Williams did a great job of laying out their switch matrix so you don't need to look at a manual to determine what columns or rows switches are on. The first number of the switch represents its column location, and the second number represents its row. This is why there are no switch numbers starting with 0 or ending with 9.

Switch 53 = Column 5 Row 3
Switch 33 = Column 3 Row 3
Switch 47 = Column 4 Row 7
etc...

The column issue is either U20 or a broken wire. A logic probe is insanely useful for working with the matrix as you can touch it to U20-13 and know instantly if you are generating your column 6 pulse.

TerryB's guide to logic probes:
https://pinside.com/pinball/forum/topic/terrybs-guide-to-logic-probes

Buy the probe:
https://www.amazon.com/Elenco-Electronics-LP-560-Logic-Probe/dp/B000Z9HAP4/ref=sr_1_1

Once you have one and know how to use it you will wonder how you ever got along without one

Take a good look at your Column 6 drive wire on pin 6 of J206/J207 to make sure its ok at that point. You can also verify your column 6 drive by disconnecting your column and row connectors from the MPU at J206/207 and J208/209 and J212. You can then momentarily install a jumper between J207-6 and pin X of J208; if the column 6 drive is present, you will see a switch closure for each switch in column 6 corresponding to the row you jumper across to on J208.

My apologies, I initially read this as a column 6 issue. If you are jumping a column pulse from J207-X to J209-7 and don't get switch closures in row 6, then this points to an issue on the MPU. You will want to take a REAL good look at the MPU for battery corrosion and ohm out the connection from J208 to U13; If you have battery corrosion the most likely break will be between U19-14 and U13-4 as there is a transition from the top side of the board to the bottom side underneath R31. If all of the connections check out fine, then the most likely component failure would be the LM339 at U19.

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