Early ZM1000 nixie failure

[franetic]

Hi all,

can anybody (of more experienced builders) explain extreme darkening of a (then new/unused) ZM1000 nixie tube in just 3 months? The failed nixie was used in a clock in the minutes position. (The clock is a multiplexed 2x2 design, 10mA peak current at 25% duty cycle - this hits the average 2.5mA cathode current.) I've built a few of these clocks, and have never seen a ZM1000 fail like this. (I also use that clock at home and another one at my office, they have been running for years with a little silvering at the bottom and only slightly noticeable darkening.) One thing that I noticed is that the failed nixie has the anode grid cage quite a bit more out of place than usual. Could this be the culprit?

Thanks,

Damir

[Jon]

A couple of possibilities to think about... Either you've got a rogue tube where there is some manufacturing flaw that is markedly increasing sputtering. Or there's a problem with the clock circuit which is allowing a significant over-current through that tube specifically - maybe a current limiting resistor is not the value that it should be (wrong component picked out of the parts bin?).

Without seeing the circuit for the clock, I can't judge whether the second explanation is possible.

Jon.

[sture nystrom]

I had a similar massive darkening on a IN-14 tube some years ago. Repalced tube and it have working good since then. I supposed air have come inside tube.

Sture

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[marta_kson]

Can You see if the mercury giver has released it's content? Did the failed tube show any sign of a different color? I have a Telefunken ZM1210 where the giver has not melted as it should. That tube lights up in a much more reddish color than the OK ones. If there was an overcurrent or too long time slot for just that tube of any significance it should have been so much brighter that You had noticed something was wrong.

[franetic]

Thanks to all for the replies. I don't think I picked a wrong value resistor (there are only 3 values of through-hole resistors in my box), so I'll go with the rogue tube explanation. The affected tube didn't glow any brighter, nor did it glow in a different color. Anyway, I cannot check the affected tube (I don't have the clock in question), I simply shipped a replacement tube to the customer. 

Best,

Damir

[GastonP]

It might also be that you got a wrong value resistor in your PTH box. If it happens again to the same customer, make him to *measure* the value. It will not be the first time one gets a wrong part.
Gastón

[gregebert]

Something to be aware-of is the elevated current used in multiplexing, in this case 10mA vs nominal 2.5mA.  Info I've read says nixie-degradation is an exponential function of drive-current (somewhere I saw 2.5).

Even if the exponent is 2.0, increasing the current 4X and multiplexing at 50% would cause tubes to degrade about 8 times faster than they would with direct-drive. For this reason, and others, I always use direct-drive for discrete tubes. The one panaplex design I did had separate pins for all anodes & cathodes, and that, too, was direct-drive. (most multi-digit, and probably all dot-matrix displays require multiplexing due to pinout).

In this particular case, where only 1 tube failed, it's more likely to be a manufacturing defect, or the tube had substantially more hours of usage than the others.

[franetic]

@Gaston: I'll definitely ask the customer to observe if the tube in same position fails again.

@Greg: Multiplexed or not, the usage parameters are within the ratings (as per data sheet) for 100.000 hour lifetime of the ZM1000 (particularly for the minutes position). Moreover, I've recently contacted a customer who bought two such clocks from me 10 years ago out of curiosity. The only complaint was that the tube in tens-of-hours position was starting to fail, which is very similar to my experience. The ZM1000 in one rugged nixie, so I hope that the above example is a rare manufacturing defect.

Thanks to all,

Damir 

[Dekatron42]

I have seen this once on a ZM1042 where the anode screen had come loose from the pin at the back, it might be the same with your tube. If you can get it back you could check it to see if the same has happened to it.

/Martin