Quoted from ddunlow:
I'm in need of a little help. I have a switch that is failed. It's the Ball Trough 1 switch.
If you've verified that the switch itself works, and that the diode works and is on the correct tabs on the switch, then the next thing to do is check all the switches in the same row and column as this switch. The column wire is the red-green one, and the row wire is the white-orange one. A break somewhere in the line on one of these two wires could be causing your malfunctioning switch. Are you sure this is the only switch in the machine that the computer doesn't recognize?
If you're certain that this is the only switch that the machine doesn't recognize, and that there are no wire breaks, the next thing to do is to break out the schematic and follow the trace from where the white-orange and green-red wires connect to the interconnect board from the playfield. Make sure you have connectivity all the way through the interconnect board into the main board.
The trace from the green-red wire should end up at SR13 before going on to U40. U40 has half of its pins connected to the switch matrix columns, and the other half of the pins go right into the PIA, which is part of the main brain circuit of the whole thing.
The trace from the white-orange wire should run through SR11 before ending up at pin 9 of U30. For both of these traces, do the best you can to make sure you have connectivity where it should be. If you find the trace coming from one of these two wires doesn't have connectivity, check the other, similar traces to see what their state is.
If everything looks OK up to here, then it's time to bust out the logic probe. Connect it to the 5v test point on your board to give it power. You may want a friend to help you trigger a switch while you put the probe against various pins on U40, U39 and U30 to see what happens when you trigger various switches. If you have connectivity all the way to the input pins of these chips, then you should see some kind of reaction on the logic probe when you trigger a switch. It might pulse, or it might just read as high. I don't know enough about this off the top of my head to say for sure.
The point is that you're looking for variant behavior when your problem switch is activated versus literally any other switch. You want to make sure the behavior on the input pins are consistent across several swtiches in the game, including your problem one. The same goes for the output pins on those chips. Those chips are talking to other chips, and if one of those chips doesn't behave properly when a switch is pressed, then that will definitely cause problems.
Give all that a shot and see what you can discover.