Tests on DCF77 reveiled, that frequency stability is not one of the big pros of the receiver. OK, no QRSS, which was shown by Des (M0AYF).
Anyway, there was one feature I never understood, and I also disliked a lot. The clarifier position. Thus, I opened the (plastic) box to see what could be done. Next to the position where the clarifier poti is wired up to, a crystal and a varicap-trimmer are to be seen. The part is the local oscillator. Ideal, lets play with the varicap first. I got what I wanted.
Since the receiver was open anyway, I took the chance and carefully played with the inductors starting from the first IF, further to the second IF and the one in the LO. The factory settings were close, but on every single trial, a bit of improvement was possible.
At the very end of the exercise, I also tweeked the reference oscillator's crystal pull. That resulted in a short back and forth between this reference oscillator and the LO.
All in all I am somewhat satisfied with the result.
As an extra bonus, the erratic jumps of several 10 to 100Hz seem to have disappeared. Seems that the reference oscillator was slightly off, causing the PLL to fail locking from time to time.
Over the time scale of about half an hour, there still seems to be a 1-2Hz drift. I can live with that. I will try the radio as QRSS receiver, but hopes are not that high. Finally, the receiver was supposed to provide me with Navtex infos, nothing more.