LOL!! Whats old?? Im 48 and will turn 49 this year.
I didn't have " a day" and the only way I would know how ops maintained their machines is by some of the operator handbooks ive acquired from some of the games ive bought.
I do have a couple of these from the EM/SS period ( 78-79etc) and they do make interesting reading.
It would seem that once a week the takings were taken from the machine... each time the tech went to a machine he wrote it down in the log book ( never the $$ amount though..LOL) at the same time, any minor faults were rectified.. bulbs, rubbers and at the end of a log it nearly always says...  gave the PF a clean with XXXX.
Locked on coils etc were phoned through by the establishment where the game was housed and tech was on his way... fault fixed, game a wipe over and off he goes. But this is the EM period... once we go into SS and beyond, its mainly a board swap and fix back at base. Problem with modern day pinball is that I don't think anyone carries spare boards and it would appear that most if not all faults are not board related, mainly mechanical and component degradation.

In all honesty, I don't think Techs today are what we would consider a tech should be. We live in a world where people make sandwiches and are labelled " sandwich artists".
I recently had to replace a transformer to some neon in an arcade that's situated inside a shopping centre here in Brisbane. Im up a step ladder with my head inside the roof cavity talking to the "Tech" whos on the floor... he says hell go and get some tools in case I need them as he has nearly every tool you can imagine. What he came back with was an Ozito cordless drill and a mish mash of screw drivers from the $1 shop.
I thanked him and continued to use my own tools