Help with vintage Nixie Tube Clock

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...

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...

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.

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.

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.