Hey jakelmnop - did you every figure out this issue?

I just did the Boston Pinball LED swap for my Sinbad - also using Pascal's PI-1 X4 board. I came across this old post of yours as I was a bit confused by Boston's instructions regarding where the new PCB connects to A2J3 as I couldn't identify where A2J3 is located on Pascal's board! So your photo helped me realize what's going on (the A2J3 connector is installed at the the A2P3 connection on Pascal's board - you remove the connector, place the new Boston PCB on A2P3, then plug the A2J3 into the PCB).

My issue was similar to yours with the fuse to the flourescent displays blowing every four or five times its started up. I added a new fuse for the flourescents when I installed the new Boston Pinball LEDs just to be safe (I'm not sure if this fuse is still in use now that the flourescent display's are removed).

It all works fine for me!