Analysis of DC zero circuit in the HP 3466A DMM

Can someone explain how the DC zero on the HP 3466A works? The manual with the full schematic is on the Keysight web-site:

The first thing to note is that the topology of the input amplifier is non-inverting on the 20 mV and 200 mV ranges, and is switched to inverting on the higher ranges. Figure 8-19 "Ohms Current Source and Input Amplifier Schematic", on page 8-31 of the manual, shows the essential parts of this circuitry. Here's an excerpt showing the switching around the plus (+) input of the input amp:

input amp.png

On the 20 mV and 200 mV ranges, Q105 is on and connects the DC input to the plus input of the amplifier. Presumably Q107 is off. On the higher ranges, Q105 is off and Q107 is on, thus connecting the plus input of the amplifier to circuit common.

Now the puzzling bit is that the DC zero control, R134, applies an offset voltage to the drain of Q107. This makes sense for the ranges where the input amplifier is in its inverting configuration. However, the manual has you set the DC zero on the 20 mV scale. As I analyze it, that means that to the first approximation, Q105 is conducting and Q107 is not, so I don't see how the DC zero can affect the non-inverting configuration of the input amplifier on the 20 mV and 200 mV ranges.

My analysis must be wrong, so I'd appreciate someone setting me straight.

© Steve Byan 2011-2019