sweetbeats
Reel deep thoughts...
I can't recall if the M-512/M-520 has bottom panels that can be removed. Can you tip yours up and take a look or does anybody know off-hand? Its been years since I've had my hands on an M-500 console.
Basically at this point you're going to need to access the guts, either from underneath, or partially remove the 4-channel modules that have the poor functioning channels, and ideally use an oscilloscope to probe for test tone you have running through the channel, and use the Block Diagram and schematics to find a point where you should have tone and you don't and start tracing back until you have tone. That's the point at which you likely have a faulty connection or failed component. As I mentioned earlier you can also strategically and carefully manipulate connectors to see if you get lucky and find a failed connection that you can either, as needed, resolder or exercise and treat with contact cleaner. I'm sorry to say it, but you are into relatively high-level technical troubleshooting at this point, at least the kind that calls for a tone generator, an audio-grade multimeter, and an ability to read and follow schematics and trace out the problems...no silver bullet, no magic elixir.
Basically at this point you're going to need to access the guts, either from underneath, or partially remove the 4-channel modules that have the poor functioning channels, and ideally use an oscilloscope to probe for test tone you have running through the channel, and use the Block Diagram and schematics to find a point where you should have tone and you don't and start tracing back until you have tone. That's the point at which you likely have a faulty connection or failed component. As I mentioned earlier you can also strategically and carefully manipulate connectors to see if you get lucky and find a failed connection that you can either, as needed, resolder or exercise and treat with contact cleaner. I'm sorry to say it, but you are into relatively high-level technical troubleshooting at this point, at least the kind that calls for a tone generator, an audio-grade multimeter, and an ability to read and follow schematics and trace out the problems...no silver bullet, no magic elixir.