I was working on the part of the circuit that deals with the tens of minutes. I had the dekatron wired up. Every time it gets an impulse, it should jumps to the next stage. I have used a GS12C, which has 12 stages. I have connected stage 6 to stage 0, stage 7 to stage 1, etc, thereby halving the number of outputs to 6, which is good, because it's the tens of minutes and after 5 it is supposed to go back to 0. So we will count 58, 59, 00, 01...

The particular problem I had is that the dekatron stepped through the stages, but got stuck on 0. It would not move from 0 to 1. I checked all solder joints, resistors etc. but didn't find a problem. The dekatron's inputs are two guides, G1 and G2, which are supposed to get an impulse, briefly shifted in time after one another. If you give an impulse to G1 and then to G2, it moves one stage up. You can also reverse the effect, by giving an impulse first to guide G2 and the to G1, which will cause the dekatron to move one stage down. 

As an experiment, I reversed G1 an G2 and the dekatron started to move in the reverse direction and - a little to my surprise - did not get stuck anywhere. So for now, I think I will leave it this way. I have not connected the nixie tube yet; in this case I consider it an advantage, because I will have to reverse the nixie wiring to compensate for the reverse dekatron.

It looks a little like cogwheels now, like in a mechanical clock. Instead of looking at this as a problem, I might see this as a feature. I'll have to reverse some of the other dekatrons also, I guess.