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…