High-pressure sodium (HPS) light ballasts are actually high voltage multi-tap transformers. Multi-tap means that a variety of single-phase voltages can feed the transformer. In most cases once a ballast is bad, it does need to be replaced. Identifying the cause to the problem will aid in the longevity of the new replacement.
Heat and Bulbs
Inside the HPS fixture is typically a very hot environment. Most fixtures are completely sealed from the outside. Over time the internal wiring will begin to break down due to the light fixture's high internal heat. Disconnect all electrical power from the light fixture before entering the enclosed area of the electrical connections. Visually inspect the HPS bulb as you unscrew it from the porcelain socket. A burned-out light bulb will show a dark black carbon tint to the interior high voltage element. Check the internal connections to the porcelain socket itself. The metal connectors can burn out prematurely from moisture entering the sealed case. Also look for any signs of arcing or burn marks on the metal screw connectors of the light bulbs. Marks in this area may indicate moisture leakage. If these marks are present the sealed case may have been compromised. Find the leaks and seal them with a high quality silicone sealant.
Remove the heat shield from in front of the electrical makeup area. There is a large aluminum box that will conceal the wiring, ballast and a capacitor found in most HPS fixtures. Inspect all wiring for any signs of brittle or burned insulation. These signs will indicate a high amperage draw on the ballast. In these cases the ballast has internally shorted out. Remove all wire nuts from the incoming voltage wires. Check the connection to the low voltage side of the transformer. Ensure the correct feed voltage is connected to the proper multi-tap transformer connection. The typical low voltage taps are 120 VAC, 208 VAC, 240 VAC and 277 VAC. Replace any burned portions of the feed wiring.
Remove all wire connections to the transformer on both the low voltage and high voltage sides. Use a volt ohmmeter and check the transformer for continuity to the low voltage side first. Switch the meter to the “ohms” position. Insert the red lead into the “ohms” connector and the black lead into the “common” connection. Touch the red lead to one of the low voltage feed wires and the black lead to the neutral wire on the transformer. The meter should read between five ohms to 15 ohms. Any higher or lower reading and the ballast is bad. Perform the same check on the high voltage side of the ballast. The ohm readings should be in the same vicinity. The ballast should be clean in appearance. If any signs of scorching appear on the transformer wires, the ballast is burned and needs to be replaced.