This is getting weird. The clock has sat to the side of my bench for a while now, so I figured I better start on it again.
Recently I purchased second hand, a basic oscilloscope, and so I measured from Pin 12 of the ATMega168. I saw a square wave.
Since said oscilloscope has unmarked controls (I said it was basic!
) I also borrowed a frequency meter, and measured the boost output at 32.4Khz (a bit high perhaps, seems close enough? - but the meter is old, and calibration might be off too)
I guess it would appear the boost signal is working OK, though.
I then pulled the 22uF capacitor and it seemed to test OK at 17.5uF on my meter.
I also pulled the inductor, and measured resistance at 4.4 ohms (don't have inductance meter) but I guess it's OK too.
I put the capacitor and inductor back in, and my next suspect was the IRFD110 FET.
I swapped this out with a new one, and tried again.
I saw '50' briefly flash on my meter before it came down to 11.5v and stayed there.
This is with no MAX6921 or Tube plugged in, so they can't be the problem now. Checked MOSFET ground as suggested before, all OK. Perhaps the Zener has gone funky? Can't see any issues with my soldering, and the thing did work fine the first time I tried it. Gah.....