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Starting up a probable dormant vintage amp

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  • #91
    Originally posted by g1 View Post
    The bus wire connects to the pot casings, the casings are all bolted up to the chassis. So that is how the grounds connect to chassis. The Bulgin AC connector, I'm not positive about. Does the 3rd pin connect directly to chassis? Or does the green wire go to the bus wire which is then connected to chassis by the pots?

    https://music-electronics-forum.com/...3&d=1586982570
    Originally posted by eschertron View Post
    If the PE wire doesn't go straight to chassis via a reliable connection (such as a dedicated bolt) it really aughta. To meet modern code and make the shortest, highest-current-carrying path to ground from a live chassis. I don't want to bet my life on a bunch of fiddly bits between deadly voltages and protective earth. Don't bet yours, or customers, either.
    Originally posted by g1 View Post
    I don't disagree with that at all. Just pointing out how the chassis is not 'floating'. Modern standards are much more stringent. But this thing was designed with chassis 'earthed', wasn't it? Like a Fender that has the 3rd prong ground going to a transformer bolt?
    Not really acceptable, but definitely not 'floating'.
    The green wire from the Bulgin wraps around with the other wires to the front - attached at the pot bus between presence and bass. I see no connection to the chassis, nor did I get a continuity reading from that ground bus to the chassis - the ground bus is not attached to the body of the pots. With no continuity between the ground bus and the chassis - I thought this meant it was floating...??

    Of course I imagine it should be "fixed" - that's the only way I even knew to do it - shortest wire from earth to chassis - dedicated spot as close as possible - the other chassis point usually being at the input jack - at least that's how I try to build out amps. Hence the question, I wasn't sure this was... safe.
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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    • #92
      If you're not getting continuity between chassis and the green 'ground' wire, then that's an issue to fix. Is it possible that the points you chose to test on the chassis were painted, or coated with a non-conductive layer? Just throwing that out there to eliminate it as a possibility.
      If it still won't get loud enough, it's probably broken. - Steve Conner
      If the thing works, stop fixing it. - Enzo
      We need more chaos in music, in art... I'm here to make it. - Justin Thomas
      MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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      • #93
        Maybe the ONLY accetable reason to drill a new hole in an otherwise time-capsule piece that's going to be gigged, in my book... Maybe I'd consider using a tranny bolt for it cuz none of my Fenders have failed me yet, but I've been zapped by a few others...

        Tough call but unless it's going in a museum life is more precious. Cuz OF COURSE I would play it out.

        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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        • #94
          Originally posted by Justin Thomas View Post
          Maybe the ONLY accetable reason to drill a new hole in an otherwise time-capsule piece that's going to be gigged, in my book... Maybe I'd consider using a tranny bolt for it cuz none of my Fenders have failed me yet, but I've been zapped by a few others...

          Tough call but unless it's going in a museum life is more precious. Cuz OF COURSE I would play it out.

          Justin
          This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

          I'd take the green earth lead from the AC socket to the nearest tranny bolt and use a tooth washer, then slather it in anti ox grease.

          As to why the pots don't show continuity with the chassis, it looks to me that the pot case nearest the power supply has it's ground soldered to the pot case and the bus and the chassis is not painted. However, the washers for the pots look to be black in color. Is it possible that they are intentionally isolated? That might reduce the possibility of multiple tiny ground loops.?. That seems fine as long as the AC ground is on the chassis as a safety ground. I guess it's also possible that every single pot mount is exhibiting some kind of bi metal oxide acting as an insulator. As unlikely as it seems I know this has happened in some Marshall amps that relied on pot/chassis contact for circuit ground. I've never seen it myself, but I've read about it on a few occasions here and elsewhere.
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

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          • #95
            That seems fine as long as the AC ground is on the chassis as a safety ground.
            To provide shielding against outside EMI the chassis must be connected to circuit ground.

            The back shell of pots is a separate part and its electrical contact to the chassis is not always reliable.
            Last edited by Helmholtz; 04-22-2020, 06:44 PM.
            - Own Opinions Only -

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            • #96
              Thank you!!

              @Chuck H - good eye - the ground lug to the presence pot is soldered to the body of the pot. As far as checking the continuity - I tried a few pot backs to chassis connection; but the presence pot was not one of them.

              Well, either way... I agree with everything. I'm gonna drill a hole or two to secure up the grounds - it's not a museum piece and if I were to ever sell it in the distant distant ... distant future, any buyer shouldn't hold that against a seller. Safety first, sound second, the rest is just gravy.

              Thanks!!
              "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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              • #97
                Allllllriiiight - so I drilled two holes, one at the bulging for the arch and one at the inputs and ran the ground bus to there.

                It seems originally that the grounding was done through the presence pot connection to the chassis... I cut the ground wire from the ground lug to the pot body since I attached it at the input jacks.

                Thanks guys! I appreciate the responses and solid advise!
                "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                Comment


                • #98
                  Allllllriiiight - so I drilled two holes, one at the bulging for the arch and one at the inputs and ran the ground bus to there.

                  It seems originally that the grounding was done through the presence pot connection to the chassis... I cut the ground wire from the ground lug to the pot body since I attached it at the input jacks.

                  Thanks guys! I appreciate the responses and solid advise!
                  "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                  Comment

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