Help with vintage Nixie Tube Clock

5 years 7 months ago #7356 by NixieDave
My dad designed and built this Nixie Tube clock in early 1970. The tubes do light up, but several numbers light up at the same time and it doesn't cycle. I'm not sure if the tubes are getting too much power, or if the logic circuits are faulty. There are a couple of wires disconnected, but I'm not sure where they should connect to. I've tried to read the schematics he wrote, but I'm having a hard time learning it. He was an electrical engineer, I'm a banker. So, I'm a little lost! Help! I'm still learning how to use a volt meter, but again, not sure where to start. Any suggestions would be appreciated.

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5 years 7 months ago #7357 by Ian
Hi Dave, that looks like a decent piece of vintage electronics you have there. Nicely laid out and very clean (in the design sense, not the detergent sense).

When you say that multiple digits turn on, are they always the same digits, and do they turn on fully, or just "ghost"? Perhaps a short video would help. While a picture is worth a thousand words, a video is worth a thousand pictures for troubleshooting... :lol:

The schematic is not hard, it's a fairly simple but totally functional set up. The fact that the digits don't move is most likely that the "1PPS" line is disconnected somewhere, or that the drive to it (from the "LFO" page - there is a divider to change down from 60Hz mains input and a separate oscillator, with a switch to choose which one to use) has come unstuck.

If you have a basic amount of test gear, we can get this working...

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5 years 7 months ago #7362 by NixieDave
Hi Ian! Thanks for the reply. I've made a couple of videos. But then something strange happened. This first video is the second time I've plugged it in.

The second video shows the tubes lighting correctly, but they aren't advancing unless I manually click the switch in back that disconnects the logic circuit.

Let me know what you think.


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5 years 7 months ago - 5 years 7 months ago #7363 by Ian
Oh, that's pretty good news, it means the logic is working order.

My assumption at this point is that the tubes were in an undefined state because the circuit is clearly not being clocked. Note that the 2nd, 4th and 6th tubes show the same mixed "7" and "9", but tube 1 shows something different from tube 3 and 5, and tubes 3 and 5 show the same "fuzz".

So, looking at the circuit, tubes 2,4 and 6 have the same wiring, because they have to be able to show all digits from 0 - 9. Tube 1 only has to show digits 0 - 2, and tubes 3 and 5 have to show digits 0 - 5. When the circuit starts up, the logic chips might be powering up in a state that is not supported by the driver chips. Normally, this state will be cleared within 1 second when the "1 pulse per second" pulse comes in a drives the logic, but because the 1PPS signal is not there, the state is not cleared.

You can check this by manually toggling the switch to see if the invalid state is cleared, because toggling the switch appears to drive the logic.

My best guess at this point is that the single wire that is disconnected probably came from the switch. The normal position of the switch drives the logic with a divided down 60Hz (divided by 60 to give 1PPS), and the other position of the switch is a "fast forward" position to allow setting the clock.

Could you attach a photo of the switch area, so we can see if there are any obvious signs of where the loose wire came from? Also a photo of the upright board next to the tubes? I think that must be the "Low Frequency Oscillator" (LFO) circuit. If this is the case, I think we're getting really close. I would suppose that the two dangling wires the the two feeds of pulse signals.

The good news is that I think if we can re-attach those three wires, your dad's clock will be in working order again.
Last edit: 5 years 7 months ago by Ian.

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5 years 7 months ago #7365 by Ty_Eeberfest
Hi Dave,

At this point I can't really add anything to what Ian has already said. I just wanted to jump in to say that's a very good looking clock, good schematics and so on. Very nice work. Shouldn't be too hard to get it going again

Gotta admit I'm not familiar with the "946" and "960" ICs shown on the schematic. I can pretty well infer what their function is but never encountered chips with those part numbers.

Look into it later when the dust is clearing off the crater.

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5 years 7 months ago #7366 by Accutron
946 and 960 are Fairchild micrologic part numbers. The 960 is a CTL BCD decoder. I don't know what a 946 is offhand, but given its number range it should be DTL. Fairchild CTL/RTL/DTL ICs are notorious for going bad, and it would be very difficult or impossible to find 1:1 replacements.

Micah Mabelitini
The following user(s) said Thank You: Ty_Eeberfest

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