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Grace G707 Hum possible grounding issue?

Chaggy78

pfm Member
Hi guys,
I have a Grace G707 tonearm on an old Systemdek turntable and am using a Audio Technica AT3200XE MC cartridge which does sound good but when the arm is on the rest and I turn the volume up there is a very annoying humming noise. I did a continuity check with a multimeter and it appears that the arm post is grounded but the arm wand/tube and headshell is not. When I touch the headshell or arm tube with my finger the hum reduces a lot.
Is the arm tube supposed to be grounded also?
Thanks I really look forward to your replies/advice.
 
Yes. There is an internal wire that grounds the arm tube, it may have broken. You can check with a meter for continuity from the tube to the groundwire.
 
Thanks for your reply Rusty it's much appreciated! I have already done a continuity test and test and the arm tube is not grounded at all but the arm post is. I am now wondering where the wire in the arm tube may have been attached if it has broken off. I was thinking about rewiring the arm soon anyway as the old wires are not the best.
 
Does anyone have any advice on how to remove the socket from the base of the arm? I read somewhere online that you have to twist it and pull it gently out but I just thought it would be a good idea to ask if anyone else has done this. Thanks!
 
With the tonearm pillar fully removed from the base screw (a.k.a. the threaded mounting cylinder), a small set screw should be revealed near to the bottom end of the pillar.

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I am trying to solve the same problem.
I ve removed the little screw in lower part of shaft as in the pic above but i cant remove the socket that receives the plug and interconnects .
Anyone have any ideas how to take this apart ?
 
If the hum is only there with the arm in the rest, the rest is providing an earth loop path. Which does not tie-in with the arm being isolated from earth, it has to be said.
 
If the hum is only there with the arm in the rest, the rest is providing an earth loop path. Which does not tie-in with the arm being isolated from earth, it has to be said.

Nope that's wrong. There are more causes for hum than ground loops... Finding and fixing the ground wire to the arm tube is the first thing to do.
 
Thanks gents
There is hum all the time, its is worse when one touches the headshell lever .
There is the odd business too of a metal plate/foil on the side of the headshell facing the cartridge and this is connected to the Green headshell wire terminal .
I too seem to have no Ground to the ar,tube hence i want to get in to the shaft and the connections therein to see whats going on there .
 
its is worse when one touches the headshell lever .

I would say that that is common, or at least common here - your body is earthing the arm. I have never known it to mean that hum was audible when not earthed through me, unless the volume was turned to totally lunatic levels.

I have never noticed hum being exagerated with any arm in a rest though, but I am pretty certain that none of the rests have been earthed.
 
There’s a copper sleeve pressed into the arm tube at the head shell end to which is soldered a black wire. That wire travels back down the arm tube to the five pin connector in the arm pillar. Because the head shell is made of plastic a brass plate is glued to the roof of the head shell to provide some shielding. The brass plate is soldered to the green terminal at the circuit board at the back of the head shell. Maybe that joint has fractured which would explain the hum when you touch the finger lift.
 
I am trying to solve the same problem.
I ve removed the little screw in lower part of shaft as in the pic above but i cant remove the socket that receives the plug and interconnects .
Anyone have any ideas how to take this apart ?
I've not attempted to remove the female DIN socket from the pillar of G-707, however, I suspect that in order to free up enough slack to do so one may have to desolder the internal wires from back of the headshell PCB, as well as release the ground wire attachment within the tube (post removal of the headshell set screw).
 
It’s been quite a few years since I last dismantled a G 707 but I seem to remember the din socket was glued as well as having a grub screw.
 
Dead easy to lash-up an earth for the arm tube to seee if it fixes the hum - it doesn't have to allow the arm to work. If it doesn't fix the hum................ If it does, you'll have the task of dismantling it to make a proper job of it.
 
Dead easy to lash-up an earth for the arm tube to seee if it fixes the hum - it doesn't have to allow the arm to work. If it doesn't fix the hum................ If it does, you'll have the task of dismantling it to make a proper job of it.

I will run a wire from base of arm (from under the locking nut) to the phonostage Ground terminal and report back.
Earlier what i did was run wire from base of arm to the Earth/Ground terminal of the tt (td124) - that did not work.
 
I will run a wire from base of arm (from under the locking nut) to the phonostage Ground terminal and report back.
Earlier what i did was run wire from base of arm to the Earth/Ground terminal of the tt (td124) - that did not work.
Because there are (supposed to be) two internal ground paths, tube to ground plus pillar to ground, that converge before exiting the tonearm, the limitations of running a length of external wire as test are as follows:
  1. If the standard ground lead is also connected then a ground loop is likely to be created, negating the usefulness of the test.
  2. If the standard ground lead is not connected then only one ground path is being employed at a time, likely resulting in a similar hum level even if the path being tested proves to have continuity internally.
Your best test points are between tonearm tube and tonearm pillar (with the standard ground wire connected to the phono stage ground terminal). A positive result here won't tell you which of the two internal paths has no continuity, however, it will confirm an internal ground fault.

This level of investigation really merits the use of a multimeter.

Craig
 


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