eldenh wrote:ZD101 is connected to the HV+ by about 205k.
Ah, That's probably R117-R120 (51 kΩ x 4 = 204 kΩ). Perhaps consider shorting one or two of these temporarily.
It appears the voltage out of Q109 is connected to VCC of IC101 by a diode.
OK. So perhaps Q109 turning on, perhaps partially, is what "bootstraps" IC101. IC101 gets its power from the "logical OR" of Q109 or another path via another diode. That other path is probably one of the secondary windings of TX105, suitably rectified and filtered. So once IC101 is driving the MOSFET and transformer (probably actually a multi-winding inductor), it can power itself from the HV source. At that point, something will likely happen to turn off Q109. Perhaps there is a capacitor in the base circuit of that transistor that turns it off after a few hundred milliseconds.
Without proper test equipment, I've been making do with a 12v battery and a 58.7v bicycle battery charger.
Hopefully, that's a dumb battery charrger that doesn't need to see an actual battery there. Elcon charrgers for example won't turn on their output relay unless a battery is detected. An actual current limited power supply would be much better, since it can be programmed not to supply more than a few watts of power, which hopefully reduces the chances of something burning up. But of course, you have to work with what you have.