I feel like I'm on to something here.
The signal coming from pin 4, IC 23 is smeared on my scope, especially when it is compared to the pin 10 signal. Not an inverse or any relation whatsoever.
Not only that, but I looked at the signals coming from the other side of IC 19, and I found that I could see the voltage change on all connections from IC 19 to such things as MG Level, MG Decay, KBD track, etc. The signal goes up about 70% on all of them, and then falls back to the lowest voltage. MG Freq, was the only one that didn't fall back to the lowest voltage.
When I looked at that side of IC 18....nothing on all parameters.
Did the corrosion get to IC 23? There are no external components in that part of the circuit to account for the smearing.
Thanks for the pointers, Bob. I'm glad I got out my old analog scope, and am learning from it.