For future reference in case board quits working or develops random errors -- photo in post #4 shows U15 = 74LS37 (M74LS37P). A 74LS37 does not have enough current drive capability to sink the current provided by R137 so the voltage levels at his pin 3 output can be outside of valid operating range. This can cause address decoding (device selection) errors. Someday you may need to replace U15. If it comes to this, use an MC3459 (hard to find), 74S37 or 74F37 to replace it.