So the digits are running fine now. I have another question: I use 4 neon indicators. Only one half of one neon out of the four is glimming in the test mode. I guess the resistance of 120k is too high for that?
It depends. There are lots of different neon bulbs with different current requirements, so 120K may or may not be right for what you are using. I'm going to assume that you have 2 neons connected in parallel in each separator position, which is the correct way to do it. No two neons are ever exactly the same so the one with slightly lower ignition threshold will light first and tend to "steal the power" from the other one.
You can try lowering the 120K resistors a little bit at a time but a massive change would not be a good idea.
Better idea would be to give each neon its own resistor. You could, for example, replace the 120K resistors on the board with jumpers and insert a resistor in each neon's wire. The value of the resistors depends on your neons. I would start big (220K or even 330K) and work downward if necessary.
Having only 1 of the electrodes inside the neon bulb glowing is normal. When you run a neon on DC only the electrode connected to negative glows. Only on AC will both glow.
Look into it later when the dust is clearing off the crater.
Lexxar, when you say "one half glows" you mean that only both upper or both lower Neon indicators light up?
Are the Neons connected in series in two groups? If yes, you call for trouble.
I have the same problems with my Blue Dream clock which uses this wiring scheme to reduce the load on the resistors. But the power supply delivers 170V which is not enough as such Neon indicators typically have at least 90V ignition voltage according to their data sheets (I also have some with over 100V ignition voltage).
As long as the Neons are fresh the ignition voltage is most likely lower and will work for a while.
In my Blue Dream I already replaced the Neons twice and currently have given up. After replacement the Neons work for about 6..12 months and then start to fail.
I will have to change the wiring some time but I cannot simply replace the SMD resistors but will need to used through hole parts which can dissipate enough heat...