Oki PT390 Manuel D'utilisation page 17

Note: To prevent the adaptor from slipping out, the connector section is
designed to be tight to fit. When inserting, (1) pinch the base of the cable,
(2) while sliding the outer section of the connector upwards, (3) and
insert the connector until it locks in place with a "click" sound.
Base of the cable
Outer section of
the connector
(3) Connect the plug of the power cable to electrical outlet.
(4) Plug the other end of the power cord into the power outlet.
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