I once had the pleasure of reading Analog Circuit Design: Art, Science and Personalities, where Jim Williams talks about how his mentor at MIT had him fix broken test equipment to get a better understanding of practical electronics. I dimly recalled that as I wrestled a 50-pound HP 8673 Synthesized Microwave generator onto my workbench today. No, they don't build 'em like they used to.
The unit started behaving oddly, would put out a signal with a frequency offset. Strange... so I thought I'd give it a shot, got the service manual and opened 'er up. It's not every day that you get to crawl in to the mind of a genius, but man, this thing's built like a tank and had such detailed instructions on what was going on inside. I followed the service manual, isolated the fault to a malfunctioning YIG-tuned oscillator (YTO). I pulled out the YTO boards one by one, and marvelled at the elegance of the designs. Then I found a burned-out capacitor, and figured I'd swap it out for another one. Oddly, I hesitated quite a bit before applying a soldering iron to the board, I felt like I was violating some sacred artefact, the work of a true engineer, the kind that wears a pocket protector. Anyway, after replacing the cap, the synth came up perfectly, and has been working since.
While I didn't learn a whole lot directly, it did help to see how things were built by HP.