I have the dreaded hum

Well, you've already tried other phono/arm cables. And they work with your outboard. So the problem is in the preamp, whether the dealer thinks so or not.

I would be surprised if it was the RCA's since everything is brand new.

I had the same thing for a while, except mine was intermittent. I had to replace the Rca's. One had just worn out over the course of 25 years. I tried everything else first. Everything has been fine since. Good luck.

There is a problem in MC portion of the the internal phono stage. I'm not knowledgeable enough to tell you if its the MC switching or or the MC gain or step up, but there is no question that there is a problem in the internal phono stage. You know that the cart and arm wiring, and the phono interconnects are OK and you know the line and MM inputs are OK on the full function pre-amp. There's nothing else left.

To recap, i have a pre-amp with a built-in phono stage with a MC input.
When I connect the TT leads to the MC input it hums on one channel. It does not hum when the leads are disconnected. It does not hum on any other input incl
MM input. I went out and bought a stand alone MM/MC phono stage. When I use it connected to aux input of pre-amp it does not hum.
Thanks for your help. This is my first journey into MC cartridges.

So the phono preamp is a stand-alone and is OK.

I'm just trying to be clear here.

And the 'preamp' is full function and has a LOMC input? If nothing is connected to the inputs of that preamp, does it still hum when its in the MC position? Its OK in MM position? How about CD?

Atmasphere. When I use the stand alone phono stage the hum is gone. I only have hum when pre-amp is on phono input and switched to MC. That engages the internal step-up in the pre-amp. No hum with any other inputs.
All signs say it is the internal step-up, yet when I sent it back to the dealer he said he had no issues. The pre-amp was sent back to the manufacturer a few months ago for a different issue, came back repaired and apparently they checked out the phono section as well. At that time I did not yet have a TT so I didn't notice the hum. A few weeks later my brand new TT arrived and I have had a hum right from day one with the TT.
I will wait and see if I have any issues with the replacement Pre-amp the dealer is sending.

If you have a ground loop it will be in both channels. This is usually caused by poor AC ground design. Since one channel is OK, that's not likely it. Based on what I have seen so far in this thread, the problem is somewhere in the phono section itself. The swap of the phono interconnects (meaning: cables from the tone arm to phono input) rules out the table, and the problem moving from one channel to the other when you swap interconnect left for right puts it in the preamp.

Now there is a nuance worth checking: It sounds like you have a stand-alone phono section, if this is the case, what happens if you swap interconnects between the phono section and line stage? Does it move?

Yes => phono section *or* interconnect

No => line stage (input at least, unless it also hums on CD- you've not mentioned that so far...)

I tried a diff set of interconnects, no change.
The dealer is sending me another pre-amp to try, but he still thinks something else is causing it.
Could it be that the extra phono stage is isolating a ground loop?

Reconnect the old phonostage and see if the hum is gone.

Try a different set of interconnects

I agree; that circuit is usually independent and thus is isolated as the problem. Don't let those reps jerk you around and good luck.

Its your pre amp phono, MC section it need repair.Jan Olsen