tautech, thanks for the reply! When I tore it down yesterday, I did clean and re-seat the ribbon cable from the mainboard to the keyboard PCB but that didn't help. I am not sure how to know which board the issue is on, the mainboard or the keyboard PCB.
It seems the keyboard PCB has a pair of 8-bit shift register ICs (HC595 series), a pair of 8-bit analog/digital multiplexer/demultiplexer ICs (74HC4051D), and a dual 4-bit binary counter IC (HC353 series). My guess is that if anything is bad on the keyboard PCB it would be one of these ICs (since the passives appear isolated to a particular knob/button), but I haven't done a full trace to see which set is handling the rotary encoders yet. The 16-pin ribbon cable connecting the mainboard to the keyboard PCB seems to only have 8 unique signals (power and ground are two of these unique signals, taking 4 and 6 pins respectively), leaving 6 pins for actual IO through the cable. So I guess what I'm trying to figure out, again without a scope to validate, is what "half" of the IC and signal sets is dealing with the rotary encoders? I assume that they are logically split that way because all of the buttons still work, but none of the rotary encoders do. Though, even with that information, there might be some mainboard processing between the buttons and rotary encoders that is distinct as well - so how can I isolate which PCB actually has the problem before replacing parts?
Thanks!