Pro-Kit/Seck 6-2 mixer
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This cost £99.99, which seemed cheap at the time, though despite two decades of inflation you can get better mixers for less nowadays.
The adverts made a big deal of how you would save money by building it yourself, and gain an insight into mixer design. In fact the circuitry was really textbook op amp gain and equalisation stages, and hissy old 741s were used, presumably to enhance the profit margin (or achieve the sub-£100 selling point).
Having built it I then took it apart (with some difficulty) and fitted IC sockets to the circuit boards so that I could fit gourmet low-noise op amps to some of the channels (as funds gradually become available).
Later the manufacturers started selling bigger versions of this desk under the Seck name, which continued the tradition of straightforward design and execution.
My previous attempts at building a mixer having ...uh... mixed results I took some tips from the ProKit circuit board and made a six channel mixer for the drum machine, so that the individual drum sounds could be panned, faded and have effects applied separately to them. I compressed the layout a bit and dispensed with the faders, using rotary pots to make things simple.
This time I managed to avoid the fingerprints on the copper and worn out etching fluid problems which poverty and inexperience had imposed on my earlier efforts, and the thing worked as intended.
Later I built a twelve channel mixer based on the same design which had an internal power supply and hummed like a bastard. The pots were as close together as I could get them so that it would all fit onto an A3 size sheet of aluminium (which was the biggest I could get mailorder).
I only figured out several years after scrapping it that the toroidal transformer was inducing eddy currents in the face plate (to which both it and the pots were earthed). I can't remember if the Instant Automatons used this live - I know it saw active service at Meanwhile Gardens but I think that was later on… Had we carried on it would doubtless have found its place along with the Green Chipboard PA System in the sell-out tours of our imaginary future.
Or ended up dismantled in my cellar, as it is now…