K9 — occasionally written as K-9 or K•9 — was the designation given to a series of intelligent, dog-like robots who served as companions of Professor Marius, the Fourth Doctor, Leela, Romana, Sarah Jane Smith, Luke Smith, and Starkey.
K9 Mark I, II, III, and IV addressed whoever they were given to as “Master” or “Mistress” depending upon their gender, and used the formal “affirmative” and “negative” rather than “yes” or “no”. They were programmed to be both loyal and logical, with a penchant for taking orders literally, almost to a fault. Their striped collars mirrored the Fourth Doctor’s trademark scarf. More details of the K9 time line can be found here.
In the script, K9 was simply described as a ‘personal Komputa [sic] which is like a tin dog on wheels’, and was envisaged by Bob Baker and Dave Martin to be ‘performed’ by a smaller actor in a dog-shaped suit. The effects designer assigned to this story, Ian Scoones, did a series of preliminary sketches, the first of which looked somewhat like a fierce armoured doberman, but due to the heavy workload of miniatures requirements for the serial, he had to hand the final design and building of K9 to his assistant, Tony Harding.
Harding had only two weeks in which to draw up plans for the machine; his main brief that it should not look as if there were a man was inside. This meant that ‘the Dog’ (by which the production crew affectionately referred to K9) had to be remote-controlled.
Harding’s first design sketches were not that different from K9’s eventual look. From this a rough form test model of the basic shell was made, After assessing the different requirements the Dog needed to perform for the story; e.g. tail wagging, head movement, gun and probe etc., it was realised that the motor for these functions would have to be housed in the head. The original moulded head therefore had to be made larger to incorporate all the electronics for this. Once the final shell was made, the electronics could be fitted. The mechanics of K9 included rotating ear-probes, telescopic ‘eye’ probe, extendible nose, flashing lights on the top and the ‘eye’ panel, waggling tail antennae, and ticker-tape tongue. The head could also move up and down. The shell was painted in gold/grey metallic. Around K9’s neck was a tartan collar, from which hung a silver disk. Surprisingly there was nothing engraved on the disk, not even K9’s name!
K9 was operated by effects man Nigel Brackley using a radio-control unit. The chassis housed his battery power source, servos, and motor. One of the two steerable front wheels was connected by the chain drive to the motor. One major problem with K9’s radio-control was that it operated on the same wave-length as the studio cameras, causing several delays in the studio when K9, receiving jumbled signals from the cameras, went in the wrong direction and crashed into the set! K9 was originally driven by a chain-drive. Unfortunately, this was rather noisy to the extreme that composer Dudley Simpson was forced to compose loud incidental music for scenes that featured K9. One example of this is in The Sun Makers, where K9 is ‘sneaking silently’ up to a guard. We, the audience, can hear the loud whine of the motor, and the crashing music while the guard stands completely unaware of the ‘danger’ behind him!
In terms of TV continuity there have been three separate K9’s but in reality, there has been only one, but which has been repainted and re-built several times over the years. K9 Mark One was written out of the series in The Invasion of Time, staying behind on Gallifrey to look after mistress Leela. There was an ulterior motive for this. The special effects designers wanted to rebuild K9 during the recess between seasons. It would have been a cheat to not explain the difference in K9’s appearance in the opening story of the following season, so a simple scene where the Doctor produces a box containing the components for a new K9 was added to the end of The Invasion of Time.
While the shell remained the same, the interior was changed. This new K9 was given a rubber belt in place of the chain drive. Valuable studio time was still lost, however, because the drive-belt tended to snap.
While the cosmetic changes were minimal (including the new metallic charcoal paint-work), in terms of series continuity K9’s character underwent a vast change. Gone was the product of 5000 AD technology, as was the case with Marius’s version. This new K9 was a product of the Doctor’s own technology and values. The gun was now only capable of stunning – not killing – and K9 now responded to a dog-whistle. K9 also featured a vast memory-bank, seemingly a mixture of the Mark One model, the Doctor’s own memory and knowledge, and that of the TARDIS.
Also built around this time was a light-weight duplicate shell. This manageable prop was a necessity for scenes where K9 was to be carried by actors because the motorised version was far too heavy. Although the Mark Two K9 had a stronger motor, on its first location filming it still had to be either placed on hidden tracks or pulled along with string. So much for an improved drive-system!
The second K9 lasted until the end of Season Seventeen. The main operator was Nigel Brackley, who handed the task onto Mat Irvine for Season Eighteen. Before recording for the new season started, Irvine and his assistant Charlie Lumm undertook a complete overhaul on K9 which was why he was written out of the main story to allow time to work on the machine.
The entire inner workings were stripped once again and over two weeks a new drive and power system were installed. The new mechanics of K9 included larger wheels in conjunction with a front and back-wheel drive, while the radio-control was updated to be more reliable.
A year later, K9 was given a brand new coat of paint (this time a metallic-blue) plus a handle on the top of the shell to make carrying him easier, for his appearance in the K9 and Company pilot, A Girl’s Best Friend.
K9’s distinctive voice was first provided by actor John Leeson. For The Invisible Enemy Leeson’s voice was treated with a modulator to give it a more mechanical lilt, but for subsequent stories he was able to provide the required distortion naturally.