Thanks to those who gave advice on hooking the BNC connector from HP
4951B serial protocol analyzer to an external monitor. I bought an
adapter from Radio Shack Phono-to-BNC Adapter 278-254 and ran a regular
phono cable from it to a Magnavox "Computer Monitor 80" green screen
monitor.
Works great, and now I can make out the characters a bit better (though
the internal display is great, it's a bit small).
Another possibility would be to hook the thing to the video in jack on
one of my graphic cards on desktop.
-- John.