although the dimensions and colour of the tardis used in the series have changed many times, none of the bbc props has been a faithful replica of the original mackenzie trench model. this has been explained within the context of the show that chameleon circuits tend to display a bit of "drift" if left in the same setting for too long, and in any case the circuit of the doctor's tardis is malfunctioning. this was acknowledged in the episode "twice upon a time" which had the twelfth doctor meet the first doctor, and had the original incarnation and bill potts point out the visible differences between the modern and original tardises.