My 175 watt MV went dark last Sunday. Today, I lowered the pole and got to work. Photocell is fine, lamp looks fine (GE) however it was getting greener and dimmer (two years and a half, about 12 000 hours). Ballast hums quietly when photocell is taped shut. However, there is no light. All connections are tight. The ballast is Magnetek H-39 only. I tried with new lamp and still no light.

I happened to have a 175 watt MH ballast kit, I thought I could maybe replace just the cap with the one from the MH ballast but they were of different values. I ended up replacing both ballast and cap, and with a NOS Westinghouse Lifeguard lamp (R4 date code), everything works now. New ballast is a Cooper from around 2002-2004.  I didn’t try the old lamp on the new ballast for fear of causing damage.

I am wondering what could have caused the failure. The paper label on the ballast is quite charred, but there are no other signs of overheating. The ballast was humming softly when it had power. It has been mentioned here on several posts that a missing lamp on a CWA ballast can ruin it. I am thinking maybe the cap is open thereby simulating a missing lamp. The cap is not bulged or leaking. Does anyone have thoughts on this situation ?

I am now sort of looking on eBay for a spare M-57 ballast. Does anyone have particular brands to recommend or to avoid ? I am not sure I want another Magnetek.

Thanks to all who may answer.

Sounds like a bad capacitor, I've come across numerous 400W MH shoeboxes with that problem, there's not enough amperage getting to the lamp so it doesn't fire up, your GE lamp will probably still work

If there is power coming from the ballast, then the capacitor is probably bad.

Simkar used to make decent ballast kits, but I think they might import them now.

MH ballasts have higher OCV, so need higher impedance (that mean lower capacitance) to get the same arc current
Open circuit does cause higher load on the primary winding, but as there is no heat coming from the secondary (because no current there), the total power dissipation would not be worse than with lamp burning normally. And as the lamp isn't working, there is no heat coming from there, so the ballast shall stand the open circuit without troubles. And if the primary fail in that way, it will end up black and dead short, tripping the breaker

With the MV:
Have you checked the secondary voltage? (directly just the magnetic part without the capacitor) It should be around 220..250V (= the ballast OCV). If there is way lower voltage, it mean the secondary may have developed an internal short circuit.

The capacitor you may measure that way (few posts form Feb 19)

Many thanks for your responses. I was thinking it might be the cap. The trouble with this light is that the pole is very heavy, and I cannot lower it alone. I need help from either neighbours or family. That means it has to be fixed fairly quickly once the pole is down. Since I had no way of testing and then finding a replacement cap, I simply changed everything, including the lamp as I don't want another failure in a short time.

The old ballast and cap, I saved for future testing. I might replace the cap and use it elsewhere...

In the meantime, I have a Cooper MH ballast in there and on the second night, everything seems good, lamp is nice and bright. I hope the Cooper ballast lasts at least several years. I am keeping my eyes open on eBay for a good M-57 ballast such as Advance.