Well, to make troubleshooing the system simpler, I've pulled nearly every
Q-bus module from it.  I still got the same

?06 HALT INST
    PC = A0050419

error message even after I pulled everything except the M7606 CPU module,
the two 4Meg M7608 Memory boards, the RDQX3 controller, and the TK50
controller.

Taking things further, I pulled one of the memory boards... rebooted... 
Success! One of the memory boards I had was a National Semiconductor M7608
"clone" board.  It either inherently sucks or was functionally unsound.
Pulling that board fixed the problem.  Of course now I've got a 5Meg
MicroVAX II.  So now I'd better go pickup an M7609 :-)

Anyway, my machine has now happily made it to the Nboot prompt.  So my
quest continues into phase 2.

-brian.
---------------------------------------------------------------------------
Brian D. Chase         Systems Coordinator        brian.chase@carpediem.com
-- Compression, Inc. - 13765 Alton Pkwy, Suite B - Irvine, CA 92618, USA --