Pro tip: the burnt ribbon cable on a Nikon F3 in my Cincinnati shop taught me to stop guessing
I used to wiggle mirror boxes blind, chasing a stiff advance lever by feel alone. Then a customer brought in an F3 in March with a dead meter, and I found a tiny burnt spot on the flex cable only after I got a cheap USB microscope out and looked at 40x. My first guess had been the battery contact, and I almost charged the guy for a fix that wasn't the problem. Now I zoom in on every ribbon before I touch anything, and it saves me an hour a camera. What's the one tool you won't work without now?