Suggestions for debugging a dead nixie (Wemos 4 digit clock)

I just finished my Wemos 4 Digit clock, and it an excellent project. I really enjoyed putting it together, and the instructions were good. A couple of suggestions re: the instructions; partly due to the fact that I had all the parts and just bought the PCBs:

1) the 220uF 16v electroytic cap under the Wemos board (I think it is C7) which I had was a little too tall to stand up and would have prevented the Wemos board from seating. There is plenty of room for it to lay down, so you may want to update the instructions mentioning that the cap should be laid down with bent legs if it is too tall.

2) The section titled "Hint: the header strips" is a little confusing. It was not clear to me that you were discussing the pin and socket header strips used to connect the display board to main board. Pictures here would be helpful because the strips are mounted on the 'wrong' side of the board.

3) it is probably a good idea to have a picture added showing the Wemos board with the pin headers attached. It it possible to flip the board over, and solder the pins such that the ESP chip and antenna are incorrectly on the bottom of the board,

I did have some trouble soldering the IN2 nixies. Looking back, I should have cleaned up the pins on the IN2s before soldering them. The solder was not sticking well to the pins, and this lead to some solder bridges which were really annoying to remove, since the solder preferred wicking on itself than the pins.

After completing the clock, it powers up OK, and I get the expected color changes in the RGB leds. However, the N2 nixie is not working (no display) and the two neon bulbs used for the colon are also not illuminating. The other IN2 nixies seem to be working fine.

I was hoping to get some pointers on starting to debug this problem. I would like to try to identify the problem before assuming the IN2 nixie is bad and going through the hassle to replace it. Any hints about voltages and/or waveforms I should look for on the nixie, the neon tubes, and/or the opto isolators would be helpful. I have a scope and the usual test gear.

- Bill

Ian,
I will to the issues in order:

1) My mistake, the electolytic on my board is 10v not 16v. My 10v cap is a little too tall to allow the Wemos board to seat. I just bent the legs (like you do for the MOSFET) and laid it down on the board - there is plenty of room.

2) Regarding the neons: I had soldered the jumper. The Ah-Ha moment came when testing the display board against the plasma ball. The neon bulbs did not light. I tried several new ones from the package, and they also did not light when held to the plasma ball. I now think these are actually neon sparkgap devices, not neon bulbs. They look identical to neon bulbs. I attached a (crappy) picture for your viewing pleasure.

3) Using a plasma ball as a test tool!! Cracked me up - I love it! It worked great. I have attached a picture of the display board held near the plasma ball, and you can clearly see the second nixie tube (N2) is dark.

4) Your recommended tube removal procedure worked great. Breaking the thicker base glass which holds the pins was nerve-wracking. I draped a small piece of cloth over the display board to catch the glass chips as I chipped out the glass with a small diagonal cutter. After cleaning off the attachment wires, I easily removed the pins 1x1. A slightly more difficult issue was cleaning up the pads to accept the new tube. I used a spring loaded solder sucker to suck the solder out of the pin holes, and then tiny drill bits held in my fingers to open up the holes to accept the new tube.

As recommended, I dry fit the new nixie tube and it worked fine. I am going to solder the new nixie and new neon bulbs in tonight. I will measure the voltages and waveforms for fun as well.

I am going to add all your excellent suggestion to my tool box, right next to my diagnostic plasma ball!

Finally, I included a picture of the numeric filaments from the IN-2 which I had to break just for entertainment value. I can see using a flipped 6 and 9, but it still puzzles me why they bothered to use a flipped 2 for the 5 in this, and other, nixies. Could not have saved that much money.

Thanks again for the awesome help, will let you know how the clock runs after the new parts are soldered in.

- Bill

Pretty sure I found a pointer to my neon-like spark gap devices. I think I got them from the "Electronic Goldmine" surplus site some time ago. I found an old add someone had scanned and slided out the ad. Also, here is a link to some similar devices available now on Ebay:
EBay Link