I finally found a few moments to test the -350 on the bench with the case open, too see if I could find what was causing the modulation to cut out. With the radio connected to the Marconi 2955 via its mini-BNC connector, I found that the 150Hz tone was waaaay too high (some 2.5kHz!) and the mic level too low. Some tweeking put the tone modulation back to a sensible 1.5kHz (for use with the PRC-349s) and turned up the mic audio so that my typical voice level reached an average 3-4kHz max deviation. This was all done as a 'rough' guess, as the Clansman sets, despite having agressive VOGAD action, it seems do not have a limiter, so unlike the Kenwoods I used to service, I couldnt see the audio distorting at higher deviations. I suppose if i'd bothered to connect up the audio to the 2955 to provide a known steady level tone, I could have made a more accurate job of it.
Perhaps more importantly for this set, it occurred to me that the bad modulation was noted on an air test when using whip antennas. With the radio open on the bench, a quite startling discovery was made
The battle antenna connector was not connected! The wire, which is the pink one you can just see in the top left of the lower half of the radio above, had broken right at the PCB.
After quite a tricky job to remove the remains of the broken wire from the solder pad, which is not only in a difficult corner and further blocked by the flexi-PCBs and coax, but also under a shielding plate, I resoldered the wire and reconnected the battle antenna connection. All tested well on the Marconi, and a further test on air between the -350 in use by Sam and a -349 down the garden with me, showed good comms and no breakup.
So, as long as ive diagnosed that correctly and the fix is good, I now have almost everything I want in my Clansman collection!