hmm, actually, i wonder if the main contributing factor to the bandwidth, is actually just those tantalum capacitors? specifically c4/c18
if you look at the "dc balance" circuit, you'll see that twiddling the position knob on the front of the unit produces an at most ~0.2v in voltage from one extreme to another to the base of TR1B, and that's enough to more than take the trace off the edge of the entire screen, both ways, so maybe 0.01v to create 2 divisions?.
it's a really big capacitor too, implying that line needs serious decoupling, as the frequency increases, isn't the ability of that cap to decouple going to decrease a lot due to ESL, if that line needs to be stable to within <0.01v ripple under load to stay within 2 divisions,that tantalum might not be capable of that at higher frequencies?
wouldn't take a huge parasitic impedance to cause that, something on the order of 10ohms?
dunno if that's how it actually works just spitballing
,
and i don't actually have equipment good enough to test the frequency response of the scope, so just theoretical for me.