Thanks Aurora for your input. Yes, the MMIC re-work does look untidy work for an official re-work doesn't it!..and the biasing does look like select on test from the resistor chain appearance.
So, I spent the morning checking out the gain from the input through to the output of the MMIC.
Accounting for external cable losses etc, the test setup had 0dBm at 2GHz injected into the input N connector. I then removed the interconnecting coupling caps to measure the signal down the chain.
With C101, 103 removed and coax tapped onto the output of the Pi attenuator, I measured -19dBm. Looking at the Pi components, this seems high..and coincidentally 6dB more than I calculate it should be. But I'll be upsetting the match somewhat with my soldered coax tap and the removal of the coupling caps. I tried to keep the coax tap as short and tidy as possible.
I put the caps back in and then took out C102,104 and tapped onto the output of the switch, U26. I measured -22dBm. This suggests a loss of about 3dB through the six switches. At 0.5dBm per switch, this seems about what I'd expect.
I put the caps back in and then removed the two caps from the output of the MMIC and tapped onto this output. I measured -10dBm, suggesting the MMIC is providing 12dB gain. This is about 3dB lower than the data sheet suggests, but again, I'll be affecting the matching with my coax tap and removal of the coupling caps.
So here's the big surprise. I refitted all the components, and decided to re-check the error on both the 2GHz signal and the 100MHz internal cal signal....
Weirdly, the 2GHz signal was now showing only about 0.9dBm error (reading about -1.9dBm) taking into account the external cable loss of about 1dB.
I switched on the internal 100MHz calibration signal and measured 1dB error.
With all the screening covers screwed back on, the unit is still showing only about 0.9dB error, which is within the tolerances of my test kit and the unit.
Very odd... but I seem to have a correctly working unit after all of that! At least I learnt something about my analyzer
...but not why it had a 6dB error before stripping down!