Tonearm Ground Loop

Hi Everyone--

I've never posted here before, but I've hit a bit of a wall. I'm bringing an old Pioneer PL-12 back from the dead. This is a very simple circuit (relatively speaking), and it is fun to work on. I know this is not a "high end" table by any means. I have that covered with other decks (just ask my wife). But, I'm trying to learn more about what makes these things work and, in this case, not quite work.

Anyway, I've gotten the PL-12 in pretty decent working order, but I'm still getting a ground loop buzz when I touch the tone arm. I've tried most of the obvious things, I think... 

including: 

1) Connected the ground wire from the RCA to the receiver ground lug.
2) Disconnected the ground wire from the RCA to the receiver ground lug (made things worse).
2) Tried multiple different cartridges (to make sure a particular one isn't improperly grounded, etc.).
3) Tried connecting a jumper wire from the bottom of the tone arm assembly (inside the plinth area) to various parts of the metal interior. 
&
4) Tried connecting a jumper wire from the tone arm itself to a screw on the top of the metal decking.

This 4th "experiment" yielded the only "good" result. It eliminated the buzz (if not 100% then 98%). So, that led me to conclude something must be wrong with the grounding of the tonearm itself. 

As I understand it, the cartridge is independently grounded, the arm should be independently grounded, and the motor/rest-of-the-table should be independently grounded (via the RCA ground wire to receiver). That's a very fuzzy understanding on my part, I'll admit.

So...

a) am I right to conclude that the source of the issue is likely *inside* the tonearm?
b) what now? I'd rather not have an awkward jumper clipped from the anti-skate stem to the turntable deck... ;)

Thanks for any wisdom you can shed here...

Yours--

Aaron