It's time to restore the 11/45.

[tony duell]

> 
> Ah. That's the part I was missing! I (naively :-) assumed that the 'scope's
> whole input amplifier would be 'floating', and the input signal would be taken
> as referenced to the 'ground' on the probe, which could itself be floating.
> 
> Clearly not! (I'll have to look at the manuals/prints for my 'scopes -
> Tektronix 465's - to see if they are this way. Anyone happen to know,
> offhand?) Thanks!

I am darn sure the Tekky 465 does have the 'ground' of the input amplifier --
the outer of the BNC probe connector -- tied to mains ground (and the metal
chassis/case of the instrument).

-tony