Greg Arnold wrote:

The B40 does funny things when the voltage gets too low. Could the
radio transmission reduce the voltage to the B40 below the crucial point?

It is possible that your voltmeter checks the voltage very close to
the battery. It is also possible that your instrument panel, including
radio and B-40, is wired form a common source; if power is provided
from the battery to the panel with too thin a set of wires, then you
surely have a big voltage drop during transmission, and the voltmeter
can't show it.

Pure speculation, of course.

Aldo Cernezzi