In an earlier post I quoted the following description of the photo-chopper circuit from the 419A manual
4-43. Assume that DS1 lights when the input is applied to T2. Capacitor C1 charges until the oscillator switches the input, and DS1 goes off. When the oscillator switches again, the charge on C1 insures that DS2 fires, and DS1 stays off. This cycle continues with DS1 and DS2 firing as long as there is output from the oscillator. CR1 and CR2 prevent the capacitor from discharging through R1 and R2 That description also applies to the 3420B chopper. For this to work as described, George Einst says that the striking characteristics on the neons are critical, which I totally believe given what's happening in mine. AFAICT, *both* neons are striking on every +ve portion of the square wave drive signal, so the flip-flop behaviour doesn't happen. I checked on the curve tracer, and both neons strike at almost exactly 70V. Both are also totally clear which suggests to me that they are not the originals. Does anyone have any idea how much the strike voltages would need to differ for this to work, and does anyone have any suitable neons - the only ones I have are these shorter ones and they strike at about 125V. These are the longer bodied neons (glass about 5/8" long). Thank you Dave Partridge _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.