Few lines of U26 content (B and R) found corrupted. By analogy with channel A and through many attempts, I managed to restore 100% of the original values. I attach my original U4 & U26 files, maybe it will help someday. Now all 3 channels have accuracy better than 0.1dB.
*** The problem was caused by imperfect contact between EEPROM and ZIF (TL866II). The first read failed and from that moment the content was forever corrupted. So be careful if you want to backup :)
Thanks for support!
Robert
YO4HFU