Scott tends to a damaged dilithium crystal converter assembly aboard the Enterprise in 2268
The dilithium crystal converter assembly was the engineering component aboard Constitution-class starships responsible for the primary energy-production activities.
In 2268, the USS Enterprise's dilithium crystal converter assembly was sabotaged by Kryton, an Elasian working for the Klingons. Montgomery Scott discovered that the converter assembly had been completely fused as a result of Kryton's actions, making any use of warp power impossible until replacement dilithium crystals could be found. (TOS: "Elaan of Troyius")
The dilithium crystal chamber and articulation frame of Galaxy-class starships appear to be a 24th century equivalent to this component.