I was a little apprehensive about unplugging too many cables inside the M3500, so I still have to fix the broken pre-amps. I need to probably fully immerse myself in the manual so I fully understand the single path.
Have checked out the PS?
I mean...I would rule out any issues there first...that way, when you start checking voltages and levels, you know that ther PS is providing what is needed/expected.
<EDIT> OK, looks like you checked the PS.
The PFL...mmmm...god, I never use that, I just use solo.
I wsould have to check my manual to confirm if you are using it as design intends...but then, you have the manual too so I'm sure you will sort it out.
You do have a few CR options and monitoring options...so make sure you have all the right buttons engaged.
...you want the test signal levels from all tracks to be the same when you play back the recorded test tone. It was a problem for me because some channels appeared to be dead or have greatly diminished levels. It turned out that the group cards were the culprit, not the channel cards.
Yeah...if you had odd levels with the GP outs...I can see how that would be a problem.
Before I got all my outboard pres...I use to track through the 3500...but that was way back in the day, when the console was still relatively new, and worked to spec.
I'm actually quite happy that it's held up for 25 years without any major issues...but I can hear the "age" more and more.
Pots/faders getting too scratchy, too stiff, too soon after cleaning, pan pots losing their center "click" detent, EQs "chirping" when pushed too far on the + side...etc. Nothing that stops the show, but obviously things not top perfect spec and just getting old.
For my personal use, I can work around all that easy enough and not have any negative impact on the music...but if I needed to run in a more commercial environment, I would want all that resolved. which I'm sure would start with a full recap and overhaul.