Del Vento Thanks for your comments. Let me explain a bit more about my interest to tune and adjust my piano: for one part, I am a natural tinkerer. Since I was really young, 9 or so, I began to repair things, mechanically and electronically. I am Telecommunication Engineer but, instead of doing what most fellow students did, working in big telecommunication companies, deploying networks and such, I continued with my lifelong interests and my main job, which I do at my home lab, is to fix radio communication electronics. I have also done, as a hobby, some car and motorcycle mechanics over the years (I am now 53)
I am handy doing manual work and assembly and disassembly. So working on a machine as a piano is, is not an impossible thing for me. I did some adjustment to the escapement mechanism a year ago, as my fisrt work on my U3H. And, more recently, I fixed some squeak on the sustain pedal. I have good hearing and play by ear any melody I hear (or, if I have the chords, the one-note right hand melody is almost automatic for me; yesterday my end-of-year at-home night concert was all based on that 😀 )
Then, when the tuner came home last time (in March 2020, just one week before lockdown began in Spain), I was not impressed on his work. Tuning result was not bad, but there were some things which killed the experience for me: he was not careful enough and scratched the folded top cover of the piano. He used some tuning hammer movements which, I learnt later, are not too orthodox. He used an electronic tuner to set temperament and some other checkings, then aural tuned the rest. This is not specially bad, but my thought was that for other customers he would do a better job, but being myself then a two-year beginner, he decided to go the easy way. And he didn't touch up anything else but the midde pedal adjust.
So, for about 1 hour of work, he charged 100€. I think doing the tuning more carefully and using more aural tuning, he could do it in 1.5 hours. And I am sure, as he tunes concert pianos, he knew better but decided my piano (or myself) didn't deserve more.
Now, as two years have passed, the tuning needs to be redone. I can look for another technician. And will probably do sometime in the future. But would like to be able to do the work myself and learn in the process also how to adjust the mechanism. I have already the Reblitz book, so it will be my guide.
What could happen?. If I get it completely wrong (hope not to break any string, will do as you suggest), I will call a tuner and learn from my mistakes. If I can get it better than it is now, I will be happy and have some new abilities to share.
Incidentally, I began to work on vintage electronic church organs (40+ years old) some months ago, to help fixing the one in the local church, without any previous experience on it, and now it is working nicely and have already two more in the works. So, well, sometimes you just need to make a first step and get into things 😅
Thank you all for your help on this.