You might have everything your need right under your nose to figure out what is broken. You have multiple of every parts on the board, except the logic chip LS374. The same circuit is used 5 times. I would disconnect J115 , J119 , J120 and J121 if anything is connected to them. Then , using the schematic I would compare readings on the broken line with readings on a line just below and see what differs. All parts should test very similar.. Probes on R20 should be very similar results to probes on part R9, R15,R14,R17,R11. Same for R21,R9 and all the 10k in that column. Then the transistors Q17,Q9,Q13, etc.. Diode test on the transistors.

Once you identified a possible broken part (The triac mostlikely) you can swap it with the triac in the line below and see if your issue moved, and if a working triac is enough to fix it. It's not impossible that the current went back up to U1 ,the LS374 which drives these triacs. Just as you did for each line , measure pins 2,19,5,16,6 on U1 with a voltmeter , from pin to ground. They should all be very similar. With power off , you can measure continuity between the U1 pins and ground , to see if the chip grounded.

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The question I now have is how was the fuse checked ? With a meter ? Damage is not always visible by eye on a fuse , so always test with a meter. I have more experience with electronics in general than with pinballs , so if phishrace thinks the triacs rarely go bad, I would take than in consideration. Not sure the test you did is correct on the triac though. To test the triac with a multimeter you'd need a jumper to close the gate and trigger the triac for one polarity, then switch the test for the other polarity. I guess this could be done while in circuit if you disconnect both J115 and J120 to make sure both MTx pins on the triac aren't connected anywhere so that your reading is correct. You probably don't need to remove the connection to the resistor R19 , because to test you would short the gate with one of the triggers , bypassing that resistor.

You don't need to desolder anything if you want to isolate the issue though. I made an image of everything you need to see to figure out the issue. With a multimeter and if you go methodically, you can isolate it pretty quick I'm sure. The very first thing is double checking the fuse and making sure you have voltage be J115 pins 3 and 8 (AC voltage) then verify again between J115 pin 8 and J120 pin 7. With those voltages present , you need to investigate the triac driving circuit which starts at the logic IC 74LS374.

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Definitely possible that a bad connection acts as a fuse when a lot of current passes through. It's the same principle as the fuse itseft.. Wires too small to pass a lot of current that burn.

Glad to hear it's fixed!