A Type 50 TARDIS was the TARDIS that the First Doctor used prior to his life as a renegade Time Lord. (AUDIO: Prisoners of Fate) The Doctor decided to steal another TARDIS, a Type 40, when he finally departed Gallifrey with his granddaughter Susan Foreman. (TV: The Name of the Doctor)
The Type 50 was angry and hurt it had been spurned by the Doctor when he stole his Type 40, an inferior model, and called it miserable and dilapidated. The Doctor would later claim he had to take the first available TARDIS he could find.
The Type 50 TARDIS later escaped Gallifrey on its own, suffering damage in its escape as it attempted to go after the Doctor. The TARDIS had to jettison 9 tenths of its interior to escape, and travelled to Valderon, where the Doctor encountered it during his fifth incarnation in 3556. (AUDIO: Prisoners of Fate)
The TARDIS was in pain by the time it re-encountered the Fifth Doctor again. Sibor had discovered the TARDIS's capabilities of looking into the future after it had granted the people of Valderon with the Miracle, where everyone could speak with a different language. However Sibor decided to use the TARDIS for her own gain, putting it in a prison for 20 years and giving it just enough power to stay alive.
During this time the TARDIS just wanted to return home, but it generated a strong telepathic field to find someone who could repair it.