I'd like to offer up a few things I've learned over the years. You didn't specify SS or EM, and I'm an old EM guy, but I think these apply most everything.

You didn't say if you had a VOM. If you don't, get one and learn how to use it. Even if you have to start with a $7 Horrible Freight meter. Of course, the first time it lies to you, you will want to smash it with a hammer and go get something good.

Learn the basic math of electricity: I=E/R. It will serve you well. Learn what parts are. What looks like a transistor could be a SCR, or a triac. Assume nothing.

When you first open a machine, DON'T TOUCH ANYTHING! Do a detailed visual inspection. Look for things broken, burn marks, parts missing, work done by lesser technicians (we like to call those "pecker tracks").

Start at the power cord. Is it in good shape? Then to the fuse holders, they usually catch hell. Make sure you have good clean power coming out of the transformer (EM) or power supplies (SS) before you go after anything else. Flippers and lights will wait.

Listen to no one, trust no one. "I already changed that" or "I already fixed that" means nothing. That machine is your baby now. Do your own troubleshooting.

As with all types of machinery, there are repetitive failures: This machine won't do that so it's this part. This is not a magic gift from the troubleshooting gods, it's from experience. Though the internet will help you there.

Don't be a parts swapper. Shot gunning is not troubleshooting. Just replacing parts until it works is an expensive way of fixing things, and the sign of a lousy technician. Figure out what's wrong, then fix it. I realize this can be a bit more difficult in the SS world, where computers come into play.

As LTG said, don't over think things. KISS-Keep it simple, stupid. That light is not lighting up because it's not getting power, not because the planets are out of alignment or alien magnetic fields. Stick with the basics.

Don't be discouraged by the occasional set back. You will accidently let the magic smoke out now and again. We've all done it, we all still do it. Carry on.

I hope my ramblings help. The fact that you want to learn gives you a pretty good head start.

Hmmmm...the force is strong in this one.

Look at it this way: You get a machine with the player four scoring display not working. So you order up a new display board. No help. Well then the computer must not be sending the score to the display. Order up a new computer. Nothing. It's got to be the power supply. Nope. How much money have you spent to find out that the last jelly head to work on the think yanked a wire out of a connector on the harness?

Another possibility. You've got a shorted regulator in a power supply. Your 5 volt supply is now putting out 15 volts and cooking everything it touches. How many computer boards you want to fry before taking a meter to it?

Troubleshoot first, replace parts later.

Not to say that it's a bad thing to swap out a part if you have a KNOWN GOOD working spare. But don't take anybody's word on that either. (We're back to that "Listen to no one, trust no one" thing). I worked with a guy who's favorite trick was to take the old, bad part, put it back in the box, and put the box back on the shelf with the spares. Ask me how many times I wanted to strangle that SOB.

Yes, I've done some IT. But I started in medical equipment repair. I learned 'meticulous' by changing the wiring harness in a servo-ventilator. I had the guts of that thing scattered from hell till breakfast. Took my time, very careful. She went on to help others breathe again.