I finally got around to rewinding an old inductor, to get a 1.7mH inductor. I used the same schematic as the mintyboost more or less, though I used a STP55NF06 mosfet (had one lying around). I also had to pull down the switching signal from the MCU with a 4.7k resistor. Without it, the inductor heated up massively, and I couldn’t figure out why the MCU wasn’t pulling the pin down to ground properly. Ah well, it works. In hindsight, that was probably the reason the original 0.2mH inductor wasn’t working, but ah well, winding hundreds of turns of copper around a tiny bobbin is fun.
I’ll probably add a ds1307 for timekeeping, since I found one in my parts box. There’s a nice empty bit of space on the bottom right of the clock for it.
Using capacitors as spacers for the power rails has been really useful – the two copper wires running horizontal behind and above the tube are suspended at either end from small electrolytic capacitors. I went for a 600uF cap for the 40V rail, much bigger than necessary, but it does make the clock “warm up” slowly, and power off gently, whereas with a smaller one it was much more abrupt.