I have retested it with some new instruments this time. With a 9.00V power, I can only get 8.31V output at 170V. Vcc seems correct (4.96V, very stable). I still can't hear any sound from the board.
I also took the waveform at MOSFET gate, at 2 different time scales. These are really weird...
The scope traces are fine: The weird waveform is because we only drive the MOSFET when the tube output is enabled. The reason why is a bit involved:
We calibrate the HV Generator during the factory reset to hit 180V when the tubes are on full duty cycle. However, when we are dimmed, the tubes are only on for a short part of the cycle (around 10%), and if we left the HV Gen running with no load, it would ramp the voltage up while the tube is not lit, and this would cause the digits to be lit unevenly. Therefore we turn off the HVG when the tube is not lit.
The waveforms you see are exactly what they should be. There are short pulses of activity (because the LDR is not installed and the clock thinks it is dark), and inside each of these pulses, we see the HV generator square wave.
So the problem can be localised to either the MOSFET, the diode D2 or the high voltage capacitor.