I'll give a shot at reversing the leads and see what happens.
uppo: It is a DC supply. I have a picture of the board if a visual look at it helps - I noticed this new board does not have a secondary winding (plug X19) and had to use a different plug to connect to D1; could this possibly be the cause of overvoltage?
https://www.dropbox.com/sc/ds2zubdmnz4nr4v/AADDVPseCpHnYxOonEN2YVlsa Ok mate from the picture the new board does look different than what I've seen regarding a AMD drive 2. The D1 plug supplies the original power and from there to x19. I also notice, as you said, there is no x9 primary plug. So I am assuming the x6 is the direct 24DC supply, so I would confirm that supply is good, by removing it from the board and testing the voltage on the plug. If it over like you said then I would check why as you may have a floating neutral or something screwy like that.
