Replace the LM339 and the 100 uf Cap

Get out your schematic and figure out which pin on the 16 opto board is your row 1.

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I would defiantly start with the capacitor, but one thing you will have to pay close attention to is if there is the integrity of the thru holes the capacitor solders into. The dielectric in a capacitor is corrosive and will destroy these contacts just as a leaking battery does but at a much slower rate. This corrosion can break the connection of your 12V which is your reference for the LM339s that control the column reference for the other four on the board.

Getting the correct one can be a little tricky as there is no single LM339 dedicated to row 1. I wouldn't replace any of the LM339s without doing proper checks first, most will require a logic probe, but a lot can be done without one.

Start by Checking for 12V at pin 3 of all LM339s as well as Pin 1 of J2 and J4, and also check the reference voltage going to both of the comparators used to develop the column reference voltages.
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https://www.pinballlife.com/16-opto-replacement-board.html?Category_Code=

$99.95 and a bonus is the ICs are socketed to make future faultfinding easier.

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I think DumbAss can provide a very high quality board at a lower price.

Depends how its bench checked, did they happened to send you a breakdown of the tests performed?

Isn't it FANTASTIC that we actually have several options these days. I can remember when, if you couldn't fix the board, that was IT! Your machine stayed broken until someone parted out a machine and you were lucky enough to actually get a used board.

These days we have several options for this board and most are superior to the originals. The Pinball Life one I referenced is under $100 - simply amazing!

I am glad that somebody else was thinking what I was thinking. Every opto board I have built and sent out passes on my bench and nobody has mentioned any problems when received. Testing equipment is important!

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100% agree. Choice is good for the consumer. It also means that the consumer should do more research on what's available and weigh up the pros and cons of each solution. Of course, that's not required. Just like getting a second quote with a different contractor or a second opinion from another medical professional. It's 100% up to the buyer how much effort they want to put into their purchase.

If the OP (or anybody else) is interested I have two versions of the board available. The original Williams circuit (with ICs in sockets) or a re-designed circuit with discrete transistors and LED indicators for the transmitter/receiver pair.

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Please use PM if interested.