Ricoh Aficio SP C400DN Hardware Manual page 30

Installing Options
Using both hands, slide the controller board completely out.
2
D
Place the controller board on a flat surface, and then loosen the four screws
to remove the cover.
The screws cannot be fully removed.
E
Be sure to install the SDRAM module as shown.
Two slots are provided for the SDRAM modules.
The default SDRAM module is installed in the inner slot.
To install an additional memory, attach an additional SDRAM module to the
outer slot, or replace the default SDRAM module.
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AQC650S
AQC660S
AQC670S