Take a look at R78 and R79. The HRDx has a long history of channel switching troubles. You could just try re flowing the solder joints for these resistors and that may fix it. The resistors themselves might be bad. A lot of times the leads were over stressed and became open inside the component. I have replaced quite a few of those 5 watt resistors in the last few years.
Or I could be wrong, and you may have a different problem entirely. But it is an easy, quick fix to try.