Surely we are talkking J3 on the solenoid board here - top right hand .1 connector. If things blow with just J1 or J2 connected you have serious problems!!
BTW, U1,3 and 4 are not strictly IC's, they are transistors in an IC package, hence my comment on testing them with the diode function. I have never seen U2 go bad, but I guess anything can happen in pinball.
Looking at Q3, I would say the knocker coil is cooked!! Go through everything methodically on the weekend if you have time.
cheers
Ian
