I just got thru replacing capacitors in 3 old magnetics and dd a lot of research about them while working on them.
A sudden drop in light intensity can be caused by a dying capacitor so I'm getting a multimeter that can measure the capacitance to check mine. I use good bulbs like Hortilux mostly so want to get all I can out of them.
With an HPS ballast that won't fire a bulb at all it could be the ignitor. If you have an MH bulb put that in and if it fires OK then the ignitor is shot as I found out after replacing the capacitor in a 400W HPS ballast recently. I got it dead from a buddy and the guy at the electrical store in the city sold me a 21mf cap instead of the 55mf it needs so that cap went out after a year and while it worked I was likely getting much lower light from the bulb too.
Put the new cap in and a 400W HPS bulb wouldn't fire up but the MH one fired fine and looks super bright with the new cap. So now I need a new ignitor for it. Both my 1000W ballasts packed it in too. One is MH only and after putting a new cap in it still didn't work. I think the coil is shot on that one as I'm only getting 125v at the bulb end when it should be around 550v. The MH/HPS ballast works fine now with it's new cap and is running my 940W Hortilux conversion bulb in MH mode just fine. It also fired a 1000W HPS bulb great too but the Hort has a much better output than the Light EnerG 1000W Super HPS going by intensity measured with a cheapo light meter. It reads 1500 at 3ft while the other reads only 1000 and it's hard to beat a Hort for actual light quality. I did my testing on the 1000W MH/HPS with the old capacitor in it not long before it died so should do it again with the new cap and see what the results are.
I'm using that monster in the middle right now. A lot of money for those so will be getting the standard Hort SHPS when I need to replace it. Will be replacing it with a Light EnerG bulb or maybe that Sunmaster which is another conversion bulb but has only just over half the intensity but is way higher in red according to its spectral chart on the box. At day 21 of 12/12 the stretch should be over and they don't need as much light to flower well.
These PlantMax suck imo but were cheap and OK for backups.
This is the 1000W MH/HPS I bought about 2 years ago used from a hydro store in Kelowna, BC for $70. New cap was $50. lol