Re: dot matrix
My first job was with a credit union that used two part NCR paper for their receipts, printed on an Okidata 320 printer. those things were (and still are!) tanks.
In the time I was at that job (~two years) I learned how to do a complete tear down and rebuild of the original model (in all of it's iterations) and the (new at the time) 320 turbo model.
The most common thing to fail on those were the print heads and that finicky white gear clipped to the stepper that turned the main feed roller- on the really well used models, there was a visible line where the stepper gear engaged the white gear, with the teeth of the white gear getting all chewed up. Fortunately, those were cheap parts, so we bought a bunch of them and as we got the older ones in for repair that turned into a standard step in the process. the reason why it was finicky was because the gear was held on to it's shaft by three fingers, and if you were careful with an exacto, you could get the old one off without breaking them. (had to do that for the occasion where the printer had a total failure in one of the controller boards and we broke them down for spare parts.)
That was twenty years ago. even though the credit union's turned over all the customer facing hardware and use thermal paper for receipts like the rest of the world, I'm sure there's a handful of the buggers still chugging away in a back office somewhere...
Beer, because I feel old now. :)