Glad you got it sorted out. I know I got confused by some of the details, but you also did a lot of good troubleshooting steps and that helped us get to the solution more quickly.
And for what it’s worth the reason I was wanting to look at the head is from the pictures you did provide it looked like a lot of wear on the head, and I wondered if you were just getting to the point of reaching the service limit…I can’t quite explain why it would work and then stop after multiple plays, but when you followed up stating no record or playback, I figured a faulty head coil was likely the issue…the picture would have likely supported the theory.
Anyway, cool to see two machines up and running. You must use a 246 pretty regularly…clearly know your way around the guts. Anyway, glad we could help and stick around!