Shock hazard - Mr. Nixie ITS-1A clock

[Robert L]

Hi folks,

A friend received a rather nasty shock from one of his Mr. Nixie ITS-1A clocks... To add insult to injury, he dropped the clock when shocked and two tubes were broken.

I've repaired his clock and, in the process, identified and mitigated the shock hazard. Note that I am not connected with Mr. Nixie. I'm simply trying to help others avoid this nasty experience.

Here's what I found...

1) The black bottom case cover with  labels for "SET", "ADJ" and "ALM" is likely printed using a carbon black based ink... Whatever the ink used, it's conductive.

2) There are 6 trim pots on the tube carrier assembly used to individually adjust -270V supplied to each of the six tubes. Tabs on the trim pots are directly over the conductive black printed base plate.

The trim pot mounting tabs on my friends clock had come into contact with the conductive printed label. My friend touched the label and a grounded piece of the clock and was rewarded with the rather nasty shock.

The photos below show the mitigation I used on my friends clocks.

The mitigation is to assure that the trim pot tabs do not contact the black label plate. I added a triple thickness of Kapton tape between the trim pots and the label plate on his clocks. This may not be the best possible solution, but it's a starting place. I leave it to each of you to find a mitigation that you feel is safe. 

I urge you to check your ITS-1A clock for this hazard and mitigate as you see fit. 

I used a DVM to measure the voltage with one probe to the supply ground and the second probe touching the black printed label. I could also measure resistance between these two points with the clock unplugged. There should be an open circuit between these points -  no voltage / open circuit between these points.

A visual check will let you see if there's clearance between the trim pots and the label plate. I strongly recommend that you mitigate the hazard - clearance or not. At a minimum, I think that you want a non-conductive barrier between the trim pot tabs and the conductive  label.

Trim pot tabs are very close to the conductive label plate on the clock shown below... tabs were touching on the clock that shocked my friend::

This is the clock that shocked my friend. The photo shows a triple thickness of Kapton tape separating all of the trim pot tabs from the label plate. Tape is held in place by adhesive backing and is also trapped between the tube carrier and label plate:

This hazard and a possible mitigation posted here in the hope that it prevents others from receiving a nasty surprise!

All best regards,

Bob

[Terry S]

 Is this printing on a PCB? Not clear from your photos.

It seems unlikely that ink used on a PCB would be conductive.

[Dekatron42]

Would it be possible to replace the trimmers with Bourns 3362, 3386 or similar encapsulated types? Bourns also have white and grey types apart from the usual blue models.

/Martin

[Robert L]

Hi Terry,

I guess I could have been more clear... The label plate ink is only moderately conductive; black ink on a somewhat flexible white substrate... the label is not FR4 pcb material and there are no metal layers. I have no experience using this material and am just letting you know what I saw looking at the sheet on my friends clock. Both photos show pretty good detail of the sides of the label sheet. 

I measured about 18K between each pair of two side case screws securing the label plate. I measured higher resistance side to side between the more widely spaced pairs of front and back screws. Again, moderately conductive. Obviously no conductivity to speak of for the laser cut plastic case parts that the retaining screws screw into. I was able to measure the high voltage with one probe gently resting on the black ink surface. Lightly touching the black paint with the rounded side of a probe tip was sufficient. I did not need to touch the metal screws or use the sharp probe tip to break through a surface layer in order to measure voltage.

The flexibility of the printed sheet likely contributes to the problem. The sheet was able flex a bit to make contact with the pots.

Moving forward, my friends two clocks are packed and will be in the mail tomorrow. I no longer have a clock that I can look at or experiment with.

Again, all best regards, stay safe and well!

Bob

[Robert L]

I just remembered one more hazard... again an issue with the conductive black layer coming in contact with various voltages.

There is a small tab at an interior edge of the label plate sticking out toward the tube carrier pcb in the middle where the tube carrier connector is soldered to the tube carrier pcb. This is on the front side of the tube carrier pcb and is shown in circled the image below. The label plate tab is a few mm wide and is near the center of the label plate.  

Make sure this tab is clear of and above the tube carrier connector mounting pins. It is possible to catch the black conductive side of the label sheet tab on the connector pins when mounting the tube carrier. 

This issue will not be a problem if the label plate is properly installed with the tab clear of and above the connector pins.

Again, the same potential problem... there is the possibility of various voltages coming in contact with the conductive layer... possibly another shock hazard as there are moderately high voltages on pins in the center section of the connector. I could also imagine a possibility of damage to clock circuits depending on which pins touch the conductive black surface. While the -270V pin is at the side of the connector well clear of the plastic tab, there are other voltages on center pins where contact with the label sheet is possible. 

Once again, make sure the tab is completely clear of and above all of the connector mounting pins. It is possible to catch the black conductive side of the label sheet tab on the connector pins when mounting the tube carrier.

The tab is circled in red in this image copied from the assembly manual.

I think assuring that the label sheet tab clears the tube carrier pins will mitigate this additional hazard. I leave it to each of you to determine that the mitigation you adopt is safe and sufficient. My intent is to make others aware of the hazard so that they can mitigate as they see fit.

And once again, best regard, stay safe and well!

Bob

[gregebert]

If this uses an isolated supply (such as from a wall-wart), you need to touch 2 circuit nodes at different voltages in order to get shocked.

Any chance this was from electrostatic discharge (ESD) ?

[Robert L]

Hi Greg, 

No, not ESD... My friend touched the shell on the cable to an external time source (clock ground) and the label (-270VDC). Not sure exactly what he was doing at the time to touch both, but there you are.

Bob