Accuphase E-280 Voltage Conversion (100V to 120V)

Just buy a step-down AC transformer. No need to go poking around inside!

I have but I still want to change to 120V

Since you have a step-down transformer, you can confirm its transformer taps by power up the E-280 at 100VAC, using a DMM set to AC voltmeter, measure the voltage between yellow - orange, and between yellow - black, you should get both reading of 120VAC.

Swap the two wires:

RED <---> ORANGE

BLACK <---> VIOLET

That will convert the E-280 from 100V to 120V.