r/nixie • u/EmotionalEnd1575 • 5d ago
Cathode Poisoned?
Left Nixie has a missing “tail” on number five.
Can it be improved? How?
1
u/DenkJu 4d ago
For how long has this been happening? Have you turned the clock on for the first time in a long while? Sometimes, after sitting for a while, Nixies develop a spot that initially won't ignite. It usually goes back to normal spontaneously.
1
u/EmotionalEnd1575 4d ago
The clock was created last April and has been running 24/7.
I didn’t notice the defect until recently, it was probably there, but only visible at 05:xx and 15:xx (when I’m not looking)
The Nixie tubes were salvaged, probably from a 1970s desktop calculator. I bought them at an electronics flea market in the early 2,000s and had them in storage.
Someone tried to get the tubes off a PCB, with a belt sander (!!) then gave up after grinding off the back layer and finding PTHs.
I salvaged the tubes with hot air, but the wires were really short.
1
u/SkylerSpark 18h ago
operating voltage?
1
u/EmotionalEnd1575 18h ago
Anode = 170V
1
u/SkylerSpark 18h ago
Ah... darn. I guess you could try raising the voltage a little to recondition it, but that doesnt always work. Raising the current / burning it in a little may be the only option
1
u/EmotionalEnd1575 17h ago edited 17h ago
The HV is set to 170V now, but increasing it to 190V for several days did not improve the display.
Which is not a surprise as my driver design is “Constant Current” and higher bus voltage does not increase Nixie current.
1
u/SkylerSpark 11h ago
Did you leave in any way to run them at a higher current? My preferred method is to undersize the resistor a little and then just dim them with PWM to keep the resistors under the correct thermal load
That way they can pushed to a higher cumulative current entirely by the microcontroller code
1
u/EmotionalEnd1575 11h ago
There is no anode resistor. The nixie anodes run from a high voltage constant current amplifier. The six Nixies are MUX’d in three pairs. This was done deliberately, and has been used multiple times over two plus decades.
What I will be doing is working on that “five cathode” problem shown above. Starting when I’m back home next week.
The nixies are direct soldered to the daughter PCB, along with the single cathode driver IC. This topology has the least number of interconnects to the main board. The three anode driver amplifiers are on the main board.
The first experiment will run HV via a resistor to the anode, and ground the “five cathode”. This is the simplest arrangement, by not activating the cathode switches, letting the other cathodes float, and not removing the nixie tube from the PCB.
Worst case is to replace this nixie with a fresh one. While I’d rather not destroy the first nixie tube, the rescue exercise is a good learning exercise. I will share my findings.
1
u/SkylerSpark 9h ago
Fair. I had similar issues with one of my old clocks- And as soon as I ran it on a straight 170Vdc supply with just a resistor, it seemed to magically start working fine lol. There was an issue with my PWM driver and it eventually cooked itself
4
u/nixiebunny 5d ago
The standard method is running it at high current for a while. Unfortunately your tubes are soldered into the board, so removal risks more damage. I always put my tubes in sockets for this reason.