Modem connected successfully to another (I have a WinModem in this PC and a second phone line attached to it; can walk between the two to see either terminal).
The terminal software I was using, Com-It, uses a lot of reverse video. It revealed a lot of flicker (guess that's why IBM used high-persistence phosphor) and also variations in intensity (not software driven). Does this suggest power supply, i.e. capacitor, problems in the monitor? What else might it be?
Thanks again for your suggestions.
-CH-