in thse luminaires to its max allowed temperature. To get the many decades life we expect, the ballast must be used at colder ambient, which may not be the case in those wallpacks

Yup.  out of the 13 RABB 175W MH wall packs, only 5 remain on their original ballasts.  The rest have been converted by yours truly over to HPS 150W when the MH ballasts failed.  And by fail I mean fail!  They get hot enough that the wire nuts distinigrate in my fingers because of being super heated.