Dell OptiPlex 740 DCSM Quick Reference Manual page 12

4 Connect the speakers.
NOTICE:
To avoid damaging your computer, set the manual voltage-selection
switch (on the back of the computer, if your computer has a voltage selection
switch) for the voltage that most closely matches the AC power available in your
location.
NOTICE:
In Japan, the voltage selection switch must be set to the 115-V position
even though the AC power available in Japan is 100 V.
NOTE:
Your computer's power supply may or may not have a voltage selection
switch.
5 Verify that the voltage selection switch is set correctly for your location.
Your computer has a manual voltage-selection switch. Computers with a
voltage selection switch on the back panel must be manually set to operate
at the correct operating voltage.
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Quick Reference Guide