The video processor model VC2023 which replaced the original Ferranti ULA needs bodges to work correctly as it contained bugs. The bodges are done on the underside of the motherboard so it's not obvious without taking the board out whether the modifications have been done, and have been done correctly. This is from the BBC Micro Mailing list a number of years ago:
It was supposed to be a drop-in replacement, but the early ones (VC2023)
had a problem in that they didn't correctly switch settings between
teletext and non-teletext modes. To fix that, you had to remove a
jumper (S26) and solder a small wire between the VidProc pin 27 (=centre
of S26) and one of the select lines for the memory buffers (IC10 pin 1,
which is the inverted TTX teletext signal). The official fix put that
wire under the board, which caused some headscratching when people who
didn't know that swapped one VidProc for another (or for a ULA).
Statistics: Posted by BeebMaster — Thu Jun 20, 2024 12:08 am