On Feb 12, 2023, at 9:31 PM, gregebert <gregeb...@hotmail.com> wrote:
I investigated internal shorts with IN-1 tubes about 10 years ago, and never use them because of that. I never had anode shorts, but I had multiple cathode-to-cathode shorts, resulting in more than 1 numeral being on at the same time, which will cause more current and will burn a small-wattage anode resistor.
I haven’t considered that - does two cathodes being lit in ‘parallel’ reduce the voltage drop across a tube?
It’s _possible_ the control circuitry is failing and lighting two cathodes without a short in the tubes - maybe that’s how the problem starts? Too much current causes the resistor to heat up and its resistance decreases causing a runaway current which eventually also causes one anode too short to the cathode? I don’t know enough of the physics to know if that is plausible…
I've never heard of this problem with Burroughs tubes, though. Also, I havn't seen any indication that voltage surges will harm a nixie; they are current-controlled. My first clock has a +340V anode voltage, and has been running fine since 2011.
If you have a failed tube, you can confirm the internal short by measuring with an ohmmeter, then apply some current to it to burn it out like a fuse. If you do this slowly, you will see the short glow before it burns out. The tube will work for awhile, but will likely fail again after the short re-grows.
Interesting! I have one failed tube that’s not silvered, so I’ll give that a try.
I suspect the short is caused by a metal whisker that formed in the electric field. NASA did a number of studies on tin whisker growth and you can find their tech reports online. Some metals are more prone to whisker growth than others, and perhaps the ones you are seeing could be from impurities. I've also seen this on older laptops that use CCFL backlights; one of mine had a visible growth, thinner than a hair, at the HV supply.
Makes sense.
No matter the answers to the above, I’m thinking that the best I can do in practice would probably be to add a fuse to catch overcurrent after the short, but before the anode resistor burns out out. Do other folks here do that as a matter of course?
Jamie.
On Sunday, February 12, 2023 at 2:14:50 PM UTC-8 James Montgomerie wrote:
Hey folks,
Been lurking for years, but just joined properly now. I designed and made a clock myself a few years ago, using Burroughs B5750s on ‘original’ vintage carrier cards (pics attached, though I don’t think they’re important to the question I have). The clocks have been working for a year or two (and the pictures show them when they were working), but they have started to fail in a way that surprises me.
The anode resistors burn out and (unsurprisingly) the displays stop working. Analyzing at the components after a failure, the thing that sticks out to me is that the tubes themselves seem to have shorts in them. In one case, the ‘0’ has shorted to the anode. In another case the ‘4’ has shorted to the anode. Given this I think what must be happening is that the tubes develop these shorts, this causes too much current to be drawn when these numbers are meant to be illuminated, and in turn this causes the anode resistor to burn out.
Sometimes, before the failure, multiple numbers light at once, and/or the front of the tubes get silvered, but I’m not sure this is related. I think the multiple-numbers-at-once might be being caused by the ground-side control circuitry having to sink too much current from the short.
The repair is easy - replace the tube and anode resistor, but I’d like to understand the problem better and perhaps prevent it.
The power supplies seem fine, and both the supply voltage and the voltage across the anode resistor (and, by implication, current - about 2.6mA) are good when the tube and resistor are replaced.
Is a dead short internal to the tube a common fault I should be preparing for? Could it be caused by power surges or the like (in at least one case, the clock seemed to fail immediately after a power cut)? Is it likely that something in my circuit design could be is causing or encouraging this?
Thanks for any help!
Jamie.
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