When it comes to fixing guitars, amps, and cars, I am completely self-taught. I also have a lifetime of training.
According to my folks, I started playing with batteries, wires, and light bulbs when I was about six years old. Within a few years I was taking apart old TVs, radios, toasters, and anything else I could get my hands on. Eventually, I was able to put things back together and even have them work. I can't recall exactly when I got my first soldering iron, but I do vividly remember the burns. Considering that there wasn't much adult supervision, I'm amazed I survived.
In 1980, at thirteen, after being the third-closest to guessing the weight of a derby-winning salmon, I came into a small windfall and was able to buy my first guitar...electric, of course. It was a so-so Japanese Les Paul copy but before very long I had replaced the tuners and pickups, added coil-splitting and phase switches, and built fuzz boxes, phasers, and auto-wahs to go with it. My first amp was actually a small tubed table radio that I modified to take an input. I built my first "proper" guitar amplifer a few years later - a massive 120W solid-state head with reverb run through a huge Peavey vented 4x12 cab. I built a similarly huge amp for my band's keyboard player and he had the identical cab... At our first and only show, the crowd of our classmates quickly retreated to the back of the church gym to avoid going deaf.
My self-assigned high-school shop projects were always related to guitars and electronics. In my late teens, I even "boutique manufactured" an onboard preamplifier module kit that was sold (in single-digit quantities, alas) through my neighborhood music shop, Bill Lewis.
A mild obsession with vacuum tube gear followed and in addition to guitar amplifers, I eventually got into collecting and repairing tubed radios, stereos, and test equipment.
A not entirely-unrelated thread in my life as a "handy guy" is working on cars. My first was a basket-case Triumph Spitfire that I bought before I could drive and then fixed up just enough to be able to use it for my driver's license test (the examiner was not thrilled), two days before I drove my grad date to the ceremony (her dad was not impressed, either). The cross-pollination of interests eventually lead to a DIY programmable electronic fuel injection/ignition conversion on my 1973 BMW 2002, a car that has been my reliable daily driver since 1996.
As I approached middle age, my skills and interest in the non-electric aspects of luthiery grew until I was doing re-frets, neck resets, and more involved repairs in addition to the setup work I had always done.
Although my formal education and work life has taken me in many directions (I've been a paperboy, janitor, deli meat slicer, hardware store clerk, restaurant manager, camera salesman, furniture fixer, marketing guy, tech-support dude, QA manager, wedding photographer, assembly-line droid, espresso machine mechanic, web-monkey, and I even did a short stint as a BMW/Porsche/VW/Audi grease monkey), I've always kept my hand in, working on my own and friends' guitars and gear. Finally, after decades spent trying-on many ill-fitting hats and much soul-searching about what to be when I grow up, I decided to return to my youth and just do what I have always loved. So, in late 2010, I got serious and officially hung out my shingle as a Guitar and Amplifier Repair Technician and here I am.
Happy in my work, at last.