[morzh]

Another thing I noticed while investigating: in my set the screen grid of the second 24 tube (they call it 1st Detector - there is also confusion there in Riders how they call what) goes to "J" voltage, and it seems factory - there is one wire running through all tubes and the soldering was clearly original.

It is supposed to go to "H" voltage, same as 2nd Detector's screen grid. But the 2nd det. SG is the only thing going to "H".
And, in fact, before I moved the 250K resistor to it, it ALSO did go to "J" and nothing was going to "H". And the soldering was original. (we spoke of it in the very beginning).

So....I tried to move 1st detector's SG to "H". The sensitivity went way down. And then something started happening - reception started fading. So I decided to leave it where it was - on "J".