Exact same problem

working on a friend's HP d530s after a storm it wouldn't power on anymore. I tested it and sometimes everything is fine and at other times some lights on the 20-pin connector don't light up. I'm wondering if it's human error on my part. I tried a regular 500watt atx P/S on the motherboard and the green light stayed lit but it wouldn't power on. When I put the original P/S back in the green light flashes and no power (which was the problem to begin with - so no change). I even replaced the battery. The fan even runs. Can someone verify if replacing the power supply solved this issue? I wouldn't call myself a novice, but I'm no expert either. Thank you.