TARDIS tuner

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

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".

Dr Who and the Turgids
The TARDIS tuner was offered in the comic story Dr Who and the Turgids, featured in several early issues of Doctor Who Magazine beginning with Issue 2, which saw the Fourth Doctor making use of the device, which he claimed was "exclusive to Time Lords". Revealing the device to Romana within the dungeons of the Turgids, the Doctor used the TARDIS tuner first to induce sleep in a Turgid guard before sending a message to the TARDIS while leaving on the bleep signal to co-ordinate their position.

Due to this Wiki not acknowledging stories with a commercial purpose as being set in the Doctor Who universe, its narrative is not considered a valid source on this website.

Tying back to the DWU
Finally, in 2020, the TARDIS tuner was properly introduced to the mainstream, "valid" DWU by the webcast Doctors Assemble!.