Looks like a lot of folks are getting into the hobby and will be learning how to work on the machines. I thought I might share a few thoughts I believe could save anyone just starting out some time and aggravation.
1. Get the schematic and manual and spend time learning to read them.
2. Pay close attention to how things go together before taking them apart. Take photos or make sketches so you won't be staring at pieces later cursing at yourself.
3. Work on one thing at a time. That way, when it goes back together and something is wrong, you will know where to look. Leads to next point...
4. Always assume if something was working and now it isn't, you have somehow caused the problem. Think about what you were doing before the problem occurred.
5. Until you have a lot of experience, if it ain't broke, don't fix it.
6. Most problems are caused by one issue. Don't make things harder than they have to be.
7. If you fixed something and the problem is still there, don't fix the same thing again until you think it through. Your focus should probably be somewhere else.
8. Don't take switch stacks apart unless you absolutely have to. When cleaning switches, be careful to hold the stack together. If those spacers get separated, you could be in real trouble trying to get everything back the way it was.
9. Take your time and be careful! Think about what you are doing and what could go wrong beforehand.
If it sounds like this is coming from personal experience, you're right.
And the best for saving you a headache - don't assume the playfield won't fall on your head if it's tipped up while you're working in the cab. Ouch!