Now that I've determined with a certain level of confidence that the "front end" of the A Trigger circuit is OK up thru CR751 I now find myself pushing up against U780 again.
The front end
does look fine, but there are TDs and other stuff between cr751 and u780. Have you set or changed the bias trimpots for those TDs?
One problem at this stage is that there are feedback loops from u780 to the TDs, e.g. via q768 q778 and similar.
Here's a couple more images. Timebase 2ms/div, 100Hz (N.B.!) sinewave input. Note DC coupled, with 0V at the bottom of the screen. The picture at 1V/div is the "out TD" TP, the one with 2V/div is the "arm TD" TP. The amplitudes are constant, but the relative timings of the various steps depend on the input frequency, the timebase, and the holdoff (set in the normal fully ccw position).
Note that the various voltage levels are a function of whether q768 and q778 are conducting or not. I'd look closely at all the voltages around there (inc q802 etc).