A franking machine has an ink jet printing device with exchangeable ink cartridges. The lower housing shell of a security housing of the franking machine has a sealable, shaft-shaped opening in the base and is designed to accommodate assemblies that can be contaminated with ink. These assemblies are situated in a non-secure region of the franking machine and are accessible via the aforementioned opening when a cover of the shaft-shaped opening is opened. A servicing procedure allows the franking machine to be retrofitted to print with a different ink color. After moving the print carriage in one direction and opening a cartridge flap, the ink cartridges can be removed. A displacement of the print carriage takes place after closing the cartridge flap, wherein the print carriage is moved in the opposite direction so that the print carriage arrives in the sealing position for ink cartridges. A box-shaped mod can now be removed from the franking machine. Servicing of the ink-contaminated modules subsequently takes place via the aforementioned opening.