Thanks, Ed & dan. I will check R18 in particular. But I apparently have overthought the problem. It's mechanical, not electrical! I posted three pics in "Sampling with 3S2" (and set the order to most recent first, to save you thumbing through in alphabetical order).
I carefully adjusted the pots (ALL of which interact, what a PITA) as in the manual. Since I don't have a really fast edge, I used my ~600-650 ps pulser edge and set it to display ~750 through the S-1, which should be close to 350 ps risetime. Going back to the 1 MHz square wave, it looks like crap as I initially described. Not only that, suddenly the transient response pot wouldn't do anything either! But flexing the preamp board the right way caused it to pop into flatness (second pic). Now I just have to find the bad solder joint (or carbon comp resistor)...
The slight ripples in the S-2 are probably because I haven't adjusted it either - or possibly from the 114, cables and adapters, and too fast to see on my 60 MHz scope mainframe)... I'd try my delicate S-4 but it has a new hybrid and hasn't been readjusted either! Guess it's time to buy a Leo Bodnar pulser :)
