Well, I’ve managed to sort out the cause of the noise. I swapped over the Op Amp chips and the noise practically disappeared. So I imagine one of them must have been damaged in some way. The residual noise that’s remaining is only audible when the gain is whacked right up, and Izotope RX can easily sort that out.
I haven’t made any progress with getting the left channel up and running, and it’s proving difficult to find a replacement long-pole dual gang potentiometer, so have decided to cut my losses and stick with a mono output only which is fine.
I also noticed the LCD had a bit of a flicker to it, so thought I’d look again at the power output on the motherboard, now that I’ve established there was a problem with the old PSU. And I’m pleased to report that it’s outputting power, so I’ve redirected the LCD backlight back to where it should go and the flicker has totally gone.
So all in all, a success and I’m already quite a way through multi-sampling it. Will put together an audio demo once I have a full palette of sounds to choose from.