I can relate a story, a bit long, but bear with me; years ago, I was about to run a vibration survey on an aircraft propeller. The equipment was a Chadwick Helmuth 177a:
When preparing the machine for use, I noted the test function was not working,
I forget exactly what the symptoms were.
I brought the unit back inside the shop, and and was helplessly perusing the voluminous schematics, hoping to recognize a fuse (OR SOMETHING) that might be at fault. About this time, the head of our small avionics department came by, possibly drawn by the 'smell' of a schematic.
He Asked what the problem was, and then flips through the manual for a minute or two, settles on one page, and points to a component buried in this mess of lines. "There's your problem" he said. "That zener's open"
"Bullshit" is what I replied. "No way"! This whole exercise has taken maybe 5 minutes. Well, he looks up the p/n of the "zener" and says he probably has one for his bench work repairs, in stock.
I'm still not convinced, but get the Bosses' approval for some 'in house' labor. The tech opens it up, unstacks maybe four boards, and locates the culprit. It is replaced, and the box re assembled.
A quick run on the calibrator to check it, and it's good as new. I relate this story, only to illustrate that there ARE people who can think on this level. I personally believe such technicians are like the "Guild Navigators" from the old "Dune" SciFi books. They operate on a different level.