This looks like a major clue that changes everything. (Well, almost
everything.)

My ASG29 also has two main batteries and a dial switch on the panel
that selects which battery is providing power to all of the
instruments. I wonder if the transition from B1 to B2 is popping the
(fast-blow) fuse on the back of the B40. And it would explain why
there was a T500L250V fuse in the panel fuse holder; I understand that
the "T" means slow-blow ("T"imed?), where an "F" would indicate fast-
blow.

2NO