All you need is a switch to cut out the ignitor in the HPS wiring to run an MH bulb of the same power rating. They both use the same coil and capacitor. If your HPS ballast won't fire an HPS bulb but will run an MH bulb then the ignitor is shot. If neither bulb will run then it's most likely the capacitor. The caps should be checked to make sure they are still within their rated range as they will still work even if at only half but the light spectrum will be way off and the coil can overheat. I bought a $35 multimeter from Amazon that can test those caps and even tiny ones for electronics. Temp probe good to 1000F that's great for monitoring the temp when I decarb etc. Track the house wiring in the walls and all sorts of other good stuff.
I have a 1000w MH ballast that I figured the cap was shot in so got a new one and still no joy so looked it up online for the testing procedures to check the coil and it looks like the primary coil is shot. So probably was the bad cap and I fried it the last time I used it. At the time I thought it was the bulb so a good 1000w SunMaster Warm Deluxe was sitting on the shelf for 12 years after only one flowering run. Works fine in my newer ballast that does both and had it apart to replace it's cap and the wiring is just an on/off switch in the line that feeds the ignitor so dead simple. Easy to find tutorials on many sites or UTube.
No special switch needed as the switch is on the low voltage side of the transformer and just has to be rated higher than your line voltage and normal amperage. I'm no Sparky either but this is easy enough as long as you can follow simple instructions or diagrams. The hardest part for me was figuring out which holes to plug the cords in the multimeter to test the high voltages from the transformer. Those Chinese manuals translated into what is supposed to pass as English leave a lot to be desired sometimes.
The switch goes where it says Blue X3 wire on the diagram.
