I think the AUDIO2 path to the microcontroller is used to measure the signal level for the 8.7 adjustment. From looking at the schematic, AUDIO2?is going to read the signal at the wiper of R27 and that's where you are getting a low reading. That would tend to implicate things upstream of that point. I think the good news is everything after the detector IC4 is in the audio domain.
One approach to take here is audio signal injecting back up through to at least the inputs of IC 6b & 7b.?You might be able to just hold a metal probe and note the levels of induced hum you hear (or see on the bar graph) as you work back up the chain. This technique can be used through the whole audio section.
Another idea would be to rig up a small audio amplifier to probe points along each stage. It might take a little bit of doing, but if the output stage of the QCX is working a probe wire could be attached to the wiper of the volume control to probe along the audio path. But, I would caution you to put a capacitor in series with that probe wire just to isolate any DC you might encounter along the way.
Also, this advice is given from a theoretical standpoint from my experience in electronics. I haven't tried it but I think it's a valid approach.
Again, good luck!
--Al?
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