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Grace G707 Hum possible grounding issue?

Discussion in 'd.i.y.' started by Chaggy78, Oct 13, 2018.

  1. highcut28

    highcut28 pfm Member

    Thanks Craig
    I have a multimeter and will have a go tonite .

    Shahrin
     
  2. Craig B

    Craig B Re:trophile

    Just for reference, Shahrin, with DIN-5 240º sockets and plugs, the middle pin (see graphic below) is used as ground connection.

    I typically start with testing removable tonearm cables in isolation, as these are subjected to the most stress. In this case, by setting the multimeter to 'Continuity' and/or 'Ω' and touching one probe to cable DIN ground pin and the other probe to the far end of the cable ground wire. I then progress to testing from arm tube metal to tonearm DIN socket ground pin, and from arm pillar metal to tonearm DIN socket ground; in so doing, testing both internal ground paths.

    [​IMG]

    P.S. Not that it is necessarily relevant to your testing, but the Lch and Rch connections within G-707 are likely to be the reverse of those in the graphic, as the DIN connector within is female.
     
    Last edited: Jun 11, 2021
    Mike Hanson likes this.
  3. Chops54

    Chops54 pfm Member

    I have a G 707 in bits in a box so I dug it out this evening and laid it out on the bench. I’ve been meaning to fix it for a while so I had a go at repairing the damaged bearing and installing new internal wiring. I took a few pics which I’ll hopefully post tomorrow.
     
    Mike Hanson, highcut28 and Craig B like this.
  4. Chops54

    Chops54 pfm Member

    Here's my G707 in bits.

    I'll start with the sleeve that fits in the headshell end of the tonearm. You can't get this out without removing the headshell. Unfortunately the headshell is glued to the tube and may break when you try to remove it. Mine was already broken when I bought it so was of no consequence.

    [​IMG]

    Here I've threaded a wire up the armtube so I can pull the ground wire in.

    [​IMG]

    And here"s the arm ground sleeve pushed back in.

    [​IMG]

    And here's the other end of the ground wire that will do down into the arm pillar and be soldered to the din socket.

    [​IMG]

    That's the ground sorted for the armtube so now I'll go to the arm pillar and the ground connection for that.

    Here's the arm pillar stripped down.

    [​IMG]

    The black wire is the ground connection for the lower half of the arm. We need two ground wires because the bearings may not have good continuity so the arm is effectively split into two parts.

    These next shots shows the ground wire connects to the lower part of the bottom bearing spigot. There are some thin washers there that determine the height of the inner bearing ring.

    [​IMG]

    [​IMG]

    And here's the pillar back together.

    [​IMG]

    [​IMG]

    The black wire poking out of the bottom of the pillar is the other end of the ground connection. It goes to the same pin on the din socket as the other ground wire. You can see they are just long enough to be soldered to the din socket and will then be pushed back up into the pillar. I'll come back to that later.

    The main problem with my arm was the top bearing you can see in that last pic. The dust shield had come away and a minute ball bearing has gone missing. I managed to find another and I've reassembled it minus the dust shield.

    Back to the arm tube and the next pic is of the new internal wiring.

    [​IMG]

    I'll leave it there for now as I haven't finished putting the arm back together. I might get that done this evening if I don't get called out :eek:
     
    Mike Hanson and Craig B like this.
  5. Craig B

    Craig B Re:trophile

    I've seen lots of broken G-707 headshells over the years, many simply from rough handling; that, and from those who've tried to adjust azimuth.

    I've often wondered if removing the tiny set screw and letting some solvent sit in the hole, with the tonearm held upside down, might help with removal of those headshells that haven't shared such a fate.

    Not necessarily for the purposes of rewiring, more a case that, after all of these decades, there could easily have developed high enough resistance between metal ground sleeve and tube for hum to have gradually set in. Certainly, the arm pillar ground connection appears to be one that would outlast us all.

    The unknown that remains is whether or not there exists enough slack in the wires for such removal without having to desolder, say, down the DIN end?

    I'm thinking that if one could draw the ground sleeve out the end of the tube (by at least its length), this could be both cleaned and tightened (the latter by expanding the diameter).
    Looking forward to part 2!
     
  6. highcut28

    highcut28 pfm Member

    Hi Gents
    I really appreciate the posts above Craig and Chops54.
    I had prepared pics but as i dont use an image host it seems i cant post em here.
    Let me try to describe the electrical connectivity, as follows

    Headshell wires :
    Looking at the headshell from in front, as it were, wires coming from the plate behind are from Lt to Rt : Green, Red, Blue, White (G,R,B,W).
    G is also electrically connected to the brass plate at roof of headshell.

