24 April 2020

Blue dot and leg glow in Z520M Nixie

I love Nixie tubes, and I especially love round top-view ones. Perhaps that is because the first ones I bought were of this kind.

Lately I got one Z520M with the red coating quite damaged (pictures seem to make it look even worse!). Since the red coating can be restored/replicated, I proceeded to test the tube. I was not surprised to find out that it needs to be driven a bit harder than newer Nixies, both because this one is quite old and probably it has been used a lot.

Digits 0 to 2 were behaving normally. When pulling low other digit cathodes only the leg was glowing. If left ON for few minutes the glow transferred to the digit from the area close to the pin. Unfortunately if the digit is switched off for a while the malfunction comes up again the next time the same digit is lit.

Now that I own a variable HV power supply I could experiment with the current through the tube, and saw that the "leg" does not glow if the current flow is lowered closer to the datasheet value. Even 0.1 mA makes a difference. But then the digit is faint or not completely lit (like "5" and "7").

Since the red coating is cracked I could see through the plain glass and note that the leg glow releases the infamous blue dot. It is caused by mercury and overcurrent. Since it does not happen on digits 0-1-2 it could be just a deposit on unused digits/wires and might be cured throughout hours of operation of the same symbol. Perhaps. I will not go further and keep the tube as-is in the collection. (Or make a test harness where I can leave it running for days out of the way in the lab?)

Can you see the infamous blue light through the crack in the coating?