Originally posted by AstroDave Thank you Bob for the comprehensive circuit analysis. Other than the higher voltage NE2X I had it pretty much figured out. I did learn that there are 110 V Zeners, too (I normally think of these for low voltage regulation)!
Thought some here might find the explanation interesting. There are a lot of older flashes laying around which have nearly the same circuit. Note that the high voltage from the main capacitor should not be available at the flash trigger contacts (well, actually it is the same voltage value-wise, but it's across a 0.02 microfarad capacitor and not the main cap, the latter of which would pack a much larger wallop should one access that).
Zeners are available up into the 200 volt range and are made by connecting lower value Zener elements in series. In the case of this flash circuit, the Zener is used to drop the high voltage so it won't appear at the transistor circuit which disables the oscillator (which would destroy Q3, and possibly Q1, and Q2). This works as long as a small current is pulled through the Zener (always used in reverse breakdown mode, hence we're dealing with a negative voltage on the right side of D2 relative to its left side).