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How do you figure out "by simple trial and error" which piece failed, why it failed, what the original piece was supposed to look like, and finally repair it, without expert knowledge and precision tools?For a typewriter? It's not even a challenge, 14 year olds have done bigger projects in their garages. And not even particularly bright 14 year olds - and not even today, with fast access to all kinds of information, schematics and such.
Typewriters? Teenagers have been fixing electronic devices, building custom radios, amplifiers, circuits, and such for over 60 years - with "look at the shelves at Radio Shack" level of availability (and much worse in developing countries).
I've fixed MIDI equipment on my own when I was 14, and I was hardly that bright or familiar with electronics and soldering guns, or had access to the (then non-existing) web. And to be frank, I've never bothered to delve into electronics even later (though I did had some engineering classes at uni).
>At a wild guess I'd expect it'd take me a few years to do this reliably
You overblow what should be a couple of weeks or a month at most, and that's for someone mechanics-inept.