At last my tachometer is working!
It's the old MMB-house with a new "tachometer-circuit" inside. (The instrument-workshop said it's a german brand, and it has to be calibrated to be able to work - i.e. you can't do it yourself.)
Unfortunately the signal and the negative had gone mixed up, so that's why it wouldn't start. The signal from the CDI-box goes both to the ignition-coil and the tachometer, but since it was connected to the ground no spark could be produced.
After this I will not give any more effort to my tachometer. If it works it works, if it dies it dies.