TARDIS tuner

The TARDIS tuner was a device, described as "exclusive to the Time Lords, which was owned by the Fourth Doctor. When he and Romana where trapped in the steel dungeons of the Turgids, the Doctor pulled out the device and used it to stun the Turgid guard. He then sent their coordinates to the TARDIS. As they left in the time ship, Romana asked where they were going next. The Doctor responded, " Anywhere Romana! As long as we have the TARDIS Tuner!" (COMIC: Dr Who and the Turgids)

The Fourth Doctor later used his tuner to make contact with his other incarnations when the TARDIS was under siege by an pandimensional entity. (WC: Doctors Assemble!)

The Doctor of a parallel universe had a TARDIS tuner in his TARDIS, which had "mind blowing volume control" and a laser light control switch. Bernice Summerfield used it to connect the Ark of Whispers with the communications of a planet. (AUDIO: Have I Told You Lately?)

Merchandise
In the real world, the TARDIS tuner was a Doctor Who-themed radio. Released in 1979, it was presented as "the amazing Dr Who radio for all space kids". The TARDIS tuner boasted "mind blowing volume control", a "built in radio receiver, picks up radios 1, 2 & 3", a "laser light control switch", "constant flashing laser lights", a "radio tuner for crystal clear reception", a "time warp bleeper control switch", a "tough moulded matt-black casing" which "stands up to the heaviest landings" and a "sliding door for battery supplies".

A TARDIS tuner toy was featured and discussed in the 2013 documentary DOC: The Celestial Toyroom.