Originally posted by AstroDave D2 turns things off when the voltage across the NE2 gets high enough??
Yes, and your reply made me notice for the first time that this flash unit has a negative polarity at the unit foot! That's if it has a foot, and the bottom rail of the circuit is earth for the camera, because it might be a built-in flash for a Kodak P&S, but that makes no difference to the principle.
It looks like when the neon turns on, ie when its upper end connected to the xenon cathode has dropped to ~ -60v (that's a minus), it pulls down the voltage at the top of R3. C2 will continue charging until the the top of R3 reaches ~ -110v which will turn on the zener diode D2. D2 then pulls a small current from the base of the switching transistor Q3 which turns that on, effectively shorting to earth the Q2/Q1 transistor pair and the main switch S1 (via R1). That stops the oscillator and the charging.
Originally posted by Bob 256 One thing which greatly reduces any hazard (if there is one to begin with) is that touching exposed contacts, discharges the current through one's finger or hand and the current isn't routed through the heart.
I have a procedure when I know that I will be working on a circuit that could, if I have overlooked something, have live voltages in it. In something like a flash unit I short any capacitors with a 1k resistor connected to prods. I then check my multimeter works and then use it to check for voltages at salient points in the circuit. I then re-check that the voltmeter works (in case it happened to fail after the first check). Knowing that I will need to touch things sooner or later I then flick my finger over a few salient points with my other hand not touching anything - the best way to take any shock if there is one.
I also follow a rule that my father (a BBC technician) had, which was when working on mains equipment I always have its disconnected plug lying visible in front of me at the back of the workbench. I have been a ship's engineer, a railway engineer (UK sense, on a railway with ground-level live rails) and a power station engineer, so I could tell a few horror stories.
To anyone reading this generally, none of this stuff should be tackled unless you have a good understanding of electricity.