With the bottom plate off, looking underneath, you’ll see where the PSX cable plugs into the PCB, and the chip on the PCB. You’ll see two screws holding the plastic piece with the turbo switches. Remove those two screws, and lift the turbo piece up from the top; the PCB is attached. Once its remove, BE CAREFUL when separating the PCB from the plastic turbo piece; there isn’t really anything holding the slider switches in, and if you fumble it you’ll never find all of the switches again.
If you’re problem is just that one button, Im telling you now that replacing the entire PCB just isn’t worth the trouble. Even if the problem is with the wires, it’d be much easier to fix that problem than redo the whole damn thing.
First, figure out which of the two wires is the ground wire. Use a multimeter, and figure out which one is connected to the black wire coming from the sanwa harnes. Mark that wire with a sharpie.
Next, figure out which one of the wires on a nearby button is ground. Pick a button near the problem one. We’ll call this other working button the test button. Use the multimeter, and figure out which is the two wires for that test button are ground. Mark it with a sharpie.
Now, figure out if the problem connection on your bad button is the data wire, or the ground wire. With the stick plugged in in a training mode for some game, touch the ground wire of the bad button to the data wire of the test button. If this works every time to try, we can be pretty sure the ground wire on your bad button is good. Next, touch the data wire of the bad button to the ground wire of the test button. If this works every time you try, we can be pretty sure the data wire of your bad button is good. Lastly, plug in both of the wires from the test button into the physical bad button. Press the button a bunch. If that works, we can be pretty sure the actually button microswitch is good.
Congratz, you just tested both of the wires and the physical button.
If the button looks to be the problem, plug both of the bad button wires into that test switch, and press it a bunch. If it works fine, then you can be certain the problem is the button. Buy a new sanwa button to match, and replace it.
If the ground wire appears to be the problem, look to see if the QD on the end is coming loose. If so, replace the QD and repeat the tests. If the wire works sometimes, and moving the wire fixes or breaks the problem, its a bad wire. Run a piece of wire from the test ground wire long enough to plug it into both buttons; or try to remove the replace the length of wire at the pcb (much messier because of the hot glue). Problem solved.
If its the data wire, look to see if the QD on the end is coming loose. If so, replace the QD and repeat the tests. If the wire works sometimes, and moving the wire fixes or breaks the problem, its a bad wire. Bad wire would mean you have to either cut the wire close tot he PCB and connect a new length of wire, or desolder and replace the wire at the PCB (messy cause of the hot glue).