Late to the hobby, my first "repair" two years ago didn't get past my attempts at troubleshooting a broken tethering harness wire. I accidentally fried the circuit board in the process! 
My first repair that actually fixed something only involved taking the shell off a trolley, unscrewing a few things, and poking around to understand how the reverse mechanism worked. It just needed grease. Guess we all have to start somewhere.
Oh, and then there were the actual "first" first repairs. Thanks to help on this forum, I was able to file and fix two defectively manufactured turnout switches. Not a very sophisticated repair, but oh so critical to train operation.
Lastly, the Gilbert S gauge engine I remember my father maintaining at Christmas was so much simpler than any thing I have now. When the engine was apart, he'd show me the can motor, the all important wire wrappings, the solenoid?, and so on, and explain how they all worked together. And then he'd actually fix something.
That reminds me, I have an overactive smoking unit to investigate ... Fortunately it shouldn't require going anywhere near electronic test probes! 
Tomlinson Run Railroad
