Thanks God I just finished my order, sent the invoice and saved everything on CDs.
No Grmph anymore, but ROTFLMAO
So, the repairman came, we checked the cables first and there it was, something in the AC connection. He left advising me to check the first cable (from AC to transformer as he couldn't check that piece outside from the workshop) before searching for another transformer. I looked in the various boxes upstairs and found one, tried it, it works!!!
Now the fine detail. Guess which is this cable I found? Hmmm? It's the cable of the audiotape-recorder that was used to save programs from the ZX81. For the youngsters around there who never heard of that, it was our very first computer in the 80s. A tiny keyboard connected to TV. Programing in Basic, b&w only, 1Kb of memory! You could add an external memory card but it was so unstable you were losing your work all the time...