Quoted from Grandcheapskate:how does one gain the knowledge to work on and debug EM machines?
There are lots of good online information sites like:
https://pinside.com/pinball/forum/forum/electro-mechanical
http://www.pinrepair.com/em/index1.htm
https://www.pinwiki.com/wiki/index.php/EM_Repair
and many others.
I organize a free online repair clinic mentioned above where you can get help or just listen and learn:
https://pinside.com/pinball/forum/topic/an-em-pinball-online-repair-clinic
I also offer an in person repair class and remote help (not free):
https://www.funwithpinball.com/services
/Mark