No photocell, i checked the bulb and wireing, ok. I 've got a 250 w ballast which uses a big 80 µf cap and this 600 w ballast uses a small 0,33 µf. Usually a ballast uses a 3 parts circuit : transformer, capacity and solenoid. This one uses electronic circuit with a big transfo, resistors, thiristor, little caps, diodes, and so on. Yes it's shorter to buy a new one, but if i could fix mine i would save 50 bucks. No one here knows the theory of ballasts ? I don't !
Thanks