GOOD NEWS!! Stereo Master A now works!Indeed. Your problems with your M-520 just keep getting more simple. Good deal. Okay. So it’s just a problem with signal passing. Before tipping it up on its side you need to get the console to where you can tip open the meter bridge. All the way. All the wires from the meter bridge go down through part of the structural tubing I think. But I think you’ll be able to follow bundle down and then those wires plug into connector on the right-hand motherboard behind the master section. So just start by seeing if you can see where the grey wire from meters 9 & 10 plugs into the motherboard, and if you can see it and get to it then you can get out your ohm meter and start testing for continuity across the wire, then from the meter end of the wire to the pins on the motherboard with the wire plugged in, and if that’s all good then it’s time to tip it on its side and just start tracing back testing for continuity. Just follow the traces. We’ll be able to fix this.
I started with tilting the meter bridge back, as you stated, checked the connections and continuity there. All good. Realized that from that PCB the wires came off it and fed into the hole into the master section. On a lark, I thought instead of removing the console from my desk and accessing the bottom, that I'd just try pulling it from the top like I did when I fixed the headphone jack that had fallen through...
Then I spotted the little 4 conductor white plug UNPLUGGED. Cleaned it with deoxit while I was there and VOILA, they work now! I'm pretty sure it happened when I removed it to fix that jack and I didn't see it disconnect.
Thank you Cory for everything and all your help and guidance. I'm glad it turned out to be something easy, but also glad that you were there to guide me if it wasn't! It also helps me realize that sometimes it's good to look at the simplest things first, like a channel isn't out, maybe it's just the patchbay or cable! (which was my channel 7 problem!)