Denis,
First verify you aren't shorting to ground somewhere on the start button by unscrewing its and just let it dangle. If no change then disconnect the top right set of connectors from the MPU before you boot. Try starting game after it boots. If success then you'll need to narrow down which switch is faulty.
Another odd thing I've seen is a 'bad' display can mess with the machine. Try booting with all displays disconnected.
Mike V