I have a Cooper Vanguard fixture with a bad MV ballast (175W) The thermal protector cuts out for no reason. It can last from 5 to 15 minutes before cutting out. Then there is a long waiting period for a reset. It is the original ballast. I don't really have time to undo a ballast to replace the thermal protector, and I would like to keep it as a MV fitting. However, I found a NOS mercury vapour ballast for 175 H39 lamps on eBay. It is a Magnetek 1030-91. What do you think of Magnetek Mercury vapour ballasts ? Thanks in advance.

EDIT : the said ballast can be found here

That MagneTek ballast would be a HUGE upgrade for that fixture.  I don't remember the name Vanguard, but if it's a common Cooper bucket, the ballast is simply held by pressure between a bracket and the top of the housing, making replacement super easy.  The MagneTek is larger than the original, but I think it should still fit.  More of a problem maybe finding room for the large oil filled capacitor.

There is quite a bit of room inside thte fixture. It can be seen here. I will look into this replacement. The existing ballast is of an unknown brand (no label) and I suspect the quality is rather poor.

That Magnetek mercury Vapour replacement ballast should fit into your Vanguard III fixture no problem,and give you many years of service.

Got the ballast today, and installed it. I have the same problem, however. It shuts off after a while. I suspect the photocell to be at fault, although I have to do further testing. The photocell is by Area Lighting Research model number BF-120. It might be of the thermal type although I am not sure. If it can't be resolved, I will either get a Fischer Pierce PC or a shorting cap and use an astronomic timer to control the light.

On a side note but related matter, the old ballast is a Cooper branded no name MV/MH ballast for either M57 or H39 lamps. The replacement I got on eBay is dedicated to MV lamps (H39) only. Are MV only ballasts smaller than MV/MH ballasts of the same wattage ?

Probe start MH lamps do require a higher OCV to reliably start than equivalent MV lamps, and also tighter control of crest factor, so it would make sense that the ballast is somewhat larger.  My Venture M57 (175w MH) ballast is somewhat thicker than my Advance H39 (175w MV) ballast.  It's nearly the same thickness as my Advance H37 (250w MV) ballast.

Also, most mercury vapor lamps will run on same wattage probe start metal halide ballasts, which is why your Cooper ballast lists MV lamps as well as MH.

The protector trip, because the ballast overheat, so replacing the protector would not make much sense. First, you would have to solve the problem, why it overheat...

Well, well, it was not the ballast at fault ! After further testing, the photocell was causing problems while testing earlier because I was doing the testing in my basement. Light reflecting off a white surface tripped the photocell, just as daylight would. Since it is a thermal relay, there is a delay of about 30-60 seconds for the bimetal strip to bend. As the lamp warmed up, intensity increased, and the PC did it's job. I intentionnally tripped the photocell with a spot light, waited a few moments and took the photocell off the fixture and in my hand. After the usual delay, I heard a click, but the PC was in my hand. The next test will be when the fixture is outdoor atop the pole. If problems persist, I will replace it.

On the plus side, I upgraded to a MagneTek ballast, which is superior than the Cooper ballast. Being MV only makes it lighter for a pole top fixture.

Many thanks to all who tried to help.