Step down voltage for Japan to US turntable

Transformers don't limit the voltage, they only change it by a fixed ratio. If you buy a 120-100 then that ratio is 6-5. If you feed it 125 it will output 104.2V

Since the voltage in Japan varies as much or more than it does here I think that is acceptable.

Many times the internal transformer has multiple taps, which allow you to use it with a 120 or 220 volts mains. This should be a simple job for any decent technician and save you the hassle of yet another gadget in the system.
On the other hand, if you are buying an adapter the make sure that it is NOT an auto-transformer. It would then behave as an isolation tranformer and there are benefits to having such a gadget...

What I wonder about is the 50hz. Typically motors use the line frequency to establish rpm. Does it have a strobe light or other method of adjusting speed?

4est..it's a 50/60hz unit. I'll use a strobe, and it's adjustable.

Penny-wise and dollar -foolish!!!

Use the correct model for your county's voltage & frequency. X-former conversion represents a definite DOWNGRADE in audio performance. Manufacturers are really starting to crack down on "gray market sales", and are NOT obligated to repair units purchased in one country, to be used in another country...either under warranty or not!

Yeah, they aware of the currency exchange game, as well!