Well thank you, Phreakout !

I (of course) read that page, but I guess I must have overlooked that paragraph.

The turning of the pots didn't work, so I'll measure the resistors and replace them if needed.

I think I will resolder the whole board, because, in the glimpse that I saw the board, the solder joints aren't as good as they used to be anymore.

The Mac isn't really important, so it can take a while before I go further with it.