I finally had a chance to look at this and some of the conductive trace on the membrane tail had flaked off. Very easy to do if you are repeatedly plugging and unplugging the tail from the connector -- but fairly easy to fix with some conductive ink (I used a copper based ink from a car defogger repair kit). Thanks, Paul.
Both membranes are working fine but a few left panel board problems have cropped up:
Osc 1 PW, Osc 2 PW and PWM, Sweep Rate, Sweep Depth, and Keyboard tracking silders don't work.
Master volume slider at lowest point still has volume
filter cutoff slider curiously affects both cutoff and sweep rate!
My first thought is that the 4051 multiplexers are damaged but I swapped them from another left panel board and the same problem remains. My second thought is it's actually a right panel board issue but other than the single 4051 on the right board I'm not sure where else to look.
Every slider was desoldered and tested working or revived. Right panel sliders all appear to be functioning normally. I double checked the panel board interconnect and that is fine. I have a second complete top panel that works as expected so I'm pretty sure the problem is confied to the panel boards.
Any thoughts?