Well the +12 and -12 lines are probably for op-amps and comparator cirucits in the video amp. circuits. If they aren't reading right, then that would cause a video problem. Your hard drives run on the 5V line so that shouldn't be a problem. Are you checking the voltages out of the power supply with the GND on the power supply cable? Also, how many cables go into the connector? It could be possible that each power line has its own GND line associated with it. Then you would need to read between these two to get the correct voltage out.
