I found that the BR1 was bad as it tested open, so no blown fuse as a more obvious failure. Replaced it, but still have no controlled lamps in either the playfield or the backbox light board. The fuse holders on the power rectifier board are good, and the TP1 and TP2 on the lamp driver boards test properly.
I can run a jumper wire between TP1 (power) on the Lamp Driver board to the power side of the socket in the backbox (quick touch) and see the lamp illuminate.
I also swapped in a known working Lamp Driver board with no change.
I am still stumped.
I did cross the wire for the TP1 to the ground side of the one bulb at one point over the weekend and miraculously both the backbox and playfield insert lamps came on and functioned properly for a few games.
I turned the machine off and back on a while later and they were dead again.
Any other suggestions of what to look for or test?