While I had all the cards pulled I naturally brushed out cobwebs and hit the edge connectors with contact cleaner, so it was only logical to fully reassemble the unit and see if a good cleaning did the trick. No change.
Next step was to narrow down point of focus. For brevity sake I'm just going to observe the behaviour of the VU meter on power up and power down, since in consulting a friend who has a very functional version of the same machine I was informed that meter deflection of any sort in response to the on/off switch was unusual. So I swapped cards 4 & 5.
No change in 5-8 deflection at power up, though now #4 was pegging the channel 5 meter as a steady state while the natives along 6-8 returned to zero. Quickly power down & confirm card was seated correctly. I pull channel 4 card & channel 6 card to compare, they are, for all intents & purposes identical (beyond the fact that they carry factory stickers indicating their home channel).
I then install the channel 4 card into channel 6 slot, placing channel 6 card into channel 5 slot. 5-8 full deflection tap and now channel 6 is pegged. Power down.
While channels 1-4 have not been considered suspect, due to them actually functioning (and bolstered by the fact that channel 4 has in fact recorded to tape and passed the listening test, which I would like to think is pretty stringent). I then swap Channel 3 with #4 in 6. 5-8 full deflection tap and channel 6 remains pegged. Power down.
At this point I overlook a step that could have saved me a little time and effort. It should be obvious that I am not a factory trained technician, nor am I intimately familiar with the workings of this machine (getting there). I'm pretty much working in the dark with my empirical approach and a set of freely downloaded schematics.
Make no mistake, I am brimming over with a sense of gratitude toward the good people at AnalogRules.com, for without these highly useful documents I would quite simply be fucked. However, there came a point at which deep circuit analysis turned into a hide and seek sort of affair due to a lack of clarity in the scan. So yeah, I'm pretty much just making this process up as I go along.
So, my next step was to remove all the cards, set 1-4 aside as known functional and install 5-8 into the 1-4 slots. Anyone care to guess what happened?
Nothing, unless you count 1-4 meters deflecting fully on power up. I run signal test to confirm a complete lack of function.
Based on this observation (and brownie points to whoever catches my mistakes here) I shift focus on channel cards 5-8 themselves, and ignore for the moment the steady pegging of the low number cards in high number slots (another hint). I had deemed the probability of discrete failures on the four cards, right next to each other, as highly unlikely, but it's worth a look anyway.
All my analog ICs are looking a touched charred (this batch is among the cleaner looking). Granted, there's patina on the analog ICs in channel 1-4 too, but I'm not paying them much scrutiny, as they are not problematic. Incidentally, all the digital ICs are looking quite fresh & nickel plated, so it's a metallurgy thing, perhaps there's a high content of silver in the legs. Whatever.
So, channel 8 is my guinea pig, I've plucked the chips and go to work.
600 grit sandpaper and a little time, before and after shot here.
The ICs cleaned up pretty well, flushed the sockets with contact cleaner, reseated the ICs into channel 8 card and sink that channel into channel 8 slot - with no other cards with the exception of the logic control. Power the machine up. Minor deflection on channel 8. The card passes sound!
It is at this point that, flushed with a sense of progress and ready for a break I happily report to my magnificent and wonderfully patient wife that I've finally secured what appears to be a solid lead amidst a convoluted mess of red herrings. HAHAHAHA, oh the humanity.