I am testing several 556's that will be for sale and one has a lower beam that behaves badly with different plugins installed, so the gain in the mainframe is the problem. I can vary the position of a square wave from the cal out up and down with either the lower or upper CRT zone and time base, but on the lower beam the height of the square wave decreases as I position it toward the bottom of the CRT face. I'm going to start reading the manual soon today, but wonder if this is a common thing with an explanation like, "has to be in the last amp before driving the CRT pins", or "Can be the CRT internally damaged...".
When driving the upper beam on the upper CRT zone, height of the square wave stays constant all the way until part of it is off screen.
