[volt-nuts] HP419A Temporary Chopper Fix

Stan Katz stan.katz.hk at gmail.com
Sun Mar 1 01:53:52 EST 2015

For those of us who are in need of a temporary fix for our HP419A null
meters, with dead neons, here is some good news.

At http://dabbledoo.weebly.com/hp419a.html you will find an intriguing
circuit that overcomes the need for SOT neons with specific strike and
breakdown voltages.

I built this circuit, and ordered NE2U chopper capable neons from
bulbtown.com to replace the dead ones in my unit. They are cheap, and made
in Taiwan, which means there's a good chance they were manufactured in the
last 1 to 15 years. Beware of purchasing genuine US made 30 to 50 year old
NE2Us. The Krypton 85 in NE2U lamps only has a half life of about 10 years.
US made neons would have marginal dark effect compensation left in them.
Not only that, they go for over 100 times the cost of the Taiwanese ones at

I aged the newly installed NE2Us for 2 weeks without the dabbledoo circuit
installed. During that time the unit continued to malfunction.  I then
connected the add-on circuit into the unit. Voila, my HP419A came back on
the air!  I inserted the circuit on a floating perf board using Bendix
connector female sockets (TE Connectivity P/N 66360-4 and male pins P/N
66361-4) thereby avoiding having to chop up any of the unit's wiring if
things didn't work out. The females were perfect, the pins just a shy too

Caveats: This fix should be considered temporary. There's the possibility
that one, or both of the replacement NE2Us breakdown voltage could climb
beyond the capability of the 419's blocking oscillator, which is still the
source of firing voltage with the add-on circuit installed. Thus, you may
have to go through additional bulbs to find ones that fire. I give no
guarantee that you're going to find well behaved bulbs. I just installed
the add-on circuit, and am going to leave the unit on for at least a day to
see if things continue to work out. Also, there is no guarantee that YOUR
HP419A has salvageable photocells. If your photocells are shot, this is not
a fix, temporary, or otherwise, for you.

I was forced to use two obsolete 13.2 volt Craftsman power tool packs to
replace the original batteries. These are outboard of the unit, and I
haven't tried to fashion a guard around them to ensure noise and offset
voltages are within spec. There is a fellow at
who not only rebuilt his HP419A (neons were still good), but built a
monumental copper guard to cover the external battery matrix sitting on his
unit. If you don't understand German, Google translate is your friend here.

Hopefully this "spare tire" will last until a permanent fix comes along.


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