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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

rondollarsign
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cleeds
5,944 posts
 
rondollarsign
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) ...
It isn't clear what you mean by "independently grounded." You want all earth grounds to either be tied together at one good ground connection or - if that isn't convenient - for each earth ground to be at the same potential (voltage). The left and right signal grounds should be kept separate from each other and from earth ground. These connections can all be verified with a DMM.
rondollarsign OP
6 posts
 
Thanks, cleeds. OK, yes, that makes sense to me. The L and R signal grounds are soldered to separate posts on a small EMI screen, which is attached to turntable base/decking. The earth ground wire is also attached to a post on this screen, as are the other two signal wires from the tonearm/headshell combination. Finally, the L and R of the RCA out are attached to posts on this screen.

With my DMM, I get a continuity beep when testing the blue tonearm wire against the ground wire on the RCA out connection for L, R, as well as the dedicated black (earth) ground wire connection. I get the same result when testing the green tonearm wire against all three of these.

Photo of EMI screen + connections: https://photos.app.goo.gl/T7pMiX1L75mPjTPv8
lewm
13,012 posts
 
Are the turntable and phono amplifier plugged into the same AC outlet? If not, try that. Ground to that outlet too. Same goes for the amp that drives the speakers.
rondollarsign OP
6 posts
 
lewm--

Both are attached to a grounded surge protector on my bench.
lewm
13,012 posts
 
This is a long shot, but some of those surge protectors may isolate the grounds for each outlet. Anyway, I would try bypassing the surge protector and plug everything direct into a single outlet. 
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