A while ago one fader stopped moving. When moving it data was sent properly and other faders linked to it followed properly. Incoming fader data/automation was audible but fader didn’t move, so I suspected a motor break down. After a late arrival of my final spare part included in the deal – a fader – I replaced it, but without success.
After some investigation by a S12 tech newbie (me) I finally found something. Under the eq/aux panel I found four vertical cards attached to a bigger, also vertical, card. On one of the four smaller cards, on the failing channel, I found a red sensor light that was lit up all the time. The same lights on the other cards only lit up when moving faders. Swapped card with another channel and problem moved with the card.
Just ordered a new card, called Fader amp card, hopefully arriving next week. Will post again when installed. Cheers!/Paul
Edit: Those hand written crosses on the red connectors ‘could’ indicate earlier problem on this refurbished part, don’t you agree?! 
Glad to have it out of the way…
Hi Paul!
Thank you for these info!
Good to know that you've eventually sorted it out! Nice also to see activity on this board. ;-)
All the best to everybody!
Maikol
Thanks Maikol!
Got that card replaced and it's finally working.
Hi Paul and everyone !
Just a note to say that I've had exactly the same problem with one fader stopping moving, and it turns out it's also the fader drive board that is dead. Fortunately I still had a spare new one, so I just replaced it. It might be time to get in touch with the guys at Harrison and order some stuff... well that is if they still have some ! :P
Cheers,
Maikol