Sony PCG-F420 Primary User Manual page 56

To supply power to your computer while it is connected to the port
replicator, you can connect the AC adapter directly to the port replicator.
✍
Turn off your computer and disconnect all peripherals.
On the bottom of the computer, slide the port replicator connector
cover open.
Attach the bottom of the computer to the port replicator connector
until it clicks.