Nixie Cathode Short - Page 1
Forum section: electronics / repair

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 03, 2018, 12:53:46 am
content: I have a B-5750 with a partial 0 to 2 short. One digit always lights brighter though. I've tried flicking it hard in all directions with no effect.From what I've read, I could smack it really hard but risk breaking it, or I could run high current through it and risk it burning out. It seems more like a whisker/debris given the difference in brightness. Given that it's still technically readable (although hard), I don't want to do anything too risky.Any ideas?

author: davy peleman
author_rank: Regular Contributor
timestamp: February 03, 2018, 11:46:10 am
content: i'm learning tubes at the moment. so i'm interested; maybe some kind of shaker table. just thinking loud i'm a newbie so i appologize if i don't understand the problem; are there metal pieces that make a bridge or are the anode and cathode lightly touching. the term whisker/ debris is unknown to me

author: andy2000
author_rank: Frequent Contributor
timestamp: February 03, 2018, 05:24:20 pm
content: Assuming you're sure it's not the driver circuit, I think the only chance is to try applying a voltage between the two shorted electrodes. The problem is, it seems to be high impedance leakage rather than a direct short. If it was a vacuum tube, you might be able to burn it off with a high voltage, but that obviously won't work here.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: February 03, 2018, 05:48:52 pm
content: I've tried the high current-method with a nixie. It resulted in one of the digits failing completely. Depending on the use, this might not be a problem. In my case "8" isn't lighting up anymore, but as only 3 digits in a clock go beyond 5, that's no problem for me.

author: SeanB
author_rank: Super Contributor
timestamp: February 03, 2018, 06:21:29 pm
content: Disconnect the driver ( easier if it is in a socket, though if not then the driver chip is a lot easier to unsolder than the display in most cases) and measure the resistance between the 2 pins. Then check the board itself for any foreign substances between the adjacent pins, and even if there is nothing visible clean well with IPA and a good scrub, then dry with warm air and check again. this will prove if the high resistance is internal to the tube or external ( most of the time external debris is the cause, or old crusty flux residue). Then you can remove the internal leakage using a high voltage supply and a current limiting resistor, to evaporate the high resistance material, though this often runs the risk of arcing in the tube if the current is high enough or the gap is small enough.Most of the time ghosting though is external.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2018, 08:31:08 pm
content: Inter-cathode shorts can be a result of cathode spluttering. It usually occurs in non Mercury doped Nixies - some ex-Soviet tubes are particularly susceptible.Some of the spluttered cathode material can end up on the ceramic spacers between the cathodes. If this is the problem, then you ought to be able to see some level of resistance between the two cathode pins. If there is measurable resistance, then you might be able to burn it off with a modest current - but only below the striking voltage of the tube. The resistance might well be too high to burn off below that voltage limit. If it won't, then that may be the end of the tube.If you can't see any sort of resistance reading between the cathodes, then I'd be looking for board leakage or even driver IC as SeanB describes.EDIT: Corrected appalling punctuation.

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 03, 2018, 08:58:24 pm
content: It's a counter, I can't blow any digits. The driver is transistors, and the tube is soldered in. I can check the chip driving the transistors by swapping them out between tube circuits. The chip is just a jellybean logic chip, so hopefully if it's bad the modern equivalent will have the same pinout.Schematic:http://www.rfcafe.com/references/popular-electronics/build-numeric-glow-tube-dcu-2-1970-popular-electronics.htm(Utilogic version)If it's not the chip, I'll have to take it apart to try the burn-off trick. The problem is I'm not sure how they got these little metal spacers for the screws under the board in unreachable places.The only thing I can think of is upside-down, but then how did they hold the screws in?

author: SeanB
author_rank: Super Contributor
timestamp: February 03, 2018, 09:09:44 pm
content: The spacers are stuck in with superglue generally.

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 03, 2018, 09:31:34 pm
content: It's not the transistor driver, swapping them did nothing. Still could be board leakage, but most likely a bad tube.No, the spacers aren't glued. I just partly unscrewed one and it was loose. I'll have to tape the screws to hold them in and fit it in upside down. I'm assuming they did something similar originally.

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 03, 2018, 10:45:08 pm
content: There is no continuity between the pins (on normal DMM resistance range), and I've tried swapping all of the chips. I didn't see any bridges on the bottom of the board (unless I missed something). The supply is 178V which doesn't seem too high. Could it still be the tube or am I missing some microscopic bridge somewhere?

author: floobydust
author_rank: Super Contributor
timestamp: February 03, 2018, 11:54:38 pm
content: Possibly you could apply HVbetweendigits to see the area (glow) of leakage, as the electric field is strongest there. Just resistor current-limit and turn up the voltage to see a hot spot, on your workbench in the dark.

author: nickds1
author_rank: Contributor
timestamp: February 04, 2018, 01:28:37 pm
content: I would ask on the Google NEONIXIE-L group - the main resource for nixies on the www - the folk there know more than they should about nixies - some of them even make new ones from scratch, e.g. Dalibor Farny - many designers, artists, professional EEs, hobbyists etc.https://groups.google.com/forum/#!forum/neonixie-lSmall admission - I'm one of the moderators - I also deal in nixies - new B5750s are not generally that expensive. Having said that, out of the 12,000 or so nixies I have in stock, I don't currently have any B5750s... though someone on the group will most certainly have them.