[volt-nuts] Neon bulbs for HP 3420B or 419A chopper circuit

[David C. Partridge]

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