Raymond,
The 7A26 has a number of small ceramic trim caps of a design that has proved to be prone to going open circuit. You may have failures at the same point of both channels, unlikely as that seems. I would suspect C1432 and C2432 in particular, since they seem likely to affect the highest frequency signals. Try turning them: if they have gone open circuit turning them will have little to no effect. If those are not the problem, try others!
--John Gord