Started looking at U740 and I think I found some issue. This is pulse at TP5 as well as at CR751. 5MV/2ms and 5MV/1ms. That doesn't look right at all. Mostly ripple noise. Quick check of other pins of both U740 and U738 are showing same crap. I'm going to pick up in this area in the AM. It definitely looks fubar. Do you agree?
All stuff around that area, i.e. before c761/r761 does not depend on whether or not it is triggered; u780 is irrelevant.
u740 etc are classic long-tail pair differential amplifiers. The inputs are differential voltage inputs, the outputs are differential current outputs. If the input voltages are equal, the output currents should be equal. The output currents are converted into input voltages for the next IC by resistors, i.e. r731, r736, then r745, r746, r749 , then r751, r752 plus others around cr751. Using a single-ended scope probe can only measure the differential output current indirectly! The differential input voltage should be <100mV for linear operation; if it is more than 200mV the outputs will be fully saturated/off.
A good way to test that is to use the 1kHz cal out driving the external trig input (as per fig 6 top left). You should see a square wave on u740.5 and u740.9. At those two points I see
unequal 200mV-800mV square waves, and the relative amplitude depends on the position of the trigger level control.
If you don't see those square waves, then start at u730.14, u730.16 (inputs) then u730.5, u730.9 (outputs), then u738, then u740.