mikeh wrote:

This is a piece of music gear, a Roland MKS-20 piano module, which has
an audio output problem where the volume of both channels is low and
very distorted when the unit is first turned on.

Gradually, over the course of 15-20 minutes, the volume surges up and
down, and the distortion becomes less and less. After the 15-20
minutes the distortion is gone, and the volume stabilizes too.

Both the left and right 1/4" jacks and the stereo headphone jack
experience the same problem, but the problem corrects usually quicker
at the headphone output.

Can anyone suggest where I might start to look for the cause. I've
thoroughly cleaned all the outputs already, even though that doesn't
seem likely to be the cause.

Sounds like the power supply to the audio output to me.

Graham