1200 Motor Genereates EMF Causing Hum
The 1200 Motor is a 3 phase motor driven by precision quartz controlled DC pulses. There is no AC voltage present that could cause a hum producing electro magnetic field.
However, in the left hand rear corner of the chassi, there is a power transformer. Until 2009, the transformer used was a low profile very well sheilded part. the EMF radiated field at the cartridge was less than 7 miligauss. In 2009 a change was made to the transformer and the EMF field increased dramatically.
So, while there is some emf radiated by the transformer, only in the later models will that be enough to produce a slight hum in unsheilded high output cartridges. It still does not present a problem for moving coils or fully shielded high output cartridges.
There could still be other unknown sonic effects of EMF, and an external power supply is a good way to eliminate this concern altogether.
In most cases of hum it can be traced to either a ground loop, bad ground, or proximity to associated equipment with big unshielded power transformers like tube amplifiers. Deciding where to place a turntable should aways take these things into consideration. An EMF meter can be useful in hunting down sources of radiated hum producing fields.