AES wrote:

Pulled a long-unused PRO-2006 Programmable Scanner out of the back of
a closet. After a battery change on the 9 V battery, seems to work
fine in all respects, except that the LCD display has become
sufficiently faint (lost enough brightness, or contrast) that it's
barely readable, with or without the built-in light on.

Any suggestions for kick-starting the display back to better
visibility?

Constant exposure over a long period, to heat or direct sunlight can
cause an LCD screen to "wear out". I have seen this happen on
occasions and a replacement screen usually does the trick.

MC