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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
14 responses Add your response
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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. 
rondollarsign OP
6 posts
 
Lewm-

I'll give it a try.
rondollarsign OP
6 posts
 
Bypassed surge protector and went directly into a single outlet, but that didn't help.
noromance's avatar
noromance
9,768 posts
 
Try disconnecting one of the negative terminals on the cartridge. Then try shorting the negatives together at the cartridge.
dover
2,243 posts
 
@rondollarsi
From the photo it looks like there is no earth wire from the tonearm tube to the post that the large black earth (to phono earth) is connected to.

From the tonearm you should have right channel +- ,left channel +-, plus earth. ( 5 wires all up ).

I cant see if the braid is connected - is this connected to the arm ??
If so try connecting that to the black earth wire post. Otherwise if its missing you need an earth wire from the armtube to the post the black earth wire is connected to. Look closely at the bottom of the arm - is there a loose wire/evidence of earth wire missing/earth lug with nothing on it ??

rondollarsign OP
6 posts
 
@dover Sorry, the picture doesn't show it well, but there is a black ground wire coming from the tone arm, which is soldered to the EMI screen. I did a continuity test between it and the black wire that leads out with the RCAs to the amp, and it gave me a confirming beep.

I think the problem is in the arm itself. So, I took that apart -- now I kind of wish I hadn't... but when I did so, I found the black wire was loose from the copper clip I assume is designed to ground the arm itself. I'm not 100% sure I didn't pull the wire loose from the clip in disassembling the arm, of course.

I soldered the wire back to the clip and tried to reassemble, but in the process the blue wire snapped... :(

So, now, I'm going to rewire the whole arm... argh... it will probably still buzz once I've done that... but, I'll have learned some lessons, I guess...
dover
2,243 posts
 
@rondollarsign

Rewiring the arm shouldn't be too hard.
Best way is to attach multiple draw strings ( cotton or slk thread ) to each wire and pull it through.
Then attach the new wire to each draw string and pull through one at a time.
That way you'll never end up with no wire in the arm.
Pre tin the ends of the wire before you start.
Dont use cardas - its too hard to strip the insulation properly unless you have a 800 degree soldering iron, or solder bath.
Make sure there are no sharp edges where the arm goes from the tube to the pillar.
Good luck.
rondollarsign OP
6 posts
 
@dover Thanks for those tips. Unfortunately, I bought some cardas before your note arrived ... I just keep learnin'... ;)... these expensive lessons...

I'm finding it a little challenging to find non-cardas tonearm wire. Do you have any recommendations on suppliers or specs? I gather 30awg is OK, even if 33awg is ideal? The really small gauges seem to be harder to obtain, while Amazon is teaming with 30awg...
lewm
13,012 posts
 
Michael Percy Audio can supply several different wires of a gauge suitable to tonearm wiring. His catalog is on line. Ditto for several other boutique parts vendors.
dover
2,243 posts
 
Michael Percy Audio, KABUSA, Parts Connexion all have a range outside of Cardas. Connex, Discovery, KAB Superflex should be easy to work with.
 If you know the length some will pre strip & tin the wires for you.
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