Today I am going to tell you about how to revive digits in Nixie tubes that do
not glow anymore. Nixie tubes are actually neon lights with 10 digits formed
from thin wire in the form of the digits, stacked on top of each other in a neon
filled tube. When some of the digits get used more than the others, the digits
that do not get used much darken over time. This is called cathode poisoning.
Material sputtering for the more used digits during operation will land on the
not-so-much used digits, darkening them.
I had recently built a Nixie clock with some old ZM1242 (Siemens) nixies that
I had salvaged from an old machine a very long time ago. It turned out that the
tubes still worked, but I had quite some cathode poisoning on the 2,3 and 5
digits on about half the tubes. Why it was especially those three will forever
be a mystery I think.
So I set out to read about cathode poisoning on the Net, and found somewhere
that when you drive the offending digits for about half an hour at twice their
normal current (5 mA in my case, the normal current is 2.4mA) then the cathode
poisoning will "burn off".
So I needed some higher voltage power supply, and I did not have that. What I
-did- have was some old stuff in my junkbox. So I connected up two small block
transformers (230Vac - 9Vac) of the same type in a back to back fashion
(230V - 9V -- 9V - 230V) in order to get a little more safe power supply, and
I fashioned a full bridge rectifier with some small high voltage diodes from
my junk box. I then smoothed it out with about (in total) 200nF of capacity
(That is all I had for that voltage) but it worked well. I guesstimated a re-
sistor value for the Nixie to run at 5mA, and connected it up. It was too low
according to my multimeter, so I came down in resistor value until I had a
current value just below 5mA.
Then I ran all the digits for about an hour (half an hour like I had read was
not enough in my case) on the higher current, and lo and behold, the cathode
poisoning was mostly gone!
It is a bit of a long winded process, as each digit takes about an hour to do,
but at least I have a Nixie clock now with all the digits working.
This was fun and educational. Now the clock sits proudly in my living room
and tells us the time in it's nice orange glow fashion.
Allright, enough Nixie nerdities. Have a nice weekend!
