Work around available although problem still there!

I've recorded the same midi file using the output from the main jacks, and from the headphones socket, and both look fine.

The output from the phono sockets is still strange (I thought I'd better double check).
I can hear a difference between good and bad, the bad one isn't as crisp and seems to lack a shade of punch. It's not a huge difference though, there's a case to be made that I'm persuading myself there's an audible difference because I know there's a waveform difference there in the first place!

Anyhow, I'm still going to take the lid off, disturb and reseat all connectors, and maybe even put a bit of contact cleaner into the main volume control (it's getting a bit crackly) but that's as far as it goes. If this doesn't resolve the issue with the phono output I'll record off the main jacks.

Thank you all for your assistance, it's made sorting this out much less of a shot it the dark!

John