Understand the basics of the "oil drop follow test" as it applies to electrical circuit theory. This theory is not obsolete, and remains the backbone of electronics today. The logic is easier not harder to see, just different components are used to present the functional operation.

The means owning the full schematic, know how to read it, and understand designed components. All electromechanical pinball machines and devices work under the same premise of startup and operation (with a few minor differences based on manufacturer). If a person does not have a schematic it's like fumbling in the dark without a flashlight, and very common error.

This means get paper copy that can be read without scrolling on an Ipad. More harm can be done by adjusting parts than not at all. Inspect first, function check second, clean third, if needed. Assume nothing. Adjustments are made last.

A person will learn more in 30 minutes with a seasoned tech, than a month by themselves. Search them out, we are around.