I'm not STERN expert,but know well Williams/Bally/DE/SEGA.
After little investigation from the manual you have Q4 at J1(Switch Columns) for the switch matrix which is 2N3904 and after that go to U1(74LV273).
2N3904 can be test easy-with DMM
Connect the multimeter’s positive probe to the transistor’s base lead(middle leg). Connect the negative probe to the transistor’s one time at to left leg after that to right leg.
After this connect red probe to left black to right.
And after that red probe to the right leg and red to the left leg.
You must not have values under .4 or full short .0
If you have under .4 your transistor is bad and you are lucky.It's cheap and easy for replace.
To be sure on 100% you can compare the values with the other transistors in switch columns-from Q1 to Q8.They are same and muse have same values
If not then may be U1 is bad.
Also trace the connections from J1(5 pin) to U1(9 pin).And from J1(5 pin) to the and of the wire whatever it end under playfield.May be to have broken wire somehow.
This can be do again with DMM.Get both probes no matter which one ant touch them on each of the begin/and trace.If you have connection you must hear "beep" or .001 or .002 lie values.
Do this on shut off machine.