    Socket at pillar base :
    Female
    I verified the connection of each individual socket
    Clockwise from 8 o clock to 4 o clock there are 5 sockets.
    G, R, Gr, W, B
    Gr is where the Ground lead is connected and is at 12 o clock.

    At the RCA plugs at end of interconnects:
    What is designated Rt by a red RCA jack the +ve terminal is connected to Blue headshell wire
    What is designated Lt by a black RCA jack the + ve terminal is connected to Green headshell wire.
    I was surprised by these findings! Certainly not conventional
    And weird that the Green headshell wire being Lt +ve is also connected to the brass plate !
    The negative of both RCA jacks have connectivity with the Red and White headshell wires.
    Also weird

    As regards the Gr lead, that has electrical connection to the pillar but NOT to the armtube .

    I think the Ground lead in the pillar which should connect to the armtube must have come loose/broken.
    I also think the male plug was soldered wrong !
    Hmm lots to put right and i dont think i have the resources here...
     
  7. Craig B

    Craig B Re:trophile

    Pass the Paracetamol!

    Working backwards from RCA plugs to headshell...

    RCA plugs:
    • Lch and Rch are reversed.
    • Signal +ve and -ve are reversed.
    F-DIN plug within tonearm:
    • Lch vs. Rch connected as per M-DIN pattern (not a problem if M-DIN on cable connected as per F-DIN).
    • Signal +ve and -ve pin connections reversed compared to standard practice. External cable must follow this convention for phase to be maintained at RCA plugs.
    Headshell:
    • Wires are connected in the standard G-707 pattern.
    Analysis:
    • None of these issues will necessarily lead to hum, that is, unless, along the length of the external lead, signal positive is connected to cable shield. Suggest you test the interconnect connections in isolation, from M-DIN to RCA plugs.
    • As long as the phono cartridge Rch -ve pin is connected to the green headshell lead, the metal headshell shield plate will be common to the phono cartridge body shield (with most cartridges). This is independent of tonearm ground paths and, therefore, unlikely to be part of the hum issue.
    Craig
     
  8. Chops54

    Chops54 pfm Member

    Highcut, your first two paragraphs seem to indicate that the internal wiring is ok. I think your external lead is at fault. I wouldn’t worry about the arm ground until you’ve sorted the arm lead. Is the arm lead original or has it been altered in any way?
     
  9. highcut28

    highcut28 pfm Member

    thank for prompt replies gents
    The chap i bought it from said he had to get new leads
    Blue Silver Sonic cables and RCA jacks
    Wrongly soldered at the male pin.
    I should be able to sort that out.
    Will report back .

    Shahrin
     
  10. Chops54

    Chops54 pfm Member

    Craig, when I was checking my arm over I found the black wire had come away from the sleeve. There was hardly any solder on the joint! If the headshell was still fitted I might have tried to fish a longer wire through and use the headshell securing screw as a ground point maybe with a longer screw.
     
    Craig B likes this.
  11. Craig B

    Craig B Re:trophile

    It is likely that this aftermarket lead is correctly soldered at both ends, only not to the Grace pattern at the M-DIN end (which appears to be opposite to standard). M-DIN and F-DIN must be bilaterally inverted wrt one another (when looked at from the mating sides), however, your internal connection tests reveal that Grace has wired F-DIN to M-DIN pattern, only phase reversed relative to standard practice. The supplied lead would have had to match this by having its M-DIN wired to F-DIN pattern (only phase reversed to match the tonearm DIN).

    The DIN end of the cable needs both channel and phase reversal.
     
    Last edited: Jun 11, 2021
  12. Chops54

    Chops54 pfm Member

    The G707 is also perculiar in having the plug and socket arse about.
     
  13. Craig B

    Craig B Re:trophile

    BTW, Shahrin, the phase reversal between Grace F-DIN and the Blue Silver Sonic cable M-DIN will have caused your interconnect to have become less of a shielded cable and more of an RFI/hum field antennae. IOW, positive will be wholly or partially connected via cable shield, rather than via that which is being shielded ('partially' if the cable has two conductors within an outer shield, mic cable style, rather than strictly coax).

    Having said that, the hum issue will be down to not having continuity between tonearm tube and ground pin within the tonearm.

    If the cable DIN plug can be disassembled, I'd rewire this to mirror the tonearm DIN and then see to the arm tube ground.

