Check for a broken wire or diode at every switch on that column. Make sure to give each diode leg and wire a light tug to make sure they're firmly connected. The fact you're getting at least 1 switch to register tells me the driver board is likely ok.

Oh shoot i didn't realize they're optos. There's usually a board or boards for the optos--it's possible you may have a bad solder joint or broken wire at the board(s)... I'm sure someone more versed in wpc games will chime in.

But yeah, I'd still track down each switch and make sure the wiring is firmly attached etc.

First rule out the MPU as being the problem. Here is a good guide about troubleshooting the switch matrix for WPC machines - http://homepinballrepair.com/index.php/digital-probe-part-2/

I had an issue recently on my STTNG which turned out to be a cold solder joint on one of the wires on the row I was having issues with.

This is either going to be a broken wire or a failing 16-Opto board, obviously checking for a broken wire is going to be easier than troubleshooting the board, so lets start there:

Each switch contains a pair of optos, one LED board (transmitter) and one Photo Transistor board (receiver). All of these optical switch board pairs are part of a larger assembly (Borg ship, VUK, Gun, etc...), these assemblies all contain Molex type connectors as they were designed to be dropped in connected to the main wiring harness with no soldering. This is important because any single wire that would effect large portions of the column would have to be broken at the molex connector on the wiring harness where they daisy chain to the next connector; there are two daisy chained wires that can do this, these are the Ground (BLACK) wire to the Photo transistors and the +12V (GRY-YEL) wire to the LED assemblies.

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On the 16 Opto PCB assembly, do a visual inspection of C6 to look for bulging or leakage, this can also appear as a darkening of the PCB around the capacitor; failure of this capacitor can cause many opto issues and is something that should be replaced if its still the original.

Good Luck.
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