Problems with the canons
Having put everything back together, replacing J121 in the process and inserting some new batteries, I turned the machine on and was met by some crazy canons which seemed to have a mind of their own and just kept on rotating. Fortunately I was prepared for this (having read up on the most common problems with a ST:TNG) and knew that if the opto in the canon wasn’t working (which it wasn’t on the left) that there was probably a wire harness problem. So, having removed the left canon covering, the opto pairĀ and unplugged the wires from under the playfield, I removed the wiring harness to reveal some very damaged wires:
So, having repaired these and put everything back together, I had a fully working ST:TNG. Wonderful!!