What does it say under the Ylw & Blu wires?
Attention something.....

Can you show where the red and black wires from transformer are going?
If it were not adjustable I would only expect 2 wires on the transformer primary.
Or maybe the blue/white/yellow is the primary side.  Show where those go on the board as well.

Here you go!

( I found that both the transformer and the board were marked 100v/117v.)

Can you upload a pic of the transformer & board markings?

Dang, well there goes that idea. Any advice as far as transformer selection? While running it at 100v I measured 13.10v where the transformer meets the board.

Usually there are still some adjustments need to be made, like moving some jumpers on the board, or using different wires on the transformer.
It's possible they designed it to work for both voltages as is, but our modern line voltage is a lot closer to 125V rather than 117V.  So there is a danger of some components being subjected to voltages that are too high for them (the first thing I would be worried about would be the main filter capacitors).
I tried to find a schematic or service manual but there doesn't seem to be any readily available online.  That would show if there is anything that needs to be changed internally to run on North American line voltage.

Hi everyone! I just wanted to double check my logic before I accidentally blow up my amp.

My dad gifted me a Japanese Roland JC-20JT for Christmas. Because I live in the US, I thought I was going to have to swap the transformer but once I opened it up I found that both the transformer and the board were marked 100v/117v. So I shouldn't need to do a transformer swap, right? Or am I missing something? I will still be swapping out the 2 prong japanese power cable for a 3 prong.

I think it's primary purpose is to protect expensive vintage amps from modern higher line voltages.  Lots of older stuff was built for 115VAC, line voltage where I am is usually 124VAC.

https://www.brownbox.rocks/products/brownbox-by-amprx-a-stock

It avoids bringing full line AC to the front panel, safer and quieter.

Running full line AC beside the board risks noise coupling to the audio,
and/or interference to the tuner.

Ok guys ya know I don't know much but does this kinda raise some eyebrows why not put the fuse and switch on the primary side

But idk so that's why I'm asking 

That box in the lower left hand corner connected to the blue and white wires is the power button it's a tuner btw

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