On power up meters 5-8 deflect fully then return to zero.
On power down meters 1-4 deflect fully then return to zero.
Setting monitor to all input, audio passes through channels 1-4 while 5-8 has nothing.
So, my thinking is naturally focused on the split point between channels 4 & 5. With that in mind, and based on the observation that a lot of the CMOS gates used in logic direction are four per IC I started my analysis there.
As you can plainly see, that angle was not fruitful. There is not a single instance of component pairs straddling the 1-4/5-8 configuration. With that, I cracked the case and began to pull cards.
Here's the control card to go along with the (partial) schematic above. The fact of the matter is that this card does not have the same part number as the schematic I've been working from. I settle into the idea that I may have to resort to a trace by trace comparison to see if I can narrow down a culprit IC, but since I'm pulling cards now I may as well go for broke and thoroughly give everything a visual inspection.
So far all the cards & traces look good. I know, that just rules out the obvious...
...having been bit in the ass by the obvious after chasing the esoteric a few times too many that's where I like to begin.
While I have the motherboard exposed like this I do a continuity check on my pin 2/3 swap on the inputs and outputs of both channels. Everything measures up well.
Without really getting into photography it's difficult to demonstrate the look of backlit traces. Let's just say that evidence of pulls, shears or cracks do not exist. Everything passes on the visual level. One thing I do notice, is that the +/-15 volt supply rails (brightened in the picture) just span 1-4 and 5-8 (with a contiguous ground reference). This is the first point of a well defined 1-4 versus 5-8 I have observed on this machine, and it may prove useful later. But for now I simply apply power and confirm that the supply rails are delivering at all locations.
Next up, chasing a series of red herrings..