One reason for blue dots in Nixie tubes found - anode mesh not connected to anode!

[Dekatron42]

Hi all,

I have found one reason for blue dots on the anode mesh in one of my ZM1042, the anode mesh is not connected to the anode!

You can see where the welding has come loose in the first photo I have included (embedding them in the message did not work for some reason), where the red arrow points, the piece from the anode mesh which has been bent outwards towards the glass is not making contact with the pin which is connected to the anode pin. You can see the welding spot if you look closely but it has come loose. There is no other point that connects to the anode mesh to the anode pins on the socket. The upper support connects to the anode mesh but it does not have any contact with the lower support so there is no contact with the anode pins on the socket.

The only thing working as an anode in this nixie is the rod between the anode pins on the nixie, the one passing through all figures at the bottom acting as a support.

It looks like the second photo when figure 2 is lit, it is lit in extreme as I have raised the voltage to be able to take some photos with my digital camera, the blue colour is very hard to catch at lower voltages even though it is clearly visible to the eye.

 

I can light all figures but I need a higher starting voltage and also higher current for them to light fully and often the pins at the base start to shine before the figures.

 

Could this be the reason for blue dots in the IN-18's too, that there is poor contact between the anode mesh and the anode pins on the socket? I have read numerous threads on the blue dots but I have so far not seen anyone claiming that they have the true explanation. Could it just be that due to poor manufacturing resulting in either poor contact or no contact at all that these blue dots appear on the anode mesh or at other points since the working conditions are wrong?

 

Cheers,

 

Martin

 

[threeneurons]

A couple of years ago I did some experimenting and generated blue dots on an IN-18, hook-up in the normal way, mostly.

I use the circuit above. The blue dot was usually reported when numeral '1', was ON. That's why that cathode is grounded, which is how you turn it ON. Most people use the 74141, or the the Russian equivalent. These chips, don't let the unselected digits drift all the way up to the supply voltage. They usually have a clamp in the 60V range. So I simulated that with 0 and 2 thru 9, tied to the wiper of a pot. I swept that voltage from 30 to 100V, and was able to create blue dots at will. And also make them disappear. Here are 3 examples:

That unattached mesh you have, may be floating at a voltage around 50V, and alter the current path (?). I believe the blue dot  indicates a point of higher than usual current density. Even longer ago, than the photos above, I hooked up a NL841, using its decimal point as the anode. The decimal point glowed blue:

I've also seen this on many neon filled trigger tubes, with small anodes.

[John Rehwinkel]

> That unattached mesh you have, may be floating at a voltage around 50V, and alter the current path (?). I believe the blue dot indicates a point of higher than usual current density.

I suspect Mike's hypothesis is valid. If you make an ordinary linear neon tube and fill it with neon, it glows red, as you'd expect. If you introduce mercury into it, the red glow is replaced with the blue glow of mercury. So something about the normal operation of neon tubes favors the mercury glow. Nixies favor the neon glow somehow, and the difference may indeed be current density. I suppose one useful test would be to take such a linear mercury/neon sign tube and run it on various currents and see if the red neon glow reappears at some point when reducing the current.

- John

[NoCampersFluffy]