Weird spot on XN-11 Anode

[Roddy Scott]

Folks,

I put a set of XN-11s into a PV Electronics QTC board and I am getting some weird things happening.

Blurred and missing numerals and a spot on the anode mesh.

Any ideas on this as I am totally in the dark as to why?

[gregebert]

How do things look when you bench-test the tube on a DC supply ?

Is the spot actually on the anode mesh itself ? Ionization occurs at the cathode, so this is indeed strange.

[Roddy Scott]

I don't have a power supply but the spot occurs on the mesh and with a different digit  there is a a different spot. The tubes were NOS and I have had them lying about for over a year in a box. I have a couple of QTC boards so I am going to try them on those boards to see if they are the same.

I have never seen anything appear on the Anode mesh with any tubes that I have used so far so this is totally new to me.

[Nicholas Stock]

I have seen similar 'bright dots' on ZM1032 tubes....

Nick

Sent from my iPhone

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[Dekatron42]

I believe that it is the same symptom as the "blue dot" effect in Nixies, you can read about "The Infamous “Blue Dot”:" a bit down on Michael Moorrees webpage: https://threeneurons.wordpress.com/nixie-power-supply/ . I've seen both orange and blue dots on several Nixies. I recently had orange dots with some Z560M nixies and the fix in that clock design was to use resistors across all cathodes and a voltage divider from anode to cathode to set the voltage to a suitable level - check the "Lower Voltage Cathode Drivers:" on the same webpage to see the effect of these "mid-pull" resistors.

/Martin

[Paul Andrews]

Never on the mesh (yet), but I did see something similar when I had one of the numerals wired as the anode rather than the actual anode (in my defense, the actual anode pin was missing).

I would check that it is wired correctly, including possible shorts. Then check it on a different QTC mother board to see if it is the motherboard. Then check a different type of tube on the original motherboard to see if you get the same effect (another check of the motherboard).

I also don't have a bench supply - it is on my list - but in the meantime, I use a NCH Nixie power supply and a breadboard. The power supply is rock solid and the voltage is easily adjustable.

Good luck.

[GastonP]

If the mesh glows (as this one seems to be doing) then suspect that there is AC instead of DV applied.
The mesh can't glow when the right potentials are applied, whichever the value of the current through it is.

Gastón

[Roddy Scott]

That was my first thought that the tube was connected wrong but as XN-11 pins run 1 to 0 instead of 0-9 and the circuit board is dedicated to them I was extremely careful to make sure it was aligned properly by even illuminating the stand tube before soldering to check all wires were correctly lined up to their relevant hole. It has been checked on another 2 QTC boards one of which is running GNP-7AH tubes and shows the same symptoms so it is a case of "He's dead, Jim!"

A power supply is high on my list of wants just now as on board testing is not the best way.

The close up shows the white spot on the mesh as well as several blue areas behind the mesh.

This is the XN-11 board configuration from Pete's QTC PDF

The pinouts of the XN-11 as opposed to those on an IN-8-2 which is 0 to 9.

[threeneurons]

Very likely, the infamous "blue dot". The unused cathodes are clamped to 62V. I believe those unused cathodes, at that potential work somewhat like the beam plates, in beam power tubes, like the 6L6. The blue, at or near an anode, seems dependent on current density. The greater it is the more likely you'll see the blue glow. On one of my "blue dot" experiments, I used the decimal point of a NL-841 as the anode, and it glowed a nice bright blue.

Crossing my fingers, knocking on wood, and rubbing my rabbits foot, I've yet to see it on one of nixie thermometers. They are clamped to 47V, and I even have one clamped to 24V ! Maybe, the nixie tubes used on that circuit (ZM1000, IN-14, IN-8-2) aren't prone to blue dot.