    Craig
     
  14. Chops54

    Chops54 pfm Member

    The arm hums when the finger lift is touched. The finger lift is grounded through the rh signal ground and not the arm tube. There's a good chance that once the external cable is correctly wired the arm will work ok.
     
  15. Craig B

    Craig B Re:trophile

    Trouble is, Shahrin also reported that he had no continuity between arm tube and tonearm DIN pin 2.

    He also described the symptoms as; "There is hum all the time, its is worse when one touches the headshell lever."

    It would be nice if it were just down to those bass ackwards DIN plugs though.
     
  16. Chops54

    Chops54 pfm Member

    It would indeed :)
     
  17. highcut28

    highcut28 pfm Member

    Hi Craig and Choppy
    You were both right in that the problem was the connecting lead .
    The aftermarket lead did not match Grace's system at the socket at the base of the pillar.
    I am not familiar with M or F DIN conventions, but I did what was necessary to achieve the conventional situation at the headshell wires of
    Red R+, Green R -, White L + ,Blue L - .
    I was able to resolder at the plug - what I did was to reverse the Rt and Lt , and to reverse the phase.
    Quite a tough job to hold the plug firmly - i needed my daughter's help- to do the resolder of stiff silvered wires onto small plug.
    It all came together nicely.
    Multimeter revealed that there was Ground connectivity at the arm pillar and at the brass plate in roof of headshell and its lever but not at the armtube.
    I put in the V15 III (which is why I bought this arm in the first place) and all seems to be well.
    There is no hum. Touching the headshell lever doesnt cause any crackling.
    I dont have a balance on my preamp, but seperation of channels and the stereo imagery are fine

    I am really most grateful. The detailed descriptions,pics and explanations really helped.
    Thank you both and others who have chirped in, very much.
    Seems i have hijacked this thread and benefitted from it .
    I wonder if thread starter Chaggy 78 sorted out his hum?

    Choppy will you be putting a new headshell onto your Grace . I think your black finish makes it a MkII.
    A suitable new head may well enhance the overall aesthetics.
    May i send you a PM?

    Cheers
    Shahrin
     
    Craig B likes this.
  18. Chops54

    Chops54 pfm Member

    Good result and I'm glad you got it sorted Shahrin :)

    I have a Hadcock headshell fitted to my 707. I've enlarged one mounting hole slightly so I can twist the cartridge a smidge to get the alignment just right. I'll post some more pics later.

    Feel free to pm :)
     
  19. Craig B

    Craig B Re:trophile

    Yes, glad to hear that all is working well now, Shahrin.

    Also, that is interesting info re yours having continuity between brass headshell plate and arm pillar, but not between arm tube and pillar. This sets my mind into signal tracing mode, however, I won't bore you with any hypotheses; rather, here's wishing you many happy hours enjoying your music.

    Craig
     
  20. Chops54

    Chops54 pfm Member

    I managed to find some time yesterday to put my arm back together. Not quite finished but I thought I might as well post my progress.

    Here I've removed the cw decoupling sleeve and the pin that takes the bias thread so the back end of the arm tube can be placed in the bearing housing.
    [​IMG]

    Arm wires are threaded through the lower bearing and into the arm pillar.
    [​IMG]

    And here's the back end of the arm tube sitting in the bearing housing.
    [​IMG]

    The bearing housing and the bearing in the arm tube are lined up so the bearing pin can be screwed in.
    [​IMG]

    Finally the bearing pin is screwed in.
    [​IMG]

    Once the pin is installed in the other side I adjust them so the arm tube is central in the housing and then I adjust them to take out the free play. There's a point at which free play becomes minimal but arm movement starts to stiffen up. It's not always possible to set them up absolutely perfect and in that case I'd rather have a trace of free play in the bearings than any trace of tightness.

    This is the arm tube all back together with the cw decoupling sleeve etc
    [​IMG]

    Here I've soldered the din socket to the arm wires. You can see the glue on the socket which can make it hard to remove.
    [​IMG]

    And with cartridge tags on the other end.
    [​IMG]

    I might have mentioned before that the headshell was damaged when I bought this arm. I had intended to make my own headshell and I might do that at some point but I had a Hadcock headshell spare so I went for that instead. Here she is finished and sitting on an old Linn armboard but I decide what to do with her :)
    [​IMG]

    Couple of pics of my arm rest for Shahrin
    [​IMG]

    [​IMG]
     
    Craig B likes this.

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