I seem to have caught the ill-fated U20, U14 chip illness that makes switches stop working. I had a single column of switches out and replaced U20. This didn't fix anything. I then socketed and replaced U14 and the switch column worked fine. However, other switches stopped working. I reinspected my U14 soldering job and found out I was lacking continuity for a few pins. I fixed this and fired the machine backup. Now everything works except for the set of four "way out of control" switches. They do not detect during gameplay but in switch edges test they read "Ground Error." I have ground errors on rows 1, 2, 3, and 4. I have read something about Opto Board under the playfield leading to this but my opto board has no chips on it that I can see. I just read that the U18 chip does this: "Rarely the CPU board's LM339 chips fail too, where U18 controls rows 1,2,3,4, and U19 controls rows 5,6,7,8 (but replace U20 first followed by U14/U23, and then look at U18/U19 last)." Any ideas would be greatly appreciated. Thanks! Is it possible I shorted the newly replaced U14 chip?

I would rate all of my skills as a little better than knowing enough to be dangerous . I'm 18 and still learning about pinballs and electronic repairs. I didn't diagnose U20 as bad, it was already socketed so I just popped a new one in.

Fixed it! Took the board out and cleaned all the flux off with some alcohol. Re soldered questionable connections and I'm up and running. I think I had so much melted flux on the board that I wasn't
getting a good connection. Thanks!