Well, I 'think' ive found one of the problems!
As expected, 5.450MHz is received 'reasonably well', with a -75dBm 70% AM signal from my Marconi 2955. The set is clearly 'deaf' though, as the signal is pretty much lost by -90dBm!
However, changing to test at 5.505MHz showed an interesting oddity! At first, as expected, it didnt receive. But then, I accidentally jogged the 10kHz switch between settings - and it started to receive!
Take a look ...
My suspicion is that the extra voltage drop caused by the anti data-contention diodes, is putting some of the synthesizer input lines on or below the threshold for the logic to detect it cleanly as either a 1 or 0. Now, this was always a risk of this modification and one that I had already addressed in the design phase. Solving this should just be a case of measuring the actual 3v supply line to see its true value, then adjusting module 5 to ensure its actually within spec. Then, if the issue still occurs, to take the supply of this line away from module 5s PSU and replace it with a custom 3v line derived from one of the higher voltage supplies, and adjusted to be just that bit higher, in order to compensate for the diodes.
Of course, its worth my checking that the pins on the switches prior to the diodes are giving the right voltages - just in case its actually a problem with mucky switch contacts!
Discovering this has also enabled me to discover that the AGC link does in fact come up to something a bit more like an AGC line voltage, and moving the S-meter connection to that point does in fact provide a meter indication! Its not right though, as the AGC line seems to ramp up very fast then dwindle away even with a steady input level, and anything above -60dBm and the AGC volts actually start going down again!