Right after ordering the 74LS174, I noticed a very small deformation under high magnification on a trace near FB12, which connects U29 pin 8 to ESP (U21) input on pin 1. The first photo is after I removed the defective area from the trace and exposed some copper to run a jumper wire as see in the second photo. This corrected the main issue with the outputs and I was able to repair all other minor issues with the SD-1. Lesson learned here (once again) is to inspect EVERY trace in the circuit for even the smallest deformation before assuming there is a failed component. Before I removed and cleaned the problem area, it was only the size of a pen dot and a darker green than the trace around it. I confirmed no continuity with my DMM before digging into the trace to repair it. I’m so glad I didn’t have to go through the trouble of replacing any chips! Whew! 
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