It took about 30 minutes of prying around the rubber gasket to free the '577 tube. I broke the gasket
too. On the other hand the 5110 tube came out very easily. On swapping it I get a pretty stable display
After running through the calibration procedures, I used it to measure the beta of a 2N4403 at
185, which seems pretty reasonable (once I realized the lamp behind the step knob is
burned out - initially I was worried it was indicating 1850).
The only issue left is when the number of steps is even, the step waveform is very steady. When
odd (and >= 5), it shimmers vertically quite a bit. Something odd in the step generator board up next I