If you can: Put in the original CPU, see if that works. Just to make sure it is not a fault with the Juno-66.
You posted a picture of the bottom of the PCB, but most of the connections are on top, traces on the bottom are just going to the DCB interface which is not used any more. Can you also post a picture of the top with the installed Juno-66 ?
I attached part of the schematics. are all the data and control pins of IC55 connected to the Juno-66 ?