Roger,
Well... I can't tell I`m 100% sure.
I can tap with a x10 oscilloscope probe onto the 11A and 11B signals coming out of the 7A26 Vertical plugin, and the waveform, although not perfectly square (it's a little roundy in the corner), it doesn't show that slowly flattening voltage over this long 3us time period...
I assume the non-perfect corner I see (while probing the plugin output) is because I cannot properly reference the probe with a short ground lead so, I cannot take seriously anything immediately following the sharp edge. but the passive probing + long ground lead could not be concealing this 3us long voltage lagging.
If the problem I would be having would be in the first 100ns or so, I reckon that any artifacts I see could be coming either from the plugin or from the mainframe misalignment (or a combination of both), but 3us is just too much.
For a circuitry of that grade... anything slower than 1us is almost the same as DC... lol.
I also reckon that in the 10s or 100s of milliseconds, there can be other disturbances at play, like thermal imbalance (between different sides of complementary designs) or power supply mains ripple... but that's not the case here.
Yesterday, while I still didn't have chance to probe the P.S. voltages (inside the board, after decoupled), I was really convinced there was a decoupling issue... that right after the fast corner which consumes a lot of current... the available voltage was depleting and the output would be - then - being limited by the available supply, until the - lacking - decoupling would have time to recover.
But yesterday evening I probed all the important voltage supplies and bias points, and they're pretty much rock steady.
Not happy, I added additional decoupling in parallel, to each of those voltages... and neither made any difference to the displayed waveform so, I think it's safe to conclude that there's no decoupling issue (at least not on the Vertical Amplifier board).
I still need to find ways to rule-out a possible decoupling issue on the Vertical Channel switching board, that's difficult to reach and to probe at.
Happy easter to you all.
FT
Fabio Trevisan