Building a mixer is much simpler than a tape recorder.
I built the chassis of my first mixer out of aluminium L corners, with just a handsaw, a hand drill and a screwdriver.
For a tape recorder, one needs a lathe and a mill, and it requires much more precision.
I built the machine when TV and production studios were literally giving away old 2" video machines, I would show up with a truck, pull off any usable parts(it was in an Ampex video frame) and sell the remainder to a scrap yard.
I had no lathe or mill and hand milled the working(and reliable) lifter assembly(and anything else that required it) that was driven by one of those cylindrical solenoids ampex used in the video machines. All the motors and guides came from old video machines.
Unfortunately there is only one pic of it and it's sort of off to side in a pic of me during a session...it was massive!
That machine sounded great!
I retired it when Herbie Hancock sold me his MM1200.
I have also built console sections, designed pcbs for existing consoles either to improve or modify some functions, I've built such things for Neve and API consoles.
There's a long list of crazy stuff I've built during sessions to meet immediate needs of artists, engineers and producers.
I have to talk myself out of building my dream console everyday...it doesn't pay and the clients do so.......but I still wanna do it even though I own vintage Neve, API and tube consoles...
If anyone REALLY is so motivated I either have enough Neve broadcast frame parts or know where to get more than enough to build a large frame console.