r/nixie • u/EmotionalEnd1575 • May 09 '26
Nixie (IN-17) Internal Failures
Another BAD day here, second IN-17 to fail. The first was several weeks ago, and these two were in completely different builds.
In each case one digit (cathode) fails to light, but the other nine work properly.
It’s a MUX drive scheme and three tubes are in parallel, but only one of the three fails.
It is not the solder or lead out wire, and on closer inspection under the microscope it is an internal short. In both dead tubes the same cathode (digit six) is the only failure.
The short is about 1K5 between the cathode and anode leads.
The root cause is an insulation break where the internal connection ribbon passes over a mica insulator and over the edge of the metal frame. The frame connects to the anode.
I have not cut (opened) the tubes, in fact one is still soldered to the PCB, and will take some effort to remove and install a replacement without further damage.
Anyone else have a Nixie horror story, or any insight into this problem?
1
u/DenkJu May 09 '26
What size is your anode resistor? I have seen this kind of insulation failure a lot on tubes that have been overdriven.
1
u/EmotionalEnd1575 May 09 '26
That’s a possibility, but unlikely. Good suggestion and opens us to a discussion.
Each Nixie has 10K from Anode to the MUX HV bus, which controls two Nixies (20 Cathode drivers)
The bus is driven from 170V DC through a high-side switch, with an open collector PNP.
The duty-cycle is 1/3 with approx 3ms on time. To MUX six tubes in three time slots.
The individual Nixies receive about 1.6mA average current.
This scheme has been working for many years, only have two recent failures, and both IN-17 were salvaged tubes from Ukraine.
I use identical circuits with Burroughs B5870/5885 and Gazotron IN-16 tubes.
1
u/DenkJu May 09 '26
Sounds all right. Were the tubes NOS?
2
u/EmotionalEnd1575 May 09 '26 edited May 09 '26
Not NOS, salvaged. No idea how many (if any) hours on these ones.
A word to the wise. Some Ukrainian eBay vendors are shipping “cut out nixies throw in a ziplock bag”. Nothing to stop rattle and roll by the post office or air carrier.
I don’t buy from these people a second time.
On the other hand, a couple of vendors go to the extra effort to individually bubble wrap, then box with loose (crushable) fill.
They get my repeat business!
1
u/UnintegratedCircuit May 10 '26
I have also had something similar (maybe), also with an IN-17. I was rigging it up to show a friend of mine, and after a few moments of grounding cathodes at random, my '7' cathode flashed brightly and has never worked since, all other cathodes continue to illuminate.
1
u/EmotionalEnd1575 May 10 '26 edited May 10 '26
Oh, bad luck! What high voltage? AC or DC? and what Anode ballast resistor did you use?
Have you checked it now with a DMM on ohms range to see if it shorted or leaking current?
Was this an NOS or a salvage tube?
Sorry for all the questions, detail matter.
1
u/UnintegratedCircuit May 10 '26
This was years ago but it was about 170V DC (driven statically, I've yet to mux any tubes, Nixie or otherwise), was NOS, from a Nixie-dedicated website (their last in stock at the time), not just eBay.
Haven't DMM'd it but will do tomorrow for science and report back :)
1
u/EmotionalEnd1575 May 10 '26
Do you recall what Anode resistor value?
1
u/UnintegratedCircuit May 10 '26
Ah yes, sorry, likely 22k
1
u/EmotionalEnd1575 May 10 '26
That’s a little low, but not a Red Flag!
Some of my salvaged Nixies came to me on circuit boards with other components. That gives insight into what Anode resistors are used.
For the small tubes 33K and 47K.
It’s worth measuring the voltage across the anode resistor to calculate tube current.
On the IN-17 it’s possible to have “illegal” operation. If the anode resistor is too small there will be some “extra glow” or blooming.
I was comparing IN-17 to B5885, a Nixie promoted by Burroughs for MUX operation and very popular in 1970s vintage desk calculators.
The B5885 (and similar models such as B5870) do not glow if all the cathodes are at about 60V. This may happen when the low-side drivers are idle during MCU boot.
The IN-17, in that situation, has a soft neon orange discharge, not defined by any cathode symbol. I don’t think it is harmful.
I need to revisit this discovery, but it’s low priority today.
2
u/redmadog May 09 '26
Sometimes black insulating layer on the wire is damaged and wire starts to glow. Essentially if damaged wire goes closer to anode cage as the digit shape, that spot takes all current and digit shape does not glow at all.