I replaced the capsule as the last one had moisture related problems. When that noise had gone away, I heard the low hum and asked the owner about it. He said he had noticed a hum before the capsule had gone whooshy
I tried a few tubes that I know to be low noise/microphonics
I notice two trim pots on the board in the mic, probably some sort of bias adjustment? They don't affect the hum level
It doesn't look to have been dropped and I have checked solder joints. I replaced electrolytic caps in PSU and mic to see if that helped. But was left with the same low hum on the output- 80-100 mv of DC being read
The other odd thing is I read around 80 mv DC between the mic body and earth on the PSU. mic body attached to the mic connector via the shield in the cable but I am not sure how the ground scheme is supposed to go as I can't find a schematic for this