OK...Have replaced both 4051s, and am now seeing the proper ins and outs from IC18, as well as IC 10.
IC 19 seems to be behaving itself also, and for the moment, I'll assume things are right with IC14 pin 1.
Discouragingly, the behavior of the synth is still essentially the same. The only difference seems to be that the Bank D Program 4 lights are permanently on now, no matter what else lights up. It will still not complete the first calibration reset step, and will not enter manual mode.
It seems as thought the signals for controlling parameters are being confused for program change signals.
Here's what I am noticing regarding the odd behavior of voltages coming from the control pots.They seem to swing from -5 to +5, but most of the pots fall back to -5 with an audible click as soon as they pass the threshold to plus values. This then scrambles the current "patch", and changes it to another, or to noise. They can be coaxed to +5 with some control twisting, and even sometimes "repair" themselves and act normally, but only until another pot is moved and "fails". A few seem to work normally all the time. This behavior can be seen on IC 19 and 18, as well as the outputs and inputs of the ICs they feed.
Something connected to ground that shouldn't be?
I guess I'm dealing with a bigger corrosion problem than is obvious from the apparent condition of the circuit board, and maybe I should just start rebuilding on one of the replacement boards, but am glad to keep digging into this one, if anyone can give me a direction to go.
At this point, I'm curious about what I should be seeing at IC33 pin 4, or on pin 1 of IC27. Right now, it shows nothing there on my scope.
As always, I'm grateful for the help, and feel I am getting somewhere, but seem to be creating more questions than answers.
Glad to see this post stirring other discussions within the group.