Makikihalo lang ng konti. Kung 100 volts yang L-505V (Jap domestic model), meron yang adjustment for 220 volts sa loob, tabi ng transformer. The unit should be unplugged when you make the conversion.
In my case, I slid the switch from 100V to 220V when I got my unit over 10 years ago -- tapos ayaw tumunog! I then uncovered the 100v/220v switch plate, removed the wires connected to the 100v terminals and connected them to the 220v terminals.
After completing this process, I realized even the two outlets at the back of the unit are already 220v. Unfortunately, I realized I have already 7 vintage Luxman amps last year so I sold my 220-volt L-505V at a price slightly higher than my acquisition cost.
Here's the undressed unit (yung merong white arrow ang switch)
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Closer view of the switch