There is another similar thread going in Unrelated and there was one in here as well, maybe you may want to try searching for them?
Japanese power is 100 V. US power is 110 V.
Japanese systems and electronics are likely designed for Japanese power variance, meaning at some point the system may be getting as little as 95V and possibly 110V at any given time. The 110V would be like a power surge and not something that would be constant.
You also have to remember that there is variance in the US as well. At some point you may be getting 110V, then as little as 105V then it may surge up into the 120V range. If your house or whatever gets these fluctuations and you are using a Japanese system, you may:
1) Fry the system
2) Notice abnormal system operation (disc read failures, system powers off for "no reason" and so on)
How long have you been running your JPN PS2? If you've been running it for any length of time without problems, then you're probably OK.
HOWEVER there are certain devices/systems that aren't as forgiving. The Neo Geo CD is supposedly a very fussy unit and is likely something you would want a step down converter for.
Long story short, if you want to ensure your system doesn't get fried or damaged, then get a step down converter. If you want to take the chance and have to rebuy your system, then leave it the way yo uhave it.
What to keep in mind is that you also get something with the proper wattage. Check the device you are plugging in to find out what wattage it is running. I think the PS2 runs at 100W, so you would need a stepdown converter to go from 110 to 100 with a MINIMUM wattage rating of 100W. You can also get something with a larger wattage rating (150-300), all this tells you is what the MAXIMUM draw on the converter is. So if you plug in a 100W device and a 150W device and have them running at the same time, it would be drawing 250W within the 300W range.
