Switch matrix issues are always so tricky to nail down. In self test, there may not be closed switches because all are open, unlike when you are playing the game. Try to figure out when it happens while you are playing. Are drop targets down, other switches closed, etc.? I often find on the older games, it is easier to diagnose by playing with the glass off and trying different configuration of switches to see what happens.
Quoted from spandol:
Also, how do the flippers come into play with the matrix? I found when nothing was happening and I flipped the right flipper, this caused the game to add credits and a new player. And also the upper right sling is firing when the switches are properly gapped during gameplay. Looking at the matrix, I wasn’t sure how I would start except for checking diodes.
It could be that the EOS switch for the flipper is in the matrix even though it isn't shown. I don't know, i'm not familiar with the particular game.
It could also be that the vibration from the flipper is triggering another switch that is gapped too closely, and that completes the rectangle indicating a shorted diode. Try banging around the playfield with your fist and see if any switches fire when they shouldn't.
Also, it could be nothing is wrong with a diode, and you have a short to ground somewhere. You could inspect all switches, including on the flippers and make sure no wires or lugs are pressed up against lamp sockets or other metal fixtures. I find shorts to ground to be the most difficult. It is like looking for a needle in a haystack.
Lastly (for now), it could be a board problem. The way to check for that is in switch test mode, you disconnect the connectors in the back box that correspond to the switch matrix and jump the pins to try to trigger the switches one at a time. If all trigger correctly, then it is not the board, and it is something on the playfield. Again, I don't know enough about this particular pin to advise you on which connectors, and I don't want to mislead you. From the diagram, it looks like a combination of J2 and J3, but I don't know for sure, and I'm not sure why there are two in each column and row. Perhaps ask someone else who owns the game to confirm.
