As other suggested, every system is different. I would also sugggest understanding the Bally games from 77-85 that used the -17 and -35 boards and once you can repair them, move on the something like System 11 or even Whitestar.

Start with pinwiki and it would also benefit you to completely understand the test modes of various games among different companies if you do not already.

The way that the boards are designed are certainly different from the various generations of boards, but there are more similarities among companies than you may realize. Understanding the choices they made back when companies had a specific budget to accomplish the simple tasks of controlling a switch or solenoid or score display or sound board is the key to knowing what is happening.

In reality, the latest pinball systems are essentially small PC’s with specialized outputs and inputs to match up with running solenoids and lamps. So, it probably would not benefit you to learn those systems unless you want to learn how a PC motherboard works.