OK, folks - Tuning clicks conquered!
Reading back on earlier messages here, it seems the regulator 7805 deserved some attention. I had fed this via 51E to drop a few volts and 'take some heat', and put a 220uf electrolytic across its output thinking it would absorb the current spikes from the Arduino as it changed state. Given the severity of the spikes as at my last missive, it didn't, so in further experimentation I tried clip-leading a 1000uF across the 7805's input and the clicks disappeared. OK, I thought, that 220uF is doing no good where it is, let's try that on the input. Bingo! Just for good measure I put a 10nF (103) disk - just what was to hand still from the old PC PSU - across the 7805 output anyway.
I've left it at that for the night. It seems the audio is more stable too, and I've heard my first valid signals! Digital stuff down the bottom of the band. Hope is on the upsurge!
73 all, de ZL2DEX
