[G0HZU_JMR]

Yes, I think that will have some effect too. Over lunch I downloaded an image of the insides of the output amplifier module from the HP memory site.

I think I've spotted the ALC diode and the associated components and I've marked up the image below. It looks like there is a gimmick shunt capacitor shape by the detector diode to deliberately add some controlled capacitance. It looks like the capacitance can be doubled by adding link wires to the left hand capacitor shape within the blue circle. Also the 200R resistor may be adjustable to another value by snipping bond wires, but that is just a guess. This shows that HP were probably experimenting with the RC time constant into the detector and this was presumably to compensate for the droop vs frequency through the step attenuator.

The junction of the 200R resistor and the 39R resistor will be the place where the source impedance will be close to zero due to the ALC feedback action and I've arrowed this in the image below.

I'm not sure the technology needs to be quite this tiny and advanced for a 1GHz diode detector but it looks impressive!