On the upper control board to the left of the voice cards, pull all of the following TL084 chips that are socketed, carefully clean the pins with a pencil eraser and reinstall.
I like to use the big artgum erasers you get at a craft store or artists supply store, but if you have a nice, new pencil with a soft eraser, that will work too.
On the upper right quadrant:
That should do it. If not, let me know.
Scott in Ohio