Problem found
The picture tube anode cable was broken internally. A quick resistance check showed no continuity. I made up a new cable with some HT wire I had on hand and now the picture is fine. The old cable must have been arcing across the break inside the wire.
I'd like to credit my trouble shooting technique, but the only reason I checked the cable was it looked really beat-up. Sometimes you get lucky
Thanks for all the good advice.
Bob D.