Now that I do ham radio and electronics as a hobby, I found great pleasure in aligning and fixing my own equipment. I always felt that a ham which only uses commercial equipment is not what I wanted to be. My goal is to one day run my entire shack off homebrewed equipment.
Anyway, I had a very pleasurable experience today. I own a beautiful Hewlett Packard 5335A programmable frequency counter which has been helpful when aligning radios and which was an eBay find long time ago. The thing suddenly started to permanently light one of the trigger LEDs and the B channel stopped working. The self test indicated a problem with the frontend and so I opened the device and did a couple of measurements. Using those, I tried to interpret the circuit and located the faulty component, a FET. I identified a suitable spare part (salvaged from another device) and now the thing is up and running again.
What I am trying to say is that this pleasure of identifying, locating and fixing a fault in a complicated device is extremely pleasurable. In fact, when something defective comes in or breaks down, I feel happy in anticipation of fixing it.
I guess that's what they call the "Knack".