Wow!  I've never seen a fuse do that before!

Just asking a silly question, is the correct SB fuse in place?

If you have a DMM with continuity setting, you can buzz the tracks from the fuse connector to make sure they are OK..

I can only think that heat - and lots of it caused an issue like that.  Are you getting the correct voltages?

We gotta stop meeting like this Marty!!! I thought +1 about fuse rating but checked and is ok. I buzzed tracks and ok. A sparky said to me poor connection can cause such heating but he's no pinball tech. Tha's why I'm thinking about improving the connection with a wire as opposed to the track alone. The pcb has one of these on the other end of the fuse holder see picture. Unsure about voltages as the moment