Tom thanks for your post, but it is not possible to post a drawing or a picture.
Imagine a 60 years old flipper with the bottom part full of relays , a motor operate cam switches and under the top 2 big solenoids to operate the fingers and next to them contacts the ball will close when hit.
I then decided to disconnect all the relays and the cams, but I cannot avoid the finger solenoids and another pair of them to restore some flags.
Counters are in the bottom upper part and I think they will be not a problem.
Doing that the question is limited to 4 solenoids using 30 v AC related magnetic fields. From the power point of view arduino and the solenoids are on separated electrical circuits.
I hope now that using shielded wire for contacts and condenser and resistors for the solenoids I will fix the problem. It will take some time to me to test because the flipper is not in my home.