Modular Nixie Clock Rev3 keeps restarting

Sorry for the late response. The usual suspect for spontaneous reboots is the external power supply. I'm not sure what voltage you are feeding to the clock board, but if you happen to a 12V supply rated at >= 0.5A (bench supply maybe!) try using it. Do a factory reset (hold down clock's button as you apply power) and do the HV calibration, see how it goes. If that doesn't fix it we can then dig deeper.

(Yes I know the instructions say to use a 9V supply, but my personal experience is that 9V is borderline. I have a Modular sitting next to me here that has been running off 12V for over a year with no problems. On 9V it was never able to reach the HV target. Just sayin'!)

Okay, so we've ruled out the easiest possibility. Problem could be on-board power or it could be noise.

Noise:

- Make sure Pin 1 (reset) on the Atmega is held high - R13 present and correct value, connections good.

- Make sure all the electrolytic capacitors are the correct values in the correct locations, installed with correct polarity. Check this twice - it seems a lot of people get C1/C3/C4 mixed up for some reason.

- Long shot: see if lightly touching the 16MHz crystal aggravates the problem. There has been some talk about bad crystals lately.

Power:

Double check values, orientation, etc. of all passive components around the voltage regulator - refer to schematic.

Do you have anything that can monitor the +5V rail and record any dips? I'm thinking of something like my Fluke 83 meter with its MIN/MAX function. It would really help to know if the +5 is dropping out, even just for a few milliseconds. Also, is there any visible flicker or dimming of the power LED?

Thanks for the clear report! The board looks okay to me. I can't see D3 in either pic so I'm going to ask you to double check its orientation.

I think the scope trace explains the resetting. I'd like to know if that downward spike is appearing on VIN like it is on VCC. My thinking is like this: you said the HV generator seems to be involved in causing the spikes. But the HV generator runs off VIN not VCC (well, except for the gate pulse). If something is goofy in the HV generator, such as maybe the MOSFET occasionally "latching up" and drawing a huge current surge, you should be able to see it affecting VIN. Read VIN right at the board, not at the external power supply.

That's all I can think of to try at the moment. This is a strange one. I'm hoping Ian is reading here and has some other ideas.