My Batman Forever recently decided it did not want to turn on other than the GI and sound. After troubleshooting it, we discovered that a leaking capacitor at C2 on the power board was the culprit. Apparently, according to Pinwiki, this a common problem with these boards.

Just wanted to give you a heads up to be sure and check once in a while. Mine was fine one day, wouldn't turn on the next. After leaking for awhile, the traces can become severed and the problem gets more involved. Luckily, mine hadn't made it to that point.

http://www.pinwiki.com/wiki/index.php?title=Data_East/Sega#Power_Supply

See section 4.7.1.

Cheers.

Out of curiosity, how much was it to replace the power supply?