1948 Motorola VX101 brightness changes picture size

[Resonance1]

I am restoring a 1948 Motorola VX101. It's fully re-capped, all the tubes are tested and good, and at this point it basically works. But it has a strange symptom. The picture is very dependent on the brightness and contrast. Too little of either and the picture goes dark - not that surprising. But too much of either and the picture "blooms," i.e. expands rapidly out of control and then disappears. If the brightness and contrast are just right, the picture looks great, but it does not seem as if the brightness or contrast should affect the raster size, and it does not seem as if the picture should be so sensitive to having just the right brightness and contrast settings. Any suggestions?