Hi all and thanks for the replies and help. I'm pretty sure it's in the PSU but I will reconnect it all (again), fire it up, and try the trick with the microphone in a plastic tube. Trouble is, I'm 90% certain it comes from inside the PSU and when I had the scope turned on in a darkened room I couldn't see any corona. My sense of smell isn't the best but I detected no ozone. But I will try again and report back.
I did quite thoroughly clean the HV lead and the connector originally, and cleaned the cap multiplier lead when I had the PSU apart. They were totally blackened with this oily grime, as were all the wiring looms. I imagined that this could lead to arcing and other unwanted stuff, so it seemed a good way to start. It's quite astonishing that the thing worked at all!
What is the dog house hole? Is it the slot in the middle of the PSU panel where the HV lead and other wiring comes out? Once I have that confirmed I might try getting another plastic tube and seeing if I can sniff ozone through that?
Re bumping the HV lead, yes, I have tried it. No change, sad to report.
Currently waiting on a supply of tantalums and other bits to replace those present on the other boards, also a tool for inserting the connectors into new molex connector plugs before the connectors totally fall apart (the blue ones are already partially absent, I'm concerned for shorts).
