In my job, it wasn't full time and it was a University research setting. Many years ago there was "electronic PM" type work that had to be preformed on instruments and older computers (pre IBM PC) that were used to control experiments broke. Gauges and power supplies broke and we used a base of "spares" to keep the critical equipment running.
Pieces that we had a lot of, I learned to fix and I could do it quickly. Sorenson switching Power supplies and gauges would basically require recapping and various minor problems.
Computers were isolated to the module level and the module was sent for repair at a "fixed cost".
I built and designed custom interfaces and automated measurement systems. I assembled "stand-alone" temperature controllers. I found easier way of doing things. I designed safety shutdown systems.
I even learned to fix an early "Scanning Electron Microscope" to the component level. The thing had lots of 741 OP amps in it. I converted a microcomputer based card to operate stand-alone with a multi-channel analyizer. That crazy think kept blowing too and I called the manufacturer on it about the lack of "transient protection". Their comment was, the power is specified as 120 VAC +-10% 60 Hz. Your problem.
I learned to fix Mass Flow Controllers, because we had lots of them and they were easy.
An upgrade to an 1950's X-ray diffraction set kept popping RS232 drivers, so I found a solution to that.
And then there was this thing (CV-8)
https://www.vesco-usa.com/ves-prints.html This one could kill in a blink of an eye. 15 Kv, 1 Amp shunt tube regulator. Plus 1000 W tube RF transmitters and 100 kV @ 0.1 A power supplies.
A lot of the stuff I did, tended to improve reliability and reduce operating costs, but it wasn't full time. Paying for travel time or sending a piece of equipment out by truck to be repaired wasn't cost effective.
I don't think management realized what I really did. You can't see the effects of "equipment not breaking".
Recently, nearly all the newer equipment was becoming very difficult to service on your own. There was no documentation and parts were non-existant.