( 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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My friend asked me if I or anyone have any experience with this device?

My concern is that while it might "change" the incoming AC voltage, I am wondering if it can handle a high current peak.  It is rated for 540W @ 120V AC, 5 amp. So it seems like its ok for lower power amps.

I guess anyone can use a Variac and Voltmeter to achieve the same results.  So this looks like a convenience item.

I guess the other thing is that do you really hear a difference if you reduce the voltage on an amp?

https://www.youtube.com/watch?v=-gaSeS6Nu8w

Any thoughts?

Holy cow I found a Helstar Remnants of War CD and popped it in.  Of course during the title track I was loving the epic scream 
But then after that came Conquest OMG            I had it SERIOUSLY CRANKED   

Yes sir thanks

After forty something years I finally broke down and got a CD player  I love my cassettes but I was plumb wore out with the whole slide switch issue   
It would start acting up every few tapes and I would have to get a tape that could be recorded and hit the play+record switch a few times 

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So I went to my friend's junk store and dug this one out.  At first it was like Thing on the Adams family when I hit "door open" it would open and shut rapidly  but I learned to just repeat and sometimes it stays open.
When I saw the orange LED or whatever it was love at first sight 
I think it's supposed to have a remote but it's long gone.

Ive played three CDs from beginning to end and it's behaving so I'm pleased.
  I had tried to make the move to CDs in the 80s and wasn't overwhelmed 
  But I like this much better