So, that would seem to make it the actual deck then - nothing else
seems to fit the facts.


Could be, see below regarding laptop test.



Motor is mains. If this mains supply is disconnected the hum level falls
by about 5 dB

Do you mean actually pulling the plug out of the wall,


Yes, unplugging.



That also suggests that the hum may have two, or more, sources, and by
doing that you are fixing a less important one.



A reduction of 5 dB is worth looking for. Subjectively, it reduces hum
from perceptible to only just perceptible on quiet passages under my
listening conditions here.

I would imagine that there are often multiple hum sources and the object
is to reduce/eliminate enough of them so that what remains is not a
problem to the listener (me).


I just had a quick skim and your mention of laptops reminded me that I
do have a laptop available that I could try on battery power.

Certainly worth a try.


I tried it with my laptop. With laptop PSU plugged in Audacity gave
exactly the same hum levels as with the desktop.

With laptop PSU unplugged the hum drops by about 6 dB. This is almost
identical to the drop in level when I unplug the turntable mains supply
when connected to desktop. At this lower level the turntable sounds very
quiet. (In this room, with these speakers...etc etc.)

(BTW, I've been calling this 'hum' but with components up to and beyond
3 kHz I know hum/noise is a better description)

Above makes me think the problem is earth loop related but I can't see
how, yet. Would be interesting to isolate the tt motor supply with a 1:1
transformer to see if that 'breaks the loop'. I may have one around
somewhere, would be good if it had an isolating screen...


I am aware of the headshell contact issues with this turntable so will
be looking at that when I get more time to play.

Good luck.


I would have expected a headshell contact problem to result in erratic
and rapid signal level changes or discrepancies in hum between channels
etc. Still, something to check if all else fails.

Thanks for the ideas.