On my 4227, usable input is +-20v and Overload protection is said to be +-100v. Is there some kind of clamping action or reduced input impedance for inputs above +20v or below -20v. Suppose, for example, that I am measuring a 500v DC level with a 100x probe (with ~99ohm series resistor into the 1meg impedance of the 4227 scope in DC mode). 500v divided by 100 is 5 volts which should be well within the +-20v input spec. But suppose that I, but accident, select AC on the 4227 for that channel. Now we have 500v through a ~99 ohm resistor into the internal 4227 AC capacitor. If there is clamping action between 20v and the overload protection of 100v, then all should be well after transient settling, but if there is no clamping action the 100v protection level could be exceeded on a transient basis or the internal AC coupling capacitor rating could be exceeded.
I would hope that there is clamping action between +-20v and +-100v