The invention relates to a device (10) for operating in a water chamber (7) of a heat exchanger (1) comprising a wall (6) of substantially hemispherical shape and, at its upper portion, a tube plate (5) traversed by a plurality of vertical holes (4). The device comprises at least one rail (11) furnished with members for attachment on the bottom face of the tube plate (5) and supporting at least one trolley (15) that can be moved along the said rail (11) and fitted with a pendular lifting means (19) and a robotic arm (20) comprising an attachment base (30) furnished with members for coupling on the bottom face of the tube plate (5) and a drive means interacting with the lifting means (19) to draw the attachment base (30) against the tube plate (5).The invention applies more particularly to the steam generator of a pressurized water nuclear reactor.