EDIT Feb. 2018 : I’ve eventually found the culprit ! I’ve added the solution at the end of this article.
We have experienced very strange behaviours on our beloved S12 recently.
We sometimes had no input from channel 1 to channel 16 right from the powering up of the console. Some other times the sound was entering the channels, but not exiting them (no routing was working). Some other times it was the total opposite: sound was present even when the channels were muted!
Everything was pretty much unpreddictable, by the way…
As it was only on the first channel carrier (ch1 to 16), I checked the processor card of that carrier, and then every channel card.
I found that, on channel 16′s assignment board, one condenser was not good looking (it had leaked).
So I replaced the whole assignment board with a spare one, and that seems to have solved the problem.
BUT, as the problem was very unpredictable, I’m not yet sure that it is definitely solved. We’ll see in use if it stays OK!
EDIT Feb. 2018 : this wasn’t the solution to the problem, as it happened again after changing the assignment board. I eventually found the culprit several years later (!), while doing some cleaning :
After I had removed every channel cards and the brain card from the ch1-16 carrier, I noticed that the backplane (the big card on the bottom of the carrier, with the connectors which the channels plug into) was not tightly secured on the rack chassis, as a number of screws had gotten loose. So I tightened them all correctly and thought “wait a minute, some of those screws are used to bond the backplane’s ground to the chassis, so maybe this is the source of the problem ?”.
And bingo, after I had put every cards back in place, I started the console, and no problem with ch1 to 16. This was last year (2017), and the problem hasn’t happened a single time since ! 
What was happening was that depending on how bad the connection to the digital ground was, sometimes the serial data was not received correctly by the brain card (it needs this ground as a reference), and the corresponding channels would not start correctly. I also understand now why sometimes just pulling some cards out and then pushing them back in would temporarily solve the problem : it was just pushing the backplane on the chassis and re-bonding the digital ground for a while !
So here it is guys : if you’re control panel acts weirdly on startup, check those backplane screws ! 
Hope this helps…
Maikol
Good work! And a little scary thinking about the life span of the caps in the S12, regarding the number of them...
Agreed!!!
Although, I was really surprised by the overall very good appearance of all the components.
Of course that doesn't mean they're not going to fail, but they look very clean and as new for quite every of them. After 15 years of use that's not bad!
Now, compare this to any "analoggy controlled" desk, you'll find that we are lucky non moving audio parts users! :-)
I hope you got that problem solved!
I'm a totall electronics idiot - do you guys think it's possible to recap a S12? Exchanging all caps for example. Not that I wanna do that but it's something I was thinking about ...
Yes it sure is possible to recap it (the caps are pretty common), but that's going to take loooooooooong time! :-/
I recapped a trident 65 console i had in about a week ... of course the S12 has waaaaayyyyy more caps though
Hi everybody,
Just a note to say that I've updated the article, as I eventually found the solution ! :P
Cheers,
M