Magnus Greel's time cabinet

Magnus Greel's time cabinet was an early experimental time machine from the 51st century which used zygma beams.

Origins
The time cabinet was developed by the Supreme Alliance. It was an unsuccessful device, as it had been based on Sa Yy Findecker's discovery of the double nexus particle, which, according to the Fourth Doctor, would lead mankind into a scientific cul-de-sac. It used the principle of a zygma beam, which if overstretched could cause a huge and catastrophic implosion. Its use could lead to severe and life-threatening mutations. It could be locked by fused molecules, and thereafter only opened by a "time key" in the form of its trionic lattice, a unique element which was central to its operation. (TV: The Talons of Weng-Chiang) Another Supreme Alliance war criminal would, however, discover means to reverse the degenerative process. (COMIC: The Time Machination)

Use of the time cabinet
Magnus Greel, the one-time Minister of Justice of the Supreme Alliance, used the experimental device when he was fleeing persecution — or prosecution — as a war criminal. This made him, as far as he knew, the first human to travel through time (AUDIO: The Butcher of Brisbane).

Greel arrived in 19th century China. He had to hide from Imperial soldiers for a long while, and his time cabinet was taken. George Litefoot's family received the cabinet as a parting gift from Emperor Tongzhi when they left China in 1873, which they thought it was a puzzle box. Litefoot was never able to open it. Greel searched for the cabinet for many years. His search led him to London, where Litefoot had settled after leaving China. Greel recovered the cabinet in 1889. He committed several murders mostly to harvest the life force of innocents to keep himself together, though this actually accelerated the process of cellular degeneration which had begun when he first used the cabinet. He was killed by this degeneration when the cabinet could no longer function correctly. The Fourth Doctor destroyed the time key, rendering the time cabinet useless. (TV: The Talons of Weng-Chiang)

Unknown to Greel, his time cabinet had nearly materialised in 1937 when Hsien-Ko attempted to generate a massive amount of Piezoelectricity to disrupt the zygma beam, but the Doctor and Romana were able to force the cabinet back on its original path by materialising the TARDIS where the cabinet was trying to materialise (PROSE: The Shadow of Weng-Chiang).

Jago and Litefoot once again discovered the time cabinet in 1968. (AUDIO: The Bloodchild Codex) When Greel's mind was later reconsituted by Guinevere Godiva in 1968 by using a Venusian crystal, he took possession of Detective Inspector Dave Sacker to continue living. When Greel was killed by Jago and Litefoot, Jago and Litefoot used the preprogrammed time cabinet to travel from 1968 to 1893 again. (AUDIO: The Final Act)

Behind the scenes
Dialogue in The Talons of Weng-Chiang indicates that the soldiers taking the cabinet served Emperor Tongzhi, whose reign began in 1861. This could be deduced as an earliest possible arrival date for Greel.

As discussed in DWM 475 by Alan Barnes, the story was originally written with the intention that "Weng-Chiang" would be revealed not as the human tyrant Magnus Greel, but as Peter Pratt's, as recently seen in The Deadly Assassin. Accordingly, the "time cabinet" would have been revealed as a new disguise of the TARDIS the Master escaped Gallifrey with at the end of The Deadly Assassin.