TT Hum

So... I have not noticed this through my speakers, but in headphones; I get a low hum depending on the location of my tonearm. When it’s resting it’s noticeable, but when I move it to cue the hum reduces. Basically, the closer to the center of the platter the less the hum. I have an SL1200, and the power supply is offboard so it shouldn’t be anything under the platter, there is no transformer, etc. It’s not the ground wire from the arm (Jelco SA750D) either. My PSU and everything else is on another shelf, so I can’t see it being interference, and when moving things around it doesn’t change. The only things I can think of could be some kind of weird interference from the pitch fader (that’s the only thing even near the arm's resting position) or I’ve yet to try yet another cart/headshell. Stumped. Thoughts?
au_lait, when it talks like a duck and walks like a duck it is usually a duck.
The Cartridge is not well shielded and there are electronics at the right side of the table that are emitting. Make sure the cartridge body is getting grounded. You may need a cartridge with better shielding. It is not too bad and you do not hear it through your speakers so you may elect to just live with it. 
Mijo, I ask this out of curiosity, because you raise an interesting question: how many cartridge bodies are grounded or even can be grounded, unless there is continuity between the cartridge body and one of the  audio grounds available at the pins?  As you know, many cartridges are made of plastic or other non-conductive materials.  Also, many headshells are non-conductive (made of wood or carbon fiber, for example), so grounding through the headshell is often not feasible.  For that matter, the only cartridges that would be shielded would be those with conductive metal bodies that are then grounded preferably to the chassis.  I am not implying that your advice is not good.  I am just curious whether I am missing the point.  Thanks.
This is not at all to say that you are not correct in guessing that the OP's cartridge is picking up some radiation from the electronic panel under the tonearm.

-What is the cartridge?

-If you have a few different cartridges do you have a hum with all of them?

-Any proper platter mat on your Technics SL1200 or just a felt slipmat or stock rubber mat ?

-Hum is noticeable on normal level or on high volume only ?

-Can you hear the hum only using your headphones and its amp or you can hear it in the speakers?

P.S. First thing to check is ground wire from your Jelco tonearm to your phono stage. You can disconnect it and try again to see is the hum is gone or increased. You have to check headshell leadwires and headshell connection (lock) with the arm. DIN connection of the phono cable to the tonearm base. This is basics. Put some nice metal mat on Technics platter and try again.

@chakster Yeah I have done/checked every one of those basic tests and have narrowed it down. It's not a ground issue - my phono cable grounded to either phonostage or preamp is all good. Its a very quiet hum (not buzz) and it really only happens when the arm is resting. Not hearing through the speakers unless its really loud, BUT I am digitizing lots of vinyl and I hear it a bit through the ADC on headpones. It's not the end of the world, of course, and I will try another cart tonight. Just curious if anyone had experienced this before. Maybe I lowered my noise floor to the point that I've uncovered something that was masked before?

Cartridge: Nagaoka MP150 (Will try with Zu DL103 tonight)
Mats: Nagaoka Crystal, will try with Herbie's, TMM Monitor, Rubber/Cork mix tonight
Almost any cartridge will produce a faint noise if it is in space, not touching the LP surface, and if you then turn up the gain high enough. That is more a function of the phono stage signal to noise ratio and/or ambient electrical noise, than it is a function of the cartridge.  But on the one hand you do say it only happens when the arm is resting, which fits my theory.  Then you imply you hear the faint noise on your digitized copies of LPs, where the stylus must have been tracing a groove, which contradicts my theory.  Which is it?  The cartridge hanging in space is like an antenna which picks up stuff and then delivers it to your phono stage, which has a lot of inherent gain.
Lewm, many cartridges have a shield inside that is grounded. Some may not. There is the noise floor of the phono section which you will hear if you turn it all the way up. But at any reasonable volume my ARC phono stage is dead quiet even with your ear right up next to the speaker.
I think the principle defect here is probably in the turntable as I have never heard of hum problems with a Nagaoka especially since it is an MM cartridge. The only cartridge I have ever had a noise problem with was a Grado which hummed as the cartridge got closer to the motor. They are notorious for this problem. 
No I don't hear it on the recordings really, it's far too faint for that, or I haven't tried any music quiet enough to notice. I'm talking solely about when there is no music playing (so maybe it just doesn't matter) ad it's loudest while resting. As I pivot the arm closer to the center of the platter, the hum lessens, almost as if it's on a fader. Will try new styles this eve and see. Thank you all for the advice!
I don’t think it has anything to do with the stylus, you’d better try one of your mats on the platter to block EMI effect. It can be tonearm internal wires in the arm tube or something related to the tonearm. It is true that some people having problem with Grado MI cartridges hum on Technics turntable, but not with Nagaoka MM. Anyway try different cartridge first and if the problem is there with another cartridge than it’s definitely turntable or tonearm.
Problem found. A dimmer on the other side of the house. It’s not on the same line as the whole system is on its own dedicated. I guess the dimmer causes a ground loop since they’re on the same bar at the box? When on: buzz. When off: dead quiet. Anyway, dimmer removed, problem solved for good!

Sorry @chakster for hassling you about the cartridge LOL
Ha! Good for you. When you say bar i am
assume you mean same leg of the panel ? Is the stereo and dimmer in same vertical row of breakers? You can seek an even lower noise floor by moving all motors and hash producers to the opposite leg of the audio side.
best to you
@tomic601 Yes, leg, thanks Jim, pardon my layman’s terms! But now that I think of it, not sure it has to do with what leg. A month ago my front end and amps were on 2 separate lines on the same leg at the panel, but now since moving my front end across the room, I ran another line to that location using a breaker slot on the other leg, but still had the buzz. The hash only cleaned up once the dimmer was gone. Noise floor is pretty damn quiet now, but I’ll try your suggestion too. It’s an old house and the panel is a mess with loads of tandem breakers.

Dimmers are notorious for generating rf noise - I wont have them in the house anywhere.