Accuphase A-80

Go to Amazon and buy a 120 to 100 volt stepdown transformer.

Get a good step down, go to acupwr.com. Make sure you check your max wattage the unit will consume. For instance mine was 350 watts so I had to get the 400 watt unit. They make units specifically for Japanese market items. I got one for my Luxman 507Z works great. Zero hum, and barely gets warm.

I mean I would like to learn how to modify it inside to convert voltage.

Pop off the top and there should be an internal switch.

All the